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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 08 Dec 2023 21:40:13 +0000Body wave synthesis for shallow earthquake sources : inversion for source and earth structure parameters
https://resolver.caltech.edu/CaltechETD:etd-09142006-142025
Authors: Langston, Charles Adam
Year: 1976
DOI: 10.7907/Z3ZM-AC93
Expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions. Useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented. The results of this procedure are used to generate synthetic P, SV and SH waveforms for various assumptions of stress drop. The Thomson-Haskell layer matrix method for computing far-field body wave displacements from shear dislocations is also formulated to complement the ray theory methods when complicated earth structures are considered. An iterative generalized inverse technique is developed using analytic partial derivatives for estimating source parameters from data sets of P and S seismograms from shallow earthquakes.
With the inverse technique and ray theory methods long period P and SH waveforms are analyzed from the Koyna, India, earthquake of 10 December 1967. Using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 seconds of the long-period waveforms are synthesized for 17 stations and a focal mechanism obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, -29° rake plunging to the northeast, and N16°E strike, all angles ± 6°. This is an eastward dipping, left-lateral oblique-slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5 ± 1 5 sec, from a triangular time pulse which has a rise time of 2.5 sec and a tail-off of 3.9 sec, source depth of 4.5 ± 1.5 km and seismic moment of 3.2 ± 1.4 x 10(25) dyne-cm. Some short period complexity in the time function is indicated by modelling short-period WWSSN records but is complicated by crustal phases. The long-period P waveforms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time. Taken together with the I. M. D. epicenter and present depth determination, it places the epicenter directly on the meizoseismal area.
Simultaneous modelling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both the ray and layer matrix formulations. The source parameters obtained are: dip 70° to the east, strike 344° rake -75°, 63 km depth, average moment of 1.4 ± 0.6 x 10(26) dyne-cm and a triangular time function with a rise time of 0.5 sec and fall-off of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15 km thick low velocity zone with a 2.5 km/sec P wave velocity contrast lower boundary situated at approximately 56 km depth. The crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short period teleseismic observations. The inferred reflection from the base of the low velocity zone is recovered from short period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short and long period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.
To substantiate the unusual structure found from the Puget Sound waveform study the structure under Corvallis, Oregon, was examined using long period Ps and Sp conversions and P reverberations from teleseismic events as recorded at the WWSSN station COR. By modelling these phases in the time domain using a data set composed of six deep and intermediate depth earthquakes a similar low velocity zone structure is again inferred. The lower boundary occurs at 45 km depth and has S and P velocity contrasts of 1.3 and 1.4 km/sec, respectively. The material comprising the low velocity zone has a Poisson ratio of at least 0.33 and is constrained by the average P and S travel times determined from the converted phases.https://thesis.library.caltech.edu/id/eprint/3540Generalized Ray Models of Strong Ground Motion
https://resolver.caltech.edu/CaltechETD:etd-04302007-150812
Authors: Heaton, Thomas Harrison
Year: 1979
DOI: 10.7907/4VEQ-8942
<p>A method for synthesizing local ground displacement from a model consisting of a finite fault located within a layered half-space is demonstrated. The response of a three-dimensional fault is evaluated by integrating the responses of point shear dislocations over the fault plane (Green's function technique). The response of each point shear dislocation is evaluated by using generalized ray theory in conjunction with the Cagniard-de Hoop technique. A basic review of these methods is given. In general, the complete solution to a three-dimensional fault in a layered half-space is complex and computationally unwieldy. Various simplifying approximations, whose validity depends upon the source to receiver geometry and seismic frequency, are discussed. The records from three Southern California earthquakes of different magnitudes and source to receiver geometries are modeled and appropriate approximations are demonstrated.</p>
<p>The smallest earthquake that is modeled is the largest earthquake (M 4.9) in the November, 1976 Brawley swarm. Long-period strong-motion instruments were located at distances of 33 km (IVC) and 36 km (ELC). The IVC record consists almost entirely of transversely polarized motion, whereas the ELC record contains an approximately equal proportion of transversely and radially polarized motion. A simplified shear wave velocity model was determined from the compressional wave refraction studies of Biehler, Kovach and Allen (1964). The epicentral location and focal mechanism (right-lateral strike-slip) computed from P wave first arrival studies were used to locate and orient a double-couple point source within the layered half-space. Essentially, the far-field time function and source depth were the only parameters without good independent constraints. A far-field time function with a duration of 1.5 seconds along with a source depth of 7 km was sufficient to model the first 25 seconds of transverse ground motion. Although it seems clear that faulting had finite dimensions, the source to receiver geometries and small source dimension make it possible to model this earthquake with a single point dislocation having the appropriate far-field time function. It appears that the effects of velocity structure on the propagation of long period SH waves are predictable in the Imperial Valley. A study of the synthetic Fourier amplitude spectra indicates that wave propagation effects should be included in studies of source spectra and seismic wave attenuation.</p>
<p>Several synthetic models are constructed to fit the first 40 seconds of transversely polarized displacement, as recorded at El Centro (ELC), of the April 9, 1968 Borrego Mountain earthquake (M 6.5). Unfortunately, there are complications involving the non-planar seismic velocity structures which lie between source and receiver. A simplified structure of a layer over a half-space is used to roughly approximate the effect of the thick sequence of sediments in the Imperial Valley. The beginning 10 seconds of the observed record is used to model the spatial and temporal distribution of faulting, whereas the remaining portion is used to determine the upper crustal structure based on surface-wave periodicity. A natural depth criterion is provided by comparing the amplitude of the direct arrival with the surface-wave excitations. Considerable non-uniqueness is present in the modeling process. If strong midcrustal seismic discontinuities are present, then it is possible to model the ground motion with a single point dislocation. Within the framework of a single layer over a half-space model, faulting of finite vertical extent is required, whereas the horizontal dimensions of faulting are not resolvable. A model which is also consistent with the teleseismic results of Burdick and Mellman (1976) indicates massive faulting near a depth of 9 km with a fast rise time producing a 10 cm displacement pulse of 1 second duration at El Centro. The faulting appears to slow down as it approaches the free surface. The moment is calculated to be approximately 7 x 10²⁵ dyne-cm which is somewhat smaller than that found from teleseismic body waves by Burdick and Mellman (1976).</p>
<p>Because of the special source to receiver geometries present for the Brawley and Borrego Mountain earthquakes, it is necessary only to model SH waves. Furthermore, near-field source terms can be neglected and problems associated with fault finiteness are relatively easy to deal with. This is not true in the case of modeling the strong-motion recordings of the February 9, 1971 San Fernando earthquake (M 6.5). Three-dimensional models of a finite fault located in a half-space are constructed to study the ground motions observed at JPL, Palmdale, Lake Hughes and Pacoima Dam. Since the duration of faulting is comparable to the travel times for various wave types, very complex interference of these arrivals makes a detailed interpretation of these waveforms difficult. By investigating the motion due to small sections of the fault, it is possible to understand how various wave types interfere to produce the motion due to the total fault. Rayleigh waves as well as S to P head waves are shown to be important effects of the free surface. Near-field source effects are also quite dramatic. Strong directivity is required to explain the difference in amplitudes seen between stations to the north and stations to the south. Faulting appears to have begun north of Pacoima at a depth of 13 km. The rupture velocity, which is near 2.8 km/sec in the hypocentral region, appears to slow to 1.8 km/sec at a depth of 5 km. Displacements on the deeper sections of the fault are about 2.5 meters. Fault offsets become very small at depths near 4 km and then grow again to 5 meters near the surface rupture. The large velocity pulse seen at Pacoima is a far-field shear wave which is enhanced by directivity. Peak accelerations at Pacoima are probably associated with the large shallow faulting. The total moment is 1.4 x 10²⁶ ergs.</p>https://thesis.library.caltech.edu/id/eprint/1552Evidence for fault asperities from systematic time-domain modeling of teleseismic waveforms
https://resolver.caltech.edu/CaltechETD:etd-12052006-144020
Authors: Ebel, John Edward
Year: 1981
DOI: 10.7907/xdeb-v591
A simple method for determining which events prior to a main shock may be "true" foreshocks, events which are caused by the same failure process as that which triggers the main shock, is proposed. An event is regarded as a "true" foreshock if it takes place within a certain time period of, is at least half a magnitude unit smaller than and occurs within the aftershock zone of a main shock. The time periods of potential foreshock occurrence computed from local and regional seismicity rates for four events were calculated to range from several days to several weeks prior to the corresponding main shocks.
A detailed analysis of two foreshocks (FS-1, M = 7, and FS-2, M = 6-1/2), the main shock (M, M = 7-1/2) and two aftershocks (A-1, M = 6-3/4, and A-2, M = 7) from the August 11, 1965 New Hebrides Islands earthquake sequence is presented. Focal mechanisms, depths, moments, time function durations and directions of rupture (if they could be inferred) for the events have been found using time domain synthetic seismograms of the far-field body waves and surface waves. The focal mechanisms of all the events except A-2 are consistent with faulting on the interface between the subducting Indian plate and the overriding Pacific plate. A-2 was an event on a steeply-dipping fault which ruptured into the underthrusting plate. The observed radiation patterns for the Rayleigh and Love waves from these events are consistent with the results of the body-wave analysis. The theoretical static vertical surface displacements computed from the teleseismic source model for M are much smaller than the observed coastal uplift indicating that very long-period deformations accompanied the earthquake. The seismicity during the sequence migrated first from northwest to southeast and then toward the southwest and northeast.
A detailed source study of the short-period P waves from the Borrego Mountain earthquake in Southern California is reported. The short-period waveforms at different stations show good coherence, indicating that the seismograms contain reliable information from the source region. From simultaneous long-period-short-period deconvolutions the sP phase was found to consist of two separate pulses. Synthetic seismograms computed from the long-period source model of Burdick and Mellman (1976) did not match the data very well while synthetics with two high-frequency point sources did. The results of a waveform inversion analysis indicate that both sources were located at a depth of 8 km, had similar focal mechanisms, had time function durations of approximately 2 seconds and occurred about 2.2 seconds apart. Synthetic displacement, velocity and acceleration records, computed from a smoothed version of the teleseismic, short-period source model, fit both the amplitudes and waveforms of the SH wavetrain from the strong-motion data from El Centro, California.
The existence of asperities on the fault zones in the two source regions is inferred. In the New Hebrides three asperities are proposed--one at the northern end of the 1965 seismic zone, one between the islands of Santo and Mallikolo, and one near the southern end of the main shock fault plane. The sequence of events reflects a pattern of the loading and breaking of asperities on the fault. For the Borrego Mountain earthquake the short-period sources represent the breaking of two asperities. The stress drops of the two events were several hundred bars each while the average stress drop for the entire event was about 20 bars. For both the New Hebrides events and the Borrego Mountain earthquake, the area of the asperities which ruptured during the main event was no more than 15% of the total fault area.
https://thesis.library.caltech.edu/id/eprint/4797Complexity of Rupture Propagation in Large Earthquakes in Relation to Tectonic Environment
https://resolver.caltech.edu/CaltechTHESIS:02042015-163349497
Authors: Stewart, Gordon Selbie
Year: 1982
DOI: 10.7907/9d84-pr40
<p>Complexity in the earthquake rupture process can result from many factors. This study investigates the origin of such complexity by examining several recent, large earthquakes in detail. In each case the local tectonic environment plays an important role in understanding the source of the complexity.</p>
<p>Several large shallow earthquakes (M<sub>s</sub> > 7.0) along the Middle American Trench have similarities and differences between them that may lead to a better understanding of fracture and subduction processes. They are predominantly thrust events consistent with the known subduction of the Cocos plate beneath N. America. Two events occurring along this subduction zone close to triple junctions show considerable complexity. This may be attributable to a more heterogeneous stress environment in these regions and as such has implications for other subduction zone boundaries.</p>
<p>An event which looks complex but is actually rather simple is the 1978 Bermuda earthquake (M<sub>s</sub> ~ 6). It is located predominantly in the mantle. Its mechanism is one of pure thrust faulting with a strike N 20°W and dip 42°NE. Its apparent complexity is caused by local crustal structure. This is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of N. America.</p>
<p>A study of several large strike-slip continental earthquakes identifies characteristics which are common to them and may be useful in determining what to expect from the next great earthquake on the San Andreas fault. The events are the 1976 Guatemala earthquake on the Motagua fault and two events on the Anatolian fault in Turkey (the 1967, Mudurnu Valley and 1976, E. Turkey events). An attempt to model the complex P-waveforms of these events results in good synthetic fits for the Guatemala and Mudurnu Valley events. However, the E. Turkey event proves to be too complex as it may have associated thrust or normal faulting. Several individual sources occurring at intervals of between 5 and 20 seconds characterize the Guatemala and Mudurnu Valley events. The maximum size of an individual source appears to be bounded at about 5 x 10<sup>26</sup> dyne-cm. A detailed source study including directivity is performed on the Guatemala event. The source time history of the Mudurnu Valley event illustrates its significance in modeling strong ground motion in the near field. The complex source time series of the 1967 event produces amplitudes greater by a factor of 2.5 than a uniform model scaled to the same size for a station 20 km from the fault.</p>
<p>Three large and important earthquakes demonstrate an important type of complexity --- multiple-fault complexity. The first, the 1976 Philippine earthquake, an oblique thrust event, represents the first seismological evidence for a northeast dipping subduction zone beneath the island of Mindanao. A large event, following the mainshock by 12 hours, occurred outside the aftershock area and apparently resulted from motion on a subsidiary fault since the event had a strike-slip mechanism.</p>
<p>An aftershock of the great 1960 Chilean earthquake on June 6, 1960, proved to be an interesting discovery. It appears to be a large strike-slip event at the main rupture's southern boundary. It most likely occurred on the landward extension of the Chile Rise transform fault, in the subducting plate. The results for this event suggest that a small event triggered a series of slow events; the duration of the whole sequence being longer than 1 hour. This is indeed a "slow earthquake".</p>
<p>Perhaps one of the most complex of events is the recent Tangshan, China event. It began as a large strike-slip event. Within several seconds of the mainshock it may have triggered thrust faulting to the south of the epicenter. There is no doubt, however, that it triggered a large oblique normal event to the northeast, 15 hours after the mainshock. This event certainly contributed to the great loss of life-sustained as a result of the Tangshan earthquake sequence.</p>
<p>What has been learned from these studies has been applied to predict what one might expect from the next great earthquake on the San Andreas. The expectation from this study is that such an event would be a large complex event, not unlike, but perhaps larger than, the Guatemala or Mudurnu Valley events. That is to say, it will most likely consist of a series of individual events in sequence. It is also quite possible that the event could trigger associated faulting on neighboring fault systems such as those occurring in the Transverse Ranges. This has important bearing on the earthquake hazard estimation for the region.</p>https://thesis.library.caltech.edu/id/eprint/8766Constraints on the Earth's Anelastic and Aspherical Structure from Antipodal Surface Waves
https://resolver.caltech.edu/CaltechTHESIS:08302019-112320091
Authors: Chael, Eric Paul
Year: 1983
DOI: 10.7907/xe9f-zz51
<p>Seismograms recorded at antipodal distances (Δ~180°) are used to measure the attenuation and dispersion of surface waves. The antipode is a focus for surf ace wave energy radiated at all directions from the source. Antipodal records thus give direct estimates of global average properties. Laterally heterogeneous Earth structure degrades the focusing and complicates the data analysis.</p>
<p>Group velocity and Q of 120-300 s Rayleigh waves are obtained from a record (station PTO) of the May 23, 1968 New Zealand earthquake. The effects of aspherical structure on this record are simulated by generating synthetic seismograms for an ellipsoidal Earth. The results show that the bias in antipodal Q measurements varies with time. This property is used to constrain the bias in the PTO measurements.</p>
<p>A seismogram of the August 19, 1977 Indonesian earthquake from station TRN is used to study 250-500 s Rayleigh waves. This record is synthesized by combining first order perturbation theory with a realistic representation of the Earth's heterogeneities. Analysis of the synthetic shows that Q measured from the TRN record is biased as much as 20% and that group velocities are virtually unbiased. The PTO and TRN data yield global average values of Rayleigh wave Q which are comparable to or slightly below those for the model PREM [Dziewonski and Anderson, 1981].</p>
<p>Antipodal Love wave and spheroidal overtone data are also presented. Synthetic seismograms match the observed arrivals well, demonstrating that these wave modes can be profitably studied at the antipode.</p>https://thesis.library.caltech.edu/id/eprint/11778Holographic In-Situ Stress Measurement in Geophysics
https://resolver.caltech.edu/CaltechTHESIS:08302019-150635775
Authors: Cohn, Stephen Norfleet
Year: 1983
DOI: 10.7907/6vsd-wa54
<p>A new and still experimental method for measuring the absolute in-situ stress field in crustal rocks offers several advantages compared to existing in-situ stress measurement techniques. It employs optical holography to record strain-relief displacements in a borehole environment. We call the prototype instrument the holographic stressmeter. It operates in an uncased borehole where it drills strain-relieving side holes into the borehole wall. An interference holographic recording system records the resulting displacements onto film. The reconstructed interference holograms contain sufficient information in their fringe patterns to determine the three-dimensional vector displacements due to strain relief at every point surrounding the side hole. Assuming isotropic, homogeneous, linearly elastic rock, these displacements define the three stresses acting at the borehole wall at a single point. The three non-vanishing stresses acting at each of three points, distributed azimuthally, on the borehole wall provide sufficient constraint to determine all six components of the desired far-field or virgin-rock stress tensor.</p>
<p>The holographic stressmeter employs an on-board side hole drilling system to produce strain relief. Thus it should be economical to operate and it is not restricted to shallow depths as are overcoring techniques. Furthermore, recording the whole displacement field resulting from drilling the side hole reduces the potential contamination of the measurement by residual stress mechanisms which often affect point strain measurements using foil resistance gauges.</p>
<p>To date a complete stress determination in the field has not been attempted. However, a prototype stressmeter has demonstrated repeatedly that the stability necessary to conduct the measurement using this approach is attainable. Results from field deployment show that the stressmeter can make qualitatively correct measurements at one azimuth in a borehole. Modifications to make measurements at the three azimuths required for a complete determination of the stress tensor components should be easily achieved. We propose additional improvements to permit measuring rock elastic properties in situ to enable accurate, quantitative stress determinations. The theoretically predicted precision of the stress component magnitudes using this measurement approach is estimated at 20%.</p>https://thesis.library.caltech.edu/id/eprint/11782Numerical Studies of Propagation of Lg Waves Across Ocean Continent Boundaries Using the Representation Theorem
https://resolver.caltech.edu/CaltechTHESIS:10232019-124415442
Authors: Regan, Janice
Year: 1987
DOI: 10.7907/vc5q-tp40
<p>The methods for Representation Theorem (RT) coupling of finite element (FE) or finite difference calculations and Harkrider's (Harkrider 1964, 1970) propagator matrix method calculations to produce a hybrid method for propagation of SH mode sum seismograms across paths that contain regions of non plane-layered structure are explained and developed. The coupling methods explained in detail use a 2-D Cartesian FE formulation. Analogous methods for the 3-D method follow directly. Extensive tests illustrating the validity and accuracy of the implementation of these coupling methods are discussed. These hybrid techniques are developed to study the propagation of surface waves across regional transition zones or other heterogeneities that exist in part of a longer, mostly plane-layered, path. The effects of a thinning or thickening of the crustal layer on the propagation of L<sub>g</sub> mode sum seismograms have been examined in this study. The thinning or thickening of the crustal layer is used as a simple model of ocean continent transitions. The L<sub>g</sub> phase is of particular interest since it is used in several important applications such as mapping the extent of continental crust, magnitude determination, and discrimination between explosive and earthquake sources. The understanding of the observations that L<sub>g</sub> wave is attenuated completely when the propagation path includes an oceanic portion of length greater than one hundred to two hundred kilometers or a region of complex crustal structure is not complete, and a clear explanation of these phenomena could have important consequences for all these types of studies. The transition model calculations done in this study show that passage through a region of thinning crustal thickness, the model for a continent to ocean transition, increases the amplitude and coda length of the L<sub>g</sub> wave at the surface, and allows much of the modal energy trapped in the crust, which forms the L<sub>g</sub> phase, to escape in to the subcrustal layers as body waves or other downgoing phases. The magnitude of both these effects increases as the length of the transition increases or the slope of the layer boundaries decrease. The passage of the wavefront exiting the continent to ocean transition region through the oceanic structure allows further energy to escape from the crustal layer, and produces a decrease in L<sub>g</sub> amplitude at the surface as the length of the oceanic path increases. The amplitude decrease is maximum near the transition region and decreases with distance from it. Passage through a region of thickening crust, the model of a ocean to continent transition, causes a rapid decrease in the L<sub>g</sub> amplitude at the surface of the crust. The energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer, and any amplitude which has been traveling through the subcrustal layer but has not reached depths below the base of the continental crust is transmitted back into the continental crust. The attenuation of L<sub>g</sub> at the crustal surface along a partially oceanic path occurs in the oceanic structure and in the ocean to continent transition region . The attenuation at the surface depends in part on the escape of energy at depth through the continent to ocean transition region into the underlaying half-space. The total attenuation of Lg due to propagation through a forward transition followed by a reverse transition is at most a factor of four to six. This is inadequate to explain the observed attenuation of Lg. Thus, additional effects, other than geometry must be considered to provide a complete explanation of the attenuation of Lg.</p>https://thesis.library.caltech.edu/id/eprint/11850Source Characteristics of Recent and Historic Earthquakes in Central and Southern California: Results from Forward Modeling
https://resolver.caltech.edu/CaltechTHESIS:03202015-092027205
Authors: Bent, Allison Lyn
Year: 1990
DOI: 10.7907/wyry-7c13
<p>The long- and short-period body waves of a number of moderate earthquakes occurring in central and southern California recorded at regional (200-1400 km) and teleseismic (> 30°) distances are modeled to obtain the source parameters-focal mechanism, depth, seismic moment, and source time history. The modeling is done in the time domain using a forward modeling technique based on ray summation. A simple layer over a half space velocity model is used with additional layers being added if necessary - for example, in a basin with a low velocity lid.</p>
<p>The earthquakes studied fall into two geographic regions: 1) the western Transverse Ranges, and 2) the western Imperial Valley. Earthquakes in the western Transverse Ranges include the 1987 Whittier Narrows earthquake, several offshore earthquakes that occurred between 1969 and 1981, and aftershocks to the 1983 Coalinga earthquake (these actually occurred north of the Transverse Ranges but share many characteristics with those that occurred there). These earthquakes are predominantly thrust faulting events with the average strike being east-west, but with many variations. Of the six earthquakes which had sufficient short-period data to accurately determine the source time history, five were complex events. That is, they could not be modeled as a simple point source, but consisted of two or more subevents. The subevents of the Whittier Narrows earthquake had different focal mechanisms. In the other cases, the subevents appear to be the same, but small variations could not be ruled out.</p>
<p>The recent Imperial Valley earthquakes modeled include the two 1987 Superstition Hills earthquakes and the 1969 Coyote Mountain earthquake. All are strike-slip events, and the second 1987 earthquake is a complex event With non-identical subevents.</p>
<p>In all the earthquakes studied, and particularly the thrust events, constraining the source parameters required modeling several phases and distance ranges. Teleseismic P waves could provide only approximate solutions. Pₙₗ waves were probably the most useful phase in determining the focal mechanism, with additional constraints supplied by the SH waves when available. Contamination of the SH waves by shear-coupled PL waves was a frequent problem. Short-period data were needed to obtain the source time function.</p>
<p>In addition to the earthquakes mentioned above, several historic earthquakes were also studied. Earthquakes that occurred before the existence of dense local and worldwide networks are difficult to model due to the sparse
data set. It has been noticed that earthquakes that occur near each other often produce similar waveforms implying similar source parameters. By comparing recent well studied earthquakes to historic earthquakes in the same
region, better constraints can be placed on the source parameters of the historic events.</p>
<p>The Lompoc earthquake (M=7) of 1927 is the largest offshore earthquake to occur in California this century. By direct comparison of waveforms and amplitudes with the Coalinga and Santa Lucia Banks earthquakes, the focal
mechanism (thrust faulting on a northwest striking fault) and long-period seismic moment (10²⁶ dyne cm) can be obtained. The S-P travel times are consistent with an offshore location, rather than one in the Hosgri fault zone.</p>
<p>Historic earthquakes in the western Imperial Valley were also studied. These events include the 1942 and 1954 earthquakes. The earthquakes were relocated by comparing S-P and R-S times to recent earthquakes. It was found that only minor changes in the epicenters were required but that the Coyote Mountain earthquake may have been more severely mislocated. The waveforms as expected indicated that all the events were strike-slip. Moment estimates were obtained by comparing the amplitudes of recent and historic events at stations which recorded both. The 1942 event was smaller than the 1968 Borrego Mountain earthquake although some previous studies suggested the reverse. The 1954 and 1937 earthquakes had moments close to the expected value. An aftershock of the 1942 earthquake appears to be larger than previously thought.</p>https://thesis.library.caltech.edu/id/eprint/8797Finite differences and a coupled analytic technique with applications to explosions and earthquakes
https://resolver.caltech.edu/CaltechTHESIS:03192015-141813901
Authors: Stead, Richard J.
Year: 1990
DOI: 10.7907/gsvf-5426
An analytic technique is developed that couples to finite difference calculations
to extend the results to arbitrary distance. Finite differences and the
analytic result, a boundary integral called two-dimensional Kirchhoff, are
applied to simple models and three seismological problems dealing with data.
The simple models include a thorough investigation of the seismologic effects
of a deep continental basin. The first problem is explosions at Yucca Flat, in
the Nevada test site. By modeling both near-field strong-motion records and
teleseismic P-waves simultaneously, it is shown that scattered surface waves
are responsible for teleseismic complexity. The second problem deals with
explosions at Amchitka Island, Alaska. The near-field seismograms are investigated
using a variety of complex structures and sources. The third problem
involves regional seismograms of Imperial Valley, California earthquakes
recorded at Pasadena, California. The data are shown to contain evidence of
deterministic structure, but lack of more direct measurements of the structure
and possible three-dimensional effects make two-dimensional modeling of
these data difficult.https://thesis.library.caltech.edu/id/eprint/8795Rupture characteristics of California earthquakes
https://resolver.caltech.edu/CaltechTHESIS:01152013-161612475
Authors: Wald, David Jay
Year: 1993
DOI: 10.7907/0efb-ty71
<p>The rupture characteristics of the 1987 Superstition Hills (M_s = 6.6), the 1989
Loma Prieta (M_s = 7.1), and the 1991 Sierra Madre (M_L = 5.8) earthquakes were
determined using a constrained, damped, least-squares inversion of strong motion and
teleseismic waveforms. Extension of the modeling procedure to employ teleseismic,
empirical Green's functions allowed determination of faulting details of a fourth
earthquake, the great 1906 San Francisco event.</p>
<p>The 1987 Superstition Hills earthquake was the second and larger of two significant
earthquakes that occurred on conjugate faults in the western Imperial Valley.
The first event (M_s = 6.2), located on the Elmore Ranch Fault, had a geometry and
mechanism favorable for triggering the larger event on the Superstition Hills Fault
some 12 hours later. The Superstition Hills event was modeled as three independent
subevents, each nucleating from a common location near the intersection of the two
faults. This required rerupturing of one fault region on the time scale of several seconds.
Slip was quite heterogeneous along strike, but fairly systematic as a function
of depth. Substantial differences between the source process as observed from strong
motion data and from teleseismic data were observed.</p>
<p>The 1989 Loma Prieta Earthquake began with a small (magnitude 4.5 to 5.0)
precursor, which preceded the main part of the rupture by about 2 sec. Rupture was
bilateral, with the overall radiation greater from the northwest portion of the fault.
Separate inversions of the teleseismic data (periods 3-30 sec) and strong motion data
(periods 1-5 sec) resulted in similar models, indicating a close correspondence of
long- and short-period radiation. Forward predictions of the local strong motions
from the teleseismic rupture model matched the distribution, duration and overall
frequency content of the recordings, suggesting that constraints on strong motions
can be made with teleseismic broadband recordings.</p>
<p>Short period and broadband teleseismic waveform data and three-component
strong-motion records were analyzed to obtain the source rupture history of the
1991 Sierra Madre earthquake. The near-field, shear-wave displacement pulse from
this event had a relatively short duration (about 1 sec) for the magnitude of the
event, requiring a particularly high-average stress drop (175 bars). The ground-motion
variations in the Los Angeles region were controlled predominantly by source
directivity. Rupture was updip and southwestward, resulting in strong motions and
heavier damage in regions to the southwest of the epicenter and near the updip fault
projection.</p>
<p>The rupture process of the 1906 San Francisco earthquake was analyzed, using
all high-quality, teleseismic recordings archived in the 1908 Carnegie Report of the
State Earthquake Investigation Commission. The recordings are relatively simple
considering the great rupture length in 1906, requiring that substantial portions of
the fault, while having large slips, radiated little 5-25 sec energy. Two regions of the
fault, one near the epicenter south of San Francisco, and one between Point Reyes
and Fort Ross were responsible for generating the greater part of the energy observed
on the teleseismic recordings. By comparison of our model for 1906 with modern,
well-studied, large strike-slip events, we found similarities in rupture style with the
relatively simple 1990 Philippines earthquake (M_s = 7.8), but contrasts with the
complexity of the 1976 Guatemala earthquake (M_s = 7.5).</p>
<p>The rupture characteristics of these events when analyzed with previous finite-fault
studies over the past decade indicate several common features. Variations
in slip are more pronounced along strike than downdip. Vertical strike-slip faults
show a systematic slip variation with depth, consistent with both shallow and deep
zones with velocity-strengthening frictional resistance; nucleation is usually at the
base of the seismogenic zone. Oblique and dip-slip events show much more depth
variation in slip, indicative of thicker, more complex seismogenic zones associated
with tectonic regimes involving crustal thickening or extension. The Superstition
Hills, Loma Prieta and Sierra Madre strong-motion data sets all require short rise
times, so only a small portion of the fault is slipping at a particular time, in agreement
with the "self-healing" model described by Heaton [1990] and in conflict with long
slip durations required by many crack-like models of dynamic rupture. With the
exception of the Superstition Hills earthquake, seismic moments and slip distributions
determined from the strong-motion data concur with moments and slips derived
from geodetic and longer-period waveforms. This indicates that the higher-frequency
data are sufficient for estimating the total slip, and therefore, the rupture durations
inferred represent the entire coseismic slip duration. The agreement between longand
short-period source models makes it possible to estimate ground motions for
important historical events from source models determined using longer-period (5-15
sec), teleseismic body waves.</p>https://thesis.library.caltech.edu/id/eprint/7398Modeling earthquakes with local and regional broadband data
https://resolver.caltech.edu/CaltechTHESIS:02172011-095833318
Authors: Dreger, Douglas S.
Year: 1993
DOI: 10.7907/es5j-ag55
Waveform modeling techniques are applied to several recent, moderate sized earthquakes
recorded by the broadband TERRAscope array in southern California. A
method for the determination of source parameters at regional distances with three-component,
sparse network data is described. The sensitivity of the method to source
mislocations and velocity model is investigated. The method is relatively insensitive
to source mislocation. The choice of velocity model can affect the inversion results,
but it appears that for a number of paths throughout southern California, a simple
plane layered velocity model derived from travel-time studies explains much of the
observed waveforms.
The broadband waveforms of two small earthquakes that occurred in 1988 near
Upland, California are forward modeled to determine Green's functions for the path
to Pasadena, California. The effects of near surface gradients, crustal interface sharpness,
and two-dimensional basin-ridge structures were studied. This analysis resulted
in a simple plane layered velocity model that best fit the data. The Green's functions
are then used to study the source characteristics of the 1990 Upland mainshock (M_L = 5.2). The long-period body waves are inverted to determine the orientation
and seismic moment. Comparisons of the 1990 mainshock with the 1988 events revealed
that the mainshock was a relatively complicated event. Multi-point source
and distributed finite slip models show that the mainshock ruptured down dip (6
km to 9km) with a non-uniform slip distribution in which 30 % of the total seismic
moment was released from a relatively small area at 9 km depth. The overall area
of the mainshock was found to be significantly smaller than the aftershock zone.
The source process of the June 28, 1991 Sierra Madre earthquake (M_L = 5.8)
is investigated using the broadband data recorded at 6 TERRAscope stations. The
long-period body waves are inverted to determine the orientation and seismic moment.
Ratios of the peak amplitudes of simulated short-period Wood-Anderson and
long-period Wood-Anderson seismograms are compared for the mainshock and the
two largest aftershocks. The ratios show that stations southwest of the epicenter
have elevated levels of short-period energy relative to stations to the east suggesting
the presence of directivity. The displacement waveforms were forward modeled using
distributed finite slip models. The best fitting model consists of an updip rupture
toward the west. This model fails however to explain the amplitudes of the short-period
waves. A non-uniform slip model was developed that better explains the
short-period amplitudes. The results of this analysis indicate that the shorter-period
energy is controlled more by the patches on the fault that experience the greatest
slip, rather than the accumulative motions due to slip on the whole fault surface.https://thesis.library.caltech.edu/id/eprint/6249Seismic structure above and below the core-mantle boundary
https://resolver.caltech.edu/CaltechTHESIS:05242013-114249496
Authors: Garnero, Edward James
Year: 1994
DOI: 10.7907/zsct-z181
<p>Seismic structure above and below the core-mantle boundary (CMB) has been studied
through use of travel time and waveform analyses of several different seismic
wave groups. Anomalous systematic trends in observables document mantle heterogeneity
on both large and small scales. Analog and digital data has been utilized,
and in many cases the analog data has been optically scanned and digitized prior to
analysis.</p>
<p>Differential travel times of S - SKS are shown to be an excellent diagnostic of
anomalous lower mantle shear velocity (V s) structure. Wavepath geometries beneath
the central Pacific exhibit large S- SKS travel time residuals (up to 10 sec), and
are consistent with a large scale 0(1000 km) slower than average V_s region (≥3%).
S - SKS times for paths traversing this region exhibit smaller scale patterns and
trends 0(100 km) indicating V_s perturbations on many scale lengths. These times are
compared to predictions of three tomographically derived aspherical models: MDLSH
of Tanimoto [1990], model SH12_WM13 of Suet al. [1992], and model SH.10c.17
of Masters et al. [1992]. Qualitative agreement between the tomographic model
predictions and observations is encouraging, varying from fair to good. However,
inconsistencies are present and suggest anomalies in the lower mantle of scale length
smaller than the present 2000+ km scale resolution of tomographic models. 2-D
wave propagation experiments show the importance of inhomogeneous raypaths when
considering lateral heterogeneities in the lowermost mantle.</p>
<p>A dataset of waveforms and differential travel times of S, ScS, and the arrival
from the D" layer, Scd, provides evidence for a laterally varying V_s velocity discontinuity
at the base of the mantle. Two different localized D" regions beneath
the central Pacific have been investigated. Predictions from a model having a V_s
discontinuity 180 km above the CMB agree well with observations for an eastern
mid-Pacific CMB region. This thickness differs from V_s discontinuity thicknesses
found in other regions, such as a localized region beneath the western Pacific, which
average near 280 km. The "sharpness" of the V_s jump at the top of D", i.e., the
depth range over which the V_s increase occurs, is not resolved by our data, and our
data can in fact may be modeled equally well by a lower mantle with the increase in
V_s at the top of D" occurring over a 100 krn depth range. It is difficult at present to
correlate D" thicknesses from this study to overall lower mantle heterogeneity, due to
uncertainties in the 3-D models, as well as poor coverage in maps of D" discontinuity
thicknesses.</p>
<p>P-wave velocity structure (V_p) at the base of the mantle is explored using the
seismic phases SKS and SPdKS. SPdKS is formed when SKS waves at distances
around 107° are incident upon the CMB with a slowness that allows for coupling with
diffracted P-waves at the base of the mantle. The P-wave diffraction occurs at both
the SKS entrance and exit locations of the outer core. SP_dKS arrives slightly later in
time than SKS, having a wave path through the mantle and core very close to SKS.
The difference time between SKS and SP_dKS strongly depends on V_p at the base
of the mantle near SK Score entrance and exit points. Observations from deep focus
Fiji-Tonga events recorded by North American stations, and South American events
recorded by European and Eurasian stations exhibit anomalously large SP_dKS -
SKS difference times. SKS and the later arriving SP_dKS phase are separated by
several seconds more than predictions made by 1-D reference models, such as the
global average PREM [Dziewonski and Anderson, 1981] model. Models having a
pronounced low-velocity zone (5%) in V_p in the bottom 50-100 km of the mantle
predict the size of the observed SP_dK S-SKS anomalies. Raypath perturbations
from lower mantle V_s structure may also be contributing to the observed anomalies.</p>
<p>Outer core structure is investigated using the family of SmKS (m=2,3,4) seismic
waves. SmKS are waves that travel as S-waves in the mantle, P-waves in the
core, and reflect (m-1) times on the underside of the CMB, and are well-suited for
constraining outermost core V_p structure. This is due to closeness of the mantle
paths and also the shallow depth range these waves travel in the outermost core.
S3KS - S2KS and S4KS - S3KS differential travel times were measured using
the cross-correlation method and compared to those from reflectivity synthetics created
from core models of past studies. High quality recordings from a deep focus
Java Sea event which sample the outer core beneath the northern Pacific, the Arctic,
and northwestern North America (spanning 1/8th of the core's surface area), have
SmKS wavepaths that traverse regions where lower mantle heterogeneity is pre-
dieted small, and are well-modeled by the PREM core model, with possibly a small
V_p decrease (1.5%) in the outermost 50 km of the core. Such a reduction implies
chemical stratification in this 50 km zone, though this model feature is not uniquely
resolved. Data having wave paths through areas of known D" heterogeneity (±2%
and greater), such as the source-side of SmKS lower mantle paths from Fiji-Tonga
to Eurasia and Africa, exhibit systematic SmKS differential time anomalies of up
to several seconds. 2-D wave propagation experiments demonstrate how large scale
lower mantle velocity perturbations can explain long wavelength behavior of such
anomalous SmKS times. When improperly accounted for, lower mantle heterogeneity
maps directly into core structure. Raypaths departing from homogeneity play
an important role in producing SmKS anomalies. The existence of outermost core
heterogeneity is difficult to resolve at present due to uncertainties in global lower
mantle structure. Resolving a one-dimensional chemically stratified outermost core
also remains difficult due to the same uncertainties. Restricting study to higher
multiples of SmKS (m=2,3,4) can help reduce the affect of mantle heterogeneity
due to the closeness of the mantle legs of the wavepaths. SmKS waves are ideal in
providing additional information on the details of lower mantle heterogeneity.</p>
https://thesis.library.caltech.edu/id/eprint/7751Regional surface wave magnitude and moment determination methods applied to nuclear explosions at the Nevada test site : implications for yield estimation and seismic discrimination
https://resolver.caltech.edu/CaltechETD:etd-04212006-165925
Authors: Woods, Bradley Brett
Year: 1994
DOI: 10.7907/P212-Q044
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
This thesis examines the use of regional surface-wave data to measure the long-period source spectrum of underground nuclear explosions for the purposes of yield determination and seismic discrimination. It is demonstrated that regional (D < 2500 km) fundamental-mode Rayleigh and Love waveforms can be modeled with considerable accuracy. The procedure for modeling regional earth structure for such seismograms by inverting surface-wave dispersion data is described. This technique is a hybrid of preexisting surface-wave analysis and inversion methods. Theoretical path corrections are determined from the Green's function for a given modeled path. A method is described to obtain consistent, stable, time-domain surface-wave magnitude ([...]) or seismic moment ([...]) measurements from poorly dispersed regional Rayleigh waves. Source parameters for 190 Nevada Test Site explosions are determined using these methods. Observations demonstrate that the measurement/detection threshold for regional surface-waves is [...] > 4.0 (Yield = 1 kt)--a significant improvement over classical teleseismic [...] measurements. The results indicate that the [...] (or log [...]) - yield scaling relationship is near unity and constant for explosions of all measurable sizes. Site effects are also investigated to determine the portability of such surfacewave measurements. Spectral-domain moment estimates also were performed on the digital portion of the data set. Besides obtaining an average scalar moment from Rayleigh wave amplitudes, the isotropic (explosive source) and deviatoric moment (double-couple source generated by tectonic release) components were determined by a joint inversion of Rayleigh and Love wave amplitude and phase data. Although in the most general case the inversion solution is non-unique, constraining the depth of the deviatoric source to be equal to that of the explosion and assuming a vertical strike-slip orientation yields a unique linear inversion solution. The spectral moment estimates are similar to the time-domain values, although the spectral-domain moment variances are appreciably smaller than the time-domain ones. A regional short-period vs. long-period seismic discriminant is developed using the ratio of the seismic moment to local magnitude ([...]). This discriminant successfully separates the explosion and earthquake populations at all measurable source sizes, so that for a given seismic moment source level, an explosion has an [...] 0.5 magnitude units larger than a comparable-sized earthquake.
https://thesis.library.caltech.edu/id/eprint/1440Structure of the earth's core and lowermost mantle from seismic PKP waves
https://resolver.caltech.edu/CaltechTHESIS:05312013-094528303
Authors: Song, Xiaodong
Year: 1994
DOI: 10.7907/k5gh-th50
<p>This thesis addresses the fine structure, both radial and lateral, of compressional wave velocity and attenuation of the Earth's core and the lowermost mantle using waveforms, differential travel times and amplitudes of PKP waves, which penetrate the Earth's core.</p>
<p>The structure near the inner core boundary (ICB) is studied by analyzing waveforms of a regional sample. The waveform modeling approach is demonstrated to be an effective tool for constrainning the ICB structure. The best model features a sharp velocity jump of 0.78km/s at the ICB and a low velocity gradient at the lowermost outer core (indicating possible inhomogeneity) and high attenuation at the top of the inner core.</p>
<p>A spherically symmetric P-wave model of the core, is proposed from PKP differential times, waveforms and amplitudes. The ICB remains sharp with a velocity
jump of 0. 78km/ s. A very low velocity gradient at the base of the fluid core is demonstrated to be a robust feature, indicating inhomogeneity is practically inevitable. The model also indicates that the attenuation in the inner core decreases with depth. The velocity at D" is smaller than PREM.</p>
<p>The inner core is confirmed to be very anisotropic, possessing a cylindrical symmetry around the Earth spin axis with the N-S direction 3% faster than the E-W
direction. All of the N-S rays through the inner core were found to be faster than the E-W rays by 1.5 to 3.5s. Exhaustive data selection and efforts in insolating
contributions from the region above ensure that this is an inner core feature.</p>
<p>The anisotropy at the very top of the inner core is found to be distinctly different from the deeper part. The top 60km of the inner core is not anisotropic. From 60km
to 150km, there appears to be a transition from isotropy to anisotropy.</p>
<p>PKP differential travel times are used to study the P velocity structure in D". Systematic regional variations of up to 2s in AB-DF times were observed, attributed primarily to heterogeneities in the lower 500km of the mantle. However, direct comparisons with tomographic models are not successful.</p>
https://thesis.library.caltech.edu/id/eprint/7788Part I. Broadband modeling of aftershocks from the Joshua Tree, Landers, and Big Bear sequences, southern California. Part II. Characteristics of the June 28, 1992, Big Bear mainshock from TERRAscope data: evidence for a multiple-event source
https://resolver.caltech.edu/CaltechTHESIS:11302011-093359502
Authors: Jones, Laura Ellen
Year: 1995
DOI: 10.7907/r040-cd43
<p>The Joshua Tree, Landers and Big Bear earthquake sequences recorded on the broadband TERRAscope array in Southern California provide an unusually large data set of
high-quality three-component broadband waveforms from small to moderately sized earthquakes. This data set offers the opportunity of detailed large-scale studies of these smaller but nonetheless potentially damaging earthquakes. What follows is a detailed study of over sixty M > 3.8 aftershocks in three regions: north and south of the
Pinto Mountain fault in the Mojave desert (associated with the Joshua Tree and Landers sequences), and within the San Bernardino mountains block (associated with the Big Bear sequence). Source parameters, including depths, moments, and durations, for sixty M_w > 3.8 earthquakes from the Joshua Tree, Landers, and Big Bear sequences are
presented here. These events occurred between April of 1992 and November of 1994; the list of events comprises nearly every aftershock above M3.8 for which we could obtain
coherent TERRAscope data and accurate timing and location information.</p>
<p>Choice of velocity model affects the accuracy of the source estimations and the error associated with estimations of moment, though it appears that for Landers events recorded at stations within or near the Mojave region, a simple one-dimensional velocity model is adequate. To minimize model-associated error, however, a velocity model for the Mojave region is developed and presented. This model is used in the computation of the synthetic Green's functions used in estimates of source parameters for many of the earthquakes presented here.</p>
<p>The existence of such a large data set from events in the same region (the Mojave desert) also allows systematic investigation of station effects for the five TERRAscope
stations used in this study; Goldstone (GSC), Isabella (ISA), Pasadena (PAS) , Pinyon Flats (PFO), and Seven Oaks Dam (SVD). For each event, moments and durations are
computed for each station, and these examined for systematic variations of moment with azimuth and with source-receiver distance.</p>
<p>Moments and durations are computed for each aftershock we study, and stress-drops inferred from these appear to vary with location, with respect to previous seismic
activity, and proximity to previous (i.e., Landers) rupture. A strong correlation of increased stress-drop with depth is noted for the Big Bear region; the same is not
observed for Mojave aftershocks.</p>
<p>The June 28, 1992, Big Bear earthquake is commonly considered to be an aftershock of the earlier M_w = 7.3 Landers mainshock, and as such has been perhaps overlooked.
However, it is a significant and enigmatic event in its own right. Its rupture history was obscured by controversy over epicentral location, lack of observed surface rupture,
and the complexity of source suggested by the mainshock waveforms themselves. From overall pat terns of seismicity and long-period focal studies, rupture is generally assumed
to have propagated northeast. However, mainshock locations from both strong-motion and TERRAscope data are consistent and do not lie on this assumed fault plane. Further,
directivity analysis suggests significant energy propagating northwest along the presumed antithetic fault-plane .A combination of directivity analysis, point-source empirical Green's function analysis, and line-source directivity analysis together indicate that a two-fault event is necessary to produce the waveforms observed during the Big Bear mainshock. These results suggest that the Big Bear earthquake comprised at least two substantial subevents, with the initial subevent rupturing towards the northwest on the presumed antithetic fault plane. Several seconds later, rupture initiated on the northeast striking plane.</p>https://thesis.library.caltech.edu/id/eprint/6742High Resolution Modeling of Regional Phases
https://resolver.caltech.edu/CaltechTHESIS:08302023-200342752
Authors: Song, Xi
Year: 1997
DOI: 10.7907/qjr7-9471
<p>It has been a long-time goal of seismologists to decouple source phenomena from propagation effects. This thesis elaborates on our effort towards this goal.</p>
<p>We start by representing earthquakes as point-sources in space and using 1-D synthetics to resolve point-source parameters. Our trial-and-error approach to obtain 1-D crustal models is summarized in a set of sensitivity tests, where regional seismograms are decomposed into segments, i.e., the Pnl segment, the SV waves, the Love wave and the Rayleigh wave, so that the impact of model parameters on each segment is the most direct. In these tests, broadband waveform data is studied in a forward modeling approach, with synthetics computed using the reflectivity method and the generalized ray theory. Applying these tests to paths sampling the Basin and Range province, we find that a simple two-layer crustal model is effective in explaining regional seismograms. Our sensitivity tests also serve to help understand, and inter pret, the many results of a source estimation method we use to obtain point-source parameters. This method desensitizes the source mechanism result from the crustal model used to generate the 1-D synthetics, by allowing relative time shifts between the various segments. With this method, we obtain source mechanisms and seismic moments for a selection of Northridge aftershocks using broadband and long-period waveform data recorded by the TERRAscope array. The source duration of these earthquakes is measured by comparing the short-period to long-period energy ratio in the data to that in the synthetics. The seismic moment and source-duration are used to estimate the relative stress drop. The depth distribution of the relative stress drop indicates that the largest stress drops are in the depth range of 5-15 km for the 24 Northridge aftershocks in our study.</p>
<p>To obtain more detailed information about large earthquakes, such as fault dimension and rupture directivity, we develop a new method of using empirical Green's functions (eGf). As an example, the January 17, 1994 Northridge mainshock is studied with one of its aftershocks as an eGf. The source duration of the mainshock, as seen from the regional surface waves observed at various stations, is obtained by searching for the trapezoidal far-field source-time function for each station which, when convolved with the aftershock data, best simulates the mainshock data. Stations to the north see shorter source durations than those to the south. Modeling these with theoretical predictions of rupture on a square fault, we constrain the effective fault dimension to be 14 km with rupture along the direction of the average
rake vector. A moment of (1.4 ± 0.9) x 10²⁶ dyne•cm with a stress drop of ~120 bars is obtained for the mainshock from our eGf study.</p>
<p>When empirical Green's functions are not available due to a difference in the source mechanisms or in the source locations, theoretical modeling plays an important role. Our approach to develop high resolution Green's functions is to convert eGfs to pseudo Green's functions (pGf). This is done by modeling the eGfs with the generalized ray theory and consists of two major steps.</p>
<p>The first step is to shift individual ray responses to account for a difference in source location. This ray-shifting technique has its own use in fast generation of synthetic seismograms for finite sources. To study the directivity for a finite source, we discretize the fault region into a set of elements represented as point-sources. We then generate the generalized ray responses for the best-fitting point-source location, and derive for each separate ray the response for neighboring point-sources using power series expansions. The response for a finite fault is then a summation over rays and fault elements. If we sum over the elements first, we obtain an effective far field source-time function for each ray, which is sensitive to the direction of rupture. These far-field source-time functions are convolved with the corresponding rays and the results summed to form the total response. A simple application of the above method is demonstrated with the tangential motions observed from the 1991 Sierra Madre earthquake. For this event, we constrain the fault dimension to be about 3 km with rupture towards the west, which is compatible with other more detailed studies.</p>
<p>The second step in the modeling of the eGfs and the development of pseudo Green's functions is to account for variations in model structure by perturbing individual generalized ray responses calculated from a 1-D model. The model is divided into blocks and velocities in the blocks are allowed to vary, which shifts the arrival time of the individual rays. The amplitudes of the rays are perturbed independently to accommodate local velocity variations in the structure. For eGfs that are moderate-sized earthquakes with known source mechanism and time history, the velocity variation in each block and the amplification factor for individual rays can be optimized using a simulated annealing algorithm. The usefulness of the pGfs is demonstrated with the 1991 Sierra Madre earthquakes as examples. The pGf technique is also useful in retrieving 2-D structure, which is essentially waveform tomography. This is demonstrated with a study of a Tibetan profile.</p>https://thesis.library.caltech.edu/id/eprint/16168Broadband waveform modeling and its application to the lithospheric structure of the Tibetan plateau
https://resolver.caltech.edu/CaltechETD:etd-12072006-152420
Authors: Zhu, Lupei
Year: 1998
DOI: 10.7907/EWXK-PQ59
This thesis presents a study of the lithospheric structure of the Tibetan Plateau. The data are broadband seismic waveforms recorded during the 1991-1992 Sino-US Tibet PASSCAL experiment. Several techniques are developed to retrieve the structural information from these waveforms at ranges from near-field to teleseismic distances.
First, a 1-D average crustal velocity model is derived from regional earthquakes, based on travel times of various phases and modeling waveforms of Love waves. The source mechanisms and depths of 62 events in Tibet and surrounding areas are determined using this 1-D model. The result is that most earthquakes occur at shallow depths, between 5 and 15 km. Thrust faulting source mechanisms are dominant on the margins of the plateau. Within the plateau but at locations with surface elevation less than 5 km, source mechanisms are a mixture of strike-slip and thrust. In areas with surface elevation higher than 5 km, all events show consistently normal faulting, which indicates that a large portion of the high plateau is under EW extension. I also found three sub-crustal earthquakes at depth range between 70 and 80 km in southern Tibet. Their existence suggests relatively cold uppermost mantle in the region.
The lateral variations are investigated using teleseismic waveforms. Crustal thickness and Vp/Vs ratio at each station are estimated using receiver function analysis. I found crustal Vp/Vs ratios to range from 1.75 to 2.0 and crustal thicknesses from 55 to 80 km. On average the northern Tibetan crust is 20 km thinner and has a higher Vp/Vs ratio than the southern part. Teleseismic P and S arrival delays exhibit strong azimuthal and lateral variation. The uniform surface elevation of Tibet coupled with large variations of crustal thickness and upper mantle velocity suggest that the north-central plateau is supported partly by a hot upper mantle. A low velocity layer is found in the mid-crust of northern Tibet. Modeling the anomalous "double-pulse" P waveform at the northernmost station reveals a 15 to 20 km Moho offset between the plateau and the Qaidam Basin.https://thesis.library.caltech.edu/id/eprint/4842High Resolution Studies of Deep Earth Structure
https://resolver.caltech.edu/CaltechTHESIS:08032023-144000226
Authors: Xiaoming, Ding
Year: 1998
DOI: 10.7907/z21r-bk54
<p>Recent advances in seismic tomography has imaged major deep structure in the lower mantle. The ring of fast velocities originally derived from global long-period inversions has been resolved into interspersed sheet-like structure which appears to be old slabs. Beneath some of the structure, there are high velocity zones (HVZ) with variable thickness approaching the core mantle boundary (CMB). Detailed broadband modeling of waveforms produced by seismic paths sampling one of these zones yields a picture of D" zone, with one thermal negative above the layer and one positive approaching the CMB, which would seem to be quite compatible with pancaked slab debris. In contrast, modeling waveforms produced by sampling the slowest velocity regions along the CMB reveals a thin ultra low velocity zone (ULVZ). The dimensions of these zones range from a few hundred km beneath Iceland to a few thousand km beneath Africa.</p>
<p>Seismic data recorded on TERRAscope and Berkeley Digital Seismic Network are used to study the HVZ beneath Central America. Modeling these waveforms (P, SV and SH) constitutes a major portion of this thesis. Two modeling strategies were employed in the thesis: (1) Assume a "Lay type" D" with a sharp velocity discontinuity; (2) Assume an upper transition zone approaching D", and a lower transition zone approaching the CMB (old slabs). Our preferred model following strategy (1) (Chapter 2) has an S discontinuity 200 km above the CMB with 3% jump and a negative gradient in the D" layer. It fits S, SKS, Scd on radial component and
S and Scd phase on tangential component in the distance range of 78° to 92°. A 2D model with decreasing thickness of D" layer toward North is also investigated. It fits the data better at the distances beyond 90° and produces reasonable good fit of the S and Scd phases. This suggests that the typical negative gradient in the "Lay type" D" may not be necessary as in SDH. P waveform modeling, on the other hand, shows no indication of a corresponding Ped phase. The PREM model fits the P travel time and waveform well. Our preferred model following strategy (2) (Chapter 3) has a positive gradient initiated 350 km above the CMB with a sharper increase near 200 km and a strong negative radient begins at about 100 km. This model can explain both P and S waveforms.</p>
<p>In Chapter 4 the ULVZ beneath Iceland and Africa are addressed. The major phases used to study the ULVZ are SKS and S_[P_(diff)]KS which travels along the CMB as P at both the core entry (S_[P_(diff)]KS) and exit (SK_[P_(diff)]S) locations. A major structure beneath Iceland (SK_[P_(diff)]S) as identified from data recorded on stations in Northern Europe appears to be shaped like a dome, 80 km high, 200 km wide with a 10% drop in P and S velocities. The data for Africa is less complete but highly anomalous. Shear wave record sections across Africa and Europe containing the cross-over from S to SKS and extended core-phases (75° to 120°) are presented from deep South American events. These are compared against corresponding synthetics for various tomography models computed with a new 2D synthesizing technique (Appendix A). Some of the most recent models, Grand [1994], explain the observation for African data better than lD models. However, considerable fine tuning is required in D" to explain abrupt changes in S and ScS waveforms and the extreme cases in SKS-S travel times. Essentially, Grand's anomaly needs to be increased to -4% with evidence for a strong plume (1500 km of vertical structure with -4% velocity drop) to explain the SKS travel times and waveform data. The plume is located along the eastern edges of the basal low velocity region.</p>
<p>By studying the various branches of the core phases PKP, it has become quite
clear that North-South paths in the inner-core appear faster than East-West paths. Moreover, the broadband seismograms associated with these paths are distinct. The reason for this difference is not known but suggests a lower (anisotropic) inner-core with an upper (isotropic) inner-core which may have variable thickness. Modeling of long period and broadband data for such structures is, also, addressed in Chapter 5 of this thesis.</p>https://thesis.library.caltech.edu/id/eprint/16150Analysis and Modeling of Seismic Ground Motions in Heterogeneous Structures in Southern California
https://resolver.caltech.edu/CaltechTHESIS:07142023-185900244
Authors: Scrivner, Craig William
Year: 1998
DOI: 10.7907/m61x-wq81
This thesis contains studies of seismic data from the 1995-1996 Ridgecrest earthquake sequence, an aftershock of the 1987 Whittier Narrows earthquake and the 1991 Sierra Madre mainshock, and aftershocks of the 1994 Northridge earthquake. The Ridgecrest data set spans southern California, including stations in the Los Angeles area basins. The Whittier Narrows/Sierra Madre and Northridge data sets consist of stations in the Los Angeles and San Fernando Valleys, respectively, and record earthquakes occurring directly adjacent to these sedimentary basins. The studies examine the variability of ground motions in the crust and details of seismic propagation from the crust into sedimentary basins. In the Ridgecrest study, amplitudes of synthetic waveforms from a 1D model are compared with data amplitudes at rock, soil, and basin sites. At rock sites, the data amplitudes are within a factor of 2 of the synthetic amplitudes. At basin and soil sites, the data are within a factor of 3 of the synthetic amplitudes. Stations beyond the trailing edge of sedimentary basins are affected by leaked basin surface waves. In the Whittier Narrows/Sierra Madre study, waveform phases generated by the edge of the Los Angeles basin are identified and modeled with a 2D structure. In the data, multiples of the direct shear wave, reflected from the surface and turned by the basin edge, are up to two times the amplitude of the direct arrival. A simple, smooth, 2D basin edge model produces the correct timing and relative amplitude of basin-trapped phases. In the Northridge study, we contrast waveforms from a shallow and a deep event. The waveforms from the shallow event include basin-generated surface waves in the basin, and a phase-shift in the direct shear wave outside the basin. A model with a strong velocity contrast at about 1 km depth in the upper basin, a depth for the entire basin just above the shallow source depth, and a gradient beneath the basin produces synthetic waveforms that match the distinctive features in the data set.https://thesis.library.caltech.edu/id/eprint/16141I. Rupture Properties of Large Subduction Earthquakes. II. Broadband Upper Mantle Structure of Western North America
https://resolver.caltech.edu/CaltechTHESIS:09122016-121754817
Authors: Melbourne, Timothy Ian
Year: 1999
DOI: 10.7907/87nd-p040
<p>This thesis contains two studies, one of which employs geodetic data bearing on large
subduction earthquakes to infer complexity of rupture duration, and the other is a
high frequency seismological study of the upper mantle discontinuity structure under
western North America and the East Pacific Rise. In the first part, we present
Global Positioning System and tide gauge data which record the co-seismic deformation
which accompanied the 1995 M<sub>w</sub>8.0 Jalisco event offshore central Mexico, the
1994 M<sub>w</sub>7.5 Sanriku event offshore Northern Honshu, Japan, and the 1995 M<sub>w</sub>8.1
Antofagasta earthquake offshore Northern Chile. In two of the three cases we find
that the mainshocks were followed by significant amounts of rapid, post-seismic deformation
which is best and most easily explained by continued slip near the co-seismic
rupture patch. In Jalisco, we find that the post-seismic deformation which occurred
during the two weeks following the mainshock amounted to as much 70% of the co-seismic
deformation, from which we estimate an additional moment release of 40%,
while in the Sanriku event an additional 30% moment release followed in the 10 days
after the mainshock. Because of the favorable geometry of the Jalisco network, we infer
that the post-seismic faulting occurred predominantly down-dip of the co-seismic
rupture plane. This is the first documented case of rapid slip migration following
a large earthquake, and is pertinent to earthquake prediction based on precursory
deformation. Following the Antofagasta mainshock there was no rapid post-seismic
displacement within the resolution of the GPS measurements, which equals roughly
1% of the co-seismic displacement. As the three GPS data sets represent the best
observations of large subduction earthquakes to date and two of them show significant
amounts of aseismic energy release, they strongly suggest silent faulting may
be common in certain types of subduction zones. This, in turn, bears on estimates
of global moment release, seismic coupling, and our understanding of the natural
hazards associated with convergent margins.</p>
<p>The second part of this dissertation utilizes high frequency body waves to infer
the upper mantle structure of western North America and the East Pacific Rise.
An uncharacteristically large M<sub>w</sub>5.9 earthquake located in Western Texas provided a
vivid topside reflection off the 410 Km velocity discontinuity ("410"), which we model
to infer the fine details of this structure. We find that, contrary to conventional
wisdom, the 410 is not sharp, and our results help reconcile seismic observations
of 410 structure with laboratory predictions. By analyzing differences between our
structure and seismic 410 structure estimates under the nearby Gulf of California, we
attempt to extract differences in temperature and mineralogy between subcontinental
and suboceanic 410 structures.</p>
<p>Extending this analysis, we utilize teleseismic events from East Pacific Rise transform
faults to model multiple S upper mantle triplications. We find that for raypaths
traversing the rise crest the 1-D model TNA [Grand and Helmberger (1984)] derived
for the western US accurately predicts differential SnS-S travel times and triplication
waveform structure, implying that there is little velocity heterogeneity along the
ridge crest along nearly its entire length. We find that for energy traversing paths
increasingly away from the ridge axis there is no discernible change in the apparent
depth of the 410 and 670 Km discontinuities. In the shallowest mantle (uppermost 75
Km), there is a strong lateral shear velocity gradient amounting to 3% over roughly
150 Km. The LID, nonexistent at the ridge crest, grows slowly in thickness beyond
150 Km from the axis. The compatible geodynamic model of these two results is that
the East Pacific Rise is not fed from the local lower mantle, rather, upper mantle
material must be transported laterally to supply the ridge axis spreading center, and
the LID reflects the source region of the East Pacific Rise magma supply.</p>https://thesis.library.caltech.edu/id/eprint/9923A Reciprocity Method for Multiple Source Simulations
https://resolver.caltech.edu/CaltechETD:etd-08302001-030958
Authors: Eisner, Leo
Year: 2001
DOI: 10.7907/Y9S0-0S33
<p>This thesis develops and applies the reciprocity method to assess the seismic site and path effects at a chosen location of interest. To do this, we show that the reciprocity theorem is valid for this application, and develop a technique to represent velocity models of sedimentary basins. Using these tools we test the accuracy of synthetic seismograms computed for southern California. Finally, we apply the reciprocity technique to evaluate the site and path effects for three selected sites in southern California.</p>
<p>The first chapter describes the reciprocity method for simulating seismograms due to multiple earthquake sources at a site of interest. It shows a theoretical proof of the method and discusses the practical implementation and accuracy for the finite difference technique. The numerical tests show that the reciprocal simulations can be performed with the same level of accuracy as the forward calculations.</p>
<p>The second chapter develops a new methodology to represent models of sedimentary basins with extremely low near surface velocities by replacing these velocities with equivalent medium parameters for a finite frequency signal. The new model has a higher minimum velocity, which makes the numerical simulations feasible, and minimizes the difference between the seismograms from the original and new model.</p>
<p>The third chapter validates the velocity model by comparing synthetics and data. It applies the reciprocity method and compares the full waveform synthetic seismograms with a large number of weak motion data. The discrepancies between the predicted waveforms and the data are interpreted by analyzing the attributes of seismograms to find regions of the model that are in error.</p>
<p>Finally the reciprocity technique is applied to calculate site and path effects in the Los Angeles area for three selected sites by simulating 75 source scenarios on 5 major southern California faults. The largest amplitudes at the selected sites are obtained from earthquakes on local faults rather than an earthquake on the San Andreas fault.</p>https://thesis.library.caltech.edu/id/eprint/3276Analysis of Complex Faulting: Wavelet Transform, Multiple Datasets and Realistic Fault Geometry
https://resolver.caltech.edu/CaltechTHESIS:02172012-155442347
Authors: Ji, Chen
Year: 2002
DOI: 10.7907/XYM3-4718
<p>This thesis presents the studies of two recent large and well-recorded earthquakes,
the 1999 Hector Mine and Chi-Chi earthquakes. A new procedure for the determination
of rupture complexity from a joint inversion of static and seismic data was
first developed. This procedure applies a wavelet transform to separate seismic information
related to the spatial and temporal slip history, then uses a simulated
annealing algorithm to determine the finite-fault model that minimizes the objective
function described in terms of wavelet coefficients. This method is then applied to
simultaneously invert the slip amplitude, slip direction, rise time and rupture velocity
distributions of the Hector Mine and Chi-Chi earthquakes with both seismic and
geodetic data. Two slip models are later verified with independent datasets. </p>
<p>Results indicate that the seismic moment of the Hector Mine earthquake is 6.28 x
10^(19) Nm, which is distributed along a "Y" shape fault geometry with three segments.
The average slip is 1.5 m with peak amplitudes as high as 7 m. The fault rupture has
an average slip duration of 3.5 sec and a slow average rupture velocity of 1.9 km/ sec,
resulting in a 14 sec rupture propagation history. The rise time appears to be roughly
proportional to slip, and the two branches of "Y" shape fault rupture together. The
Chi-Chi earthquake is the best-recorded large earthquake so far. Its seismic moment
of 2.7 x 10^(20) Nm is concentrated on the surface of a "wedge shaped" block. The rupture
front propagates with a slow rupture velocity of about 2.0 km/ sec. The average slip
duration is 7.2 sec. Four interesting results are obtained: (1) The sinuous fault plane
strongly affects both spatial and temporal variation in slip history; (2) Long-period
peak slip velocity increases as the rupture propagates; (3) The peak slip velocity
near the surface is in general higher than on the deeper portion of the fault plane as
predicted by dynamic modeling [e.g., Oglesby et al., 1998]; and (4) the complex fault
geometry and slip distribution are related to the two transfer zones obliquely across
Taiwan, which separate Taiwan into three regions with different tectonic activity. The transfer zone in the north can be explained by the slab breakoff mechanism proposed
by Teng et al. [2000] recently. </p>
https://thesis.library.caltech.edu/id/eprint/6828Regional Seismic Wavefield Propagation
https://resolver.caltech.edu/CaltechETD:etd-05262004-100247
Authors: Savage, Brian Kirk
Year: 2004
DOI: 10.7907/JQ8E-6387
Through the examination of local and regional seismic waveform data the crust and upper mantle of southern California are investigated. Using local and regional seismic phases such as Pn, Sn, PL, and surface waves, seismic wave velocities of interesting tectonic structures are determined. These structures include the southern Sierra Nevada, San Bernardino Mountains, and the Salton Trough / Imperial Valley. Detailed studies of how seismic waves propagate at local and regional distances are also undertaken. Knowledge of the seismic wave propagation through these tectonic provinces provides for a robust determination of their characteristics. Further, complex source and site-related propagation are included through an investigation of the Kursk submarine explosion and basin-related site amplification.https://thesis.library.caltech.edu/id/eprint/2078Broadband Waveform Modeling Over a Dense Seismic Network
https://resolver.caltech.edu/CaltechETD:etd-06012006-174015
Authors: Tan, Ying
Year: 2006
DOI: 10.7907/F4DB-2K48
<p>We developed a "two-way" calibration technique for studying clustered events, particularly their mechanisms and rupture directivities. First, we demonstrate that the magnitude 4 events with known source mechanisms can be used to calibrate the path effects on the short-period (0.5-2 sec) P waves, so that the corrected P waves can be modeled for determining focal mechanisms of the smaller events. The correction is formulated in terms of a station-specific "Amplitude Amplification Factor" (AAF), whose origin is mainly due to the site effect. Second, we show that the smaller events with radiation pattern corrections provide excellent empirical Green's functions (EGFs) for investigating the detailed rupture processes of the magnitude 4 events. In Chapter 2 of this thesis, we present the application of our methods to the 2003 Big Bear sequence.</p>
<p>A new technique CAPloc to retrieve full source parameters of small seismic events from regional seismograms is developed, which include origin time, epicenter location, depth, focal mechanism, and moment magnitude. In particular, we tested whether our new method could produce satisfactory results with as few as two stations, so that we can improve source estimates of poorly monitored events with sparse waveform data. We conducted the test in the Tibetan plateau. The focal mechanisms and locations determined from only two stations agree well with those from a well-distributed PASSCAL array.</p>
<p>We use 49 Tonga-Fiji events recorded at the broadband TriNet array, southern California to develop a pure path upper-mantle shear velocity model. At the epicentral distances of 70-95 degree, multi-bounce S waves up to S5, including the guided waves, are observed and modeled to constrain the radial velocity structure. Our preferred model PAC06 contains a fast lid (Vsh=4.78 km/sec, Vsv=4.58 km/sec) ~60 km thick, and a prominent low velocity zone (LVZ) with the lowest velocities Vsh=4.34 km/sec, and Vsv=4.22 km/sec. Besides the 406 km and 651 km discontinuities, PAC06 also has a small (~1%) velocity jump at ~516 km. We consider these main features of PAC06 to be well determined, since PAC06 explains a large data set from various events. Therefore, it is ideally suited for comparing with mineralogical models.</p>https://thesis.library.caltech.edu/id/eprint/2366Investigating Large Earthquake Rupture Kinematics from the Joint Analysis of Seismological, Geodetic and Remote Sensing Data
https://resolver.caltech.edu/CaltechETD:etd-05292008-113958
Authors: Konca, Ali Ozgun
Year: 2008
DOI: 10.7907/6B13-4M30
<p>This thesis presents detailed studies of 4 large earthquakes. The 2006 Mw 8.6 Nias-Simeulue earthquake and 2007 Sumatra Mw 8.4 and 7.9 earthquake sequence which occurred on the Mentawai Island area of Sunda megathrust are studied using teleseismic, long period, GPS, and field data. Two crustal earthquakes, the 2005 Mw 7.6 Kashmir and the 1999 Mw 7.1 Duzce earthquakes, are studied using satellite image cross-correlation, seismic, GPS and SAR data. </p>
<p>The 2005, Mw 8.6, Nias-Simeulue earthquake was caused by rupture of a portion of the Sunda megathrust offshore northern Sumatra. Based on the excitation of the normal modes and geodetic data, we put relatively tight constrains on the seismic moment and the fault dip, where the dip is determined to be 8o to10o with corresponding moments of 1.24 x 1022 to 1.00 x 1022 Nm, respectively. The geodetic constraints on slip distribution help to eliminate the trade-off between rupture velocity and slip kinematics. Our results indicate a relatively slow average rupture velocity of 1.5 to 2.5 km/s and long average rise time of up to 20 s. </p>
<p>Our study of the 2007 Mentawai Islands earthquakes shows the influence of permanent barriers on the extent of large megathrust ruptures, which can be a cause of some regularity of the seismic behavior, but also that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability arises mostly from the influence of nonpermanent barriers, probably zones with locally lower prestress due to the past earthquakes. The state of stress on that portion of the Sumatra megathrust was not adequate for the development of a single major large rupture at the time of this seismic crisis. However, the slip deficit that has accumulated since the 1833 and 1797 events remains large, and so is the potential for a large megahrust event in the Mentawai area.</p>
<p>We analyzed the rupture process of 1999 Mw 7.1 Duzce earthquake using geodetic and seismic data. Applying subpixel cross-correlation of SPOT images acquired before and after the event, we mapped a continuous fault trace over 55 km; 15 km longer than the field reports. We investigated the spatiotemporal evolution of the earthquake using four-segment fault geometry with constraints on surface offsets based on satellite imagery, incorporated GPS and InSAR data and four strong-motion stations in the vicinity of the rupture. Our joint modeling shows a very stable slip distribution that does not depend on constraints imposed on rupture velocity. We show that no constant rupture velocity can explain the strong-motion data. Due to constraints from fault geometry and geodetic data, the rupture velocity has to vary rapidly. The rupture starts slow, accelerates to supershear speeds toward east and subsequently slows down. Supershear rupture is local and only toward the east of the hypocenter. Teleseismic data are consistent with the joint near-field model when 2 s time shifts are applied to their hand-picked arrivals. This implies that the weak beginning of the earthquake is not observable at teleseismic distances. This appears to be a common problem with teleseismic modeling and leads to more compact models with major slip around the hypocenter than the actual phenomenon. We performed teleseismic inversion models comparing four-segment fault geometry based on satellite imagery to one-segment geometry based on CMT solution. The four-segment model gives better predictions of near-field ground motions.</p>
<p>We analyzed the Mw 7.6 Kashmir earthquake of October 8, 2005, using sub-pixel correlation of ASTER images to measure ground deformation, and modeling SAR imagery data along with seismic waveforms. The surface rupture is continuous over a distance of 75 km. The rupture lasted about 25 s and propagated up-dip and bilaterally by ~2 km/s, with a rise time of 2-5 s. The shallowness and compactness of the rupture, both in time and space, provide an explanation for the intensity of destructions. By comparing the teleseismic models with SAR data, we infer that satellite image correlation puts constraints on teleseismic models, which lead to more coherent models with the geodetic data. This kind of satellite image analysis could be achieved as soon as a postearthquake image is available, and would provide key information for early assessment of damages.</p>https://thesis.library.caltech.edu/id/eprint/2257Broad Band Modeling Earthquake Source and Upper Mantle Structure on Plate Boundary Zones
https://resolver.caltech.edu/CaltechETD:etd-05192008-121937
Authors: Song, Teh-Ru Alex
Year: 2008
DOI: 10.7907/K7RQ-MM41
<p>Broadband seismic arrays have provided unprecedented data sets for seismologists to image the slips on faults and velocity structure beneath Earth's surface at all scales. In particular, plate boundary zones are the most complicated regions on the surface and full of complexities. Often that great earthquakes occur and rapid structural changes take place.</p>
<p>In my thesis, one major effort is to use geophysical data and broadband seismic data to characterize the occurrences of great earthquakes at the subduction zone interface. Using gravity data from satellite and bathymetry model ETopo5, I recognized the strong correlation between gravity, topography and the occurrences of great earthquakes. Such a correlation gives a strong indication that lateral variations in the occurrences of great earthquakes at a given subduction zone are strongly linked to the fore-arc structure and topographic features such as basin and peninsulas. I also give robust estimates of the size and rupture extent of the recent 2004 Sumatra-Andaman giant earthquake using Earth's normal modes.</p>
<p>Another part of my thesis consists of modeling waveform distortion and interference to study lithosphere and deep upper mantle structure. Modeling multi-pathing associated with sharp structure has become a very powerful method to delineate structure and explain the complicated behavior shown in the data recorded by dense arrays.</p>
<p>Future research will focus on linking geodynamic models and seismic analysis of broadband waveform data as a way to further constrain the mantle structure and validate various geodynamic models.</p>https://thesis.library.caltech.edu/id/eprint/1878Seismic Structure of the Lower Mantle
https://resolver.caltech.edu/CaltechETD:etd-06022009-012218
Authors: Sun, Daoyuan
Year: 2009
DOI: 10.7907/BD11-ET49
<p>The lower mantle plays an important role in the thermal and chemical evolution of the earth. Although recent advanced seismological imaging displays the heterogeneous nature of the lower mantle, most results are constrained to large scale and longer wavelength structures. This thesis involved waveform modeling studies of the detailed structures of the lower mantle, especially the African Superdome and D" layer.</p>
<p>A simple uniform 3% shear velocity reduction model can explain the observed seismological anomalies for the African Superdome (also refer as Africa Large Low Shear Velocity Province or Africa Superplume), but it lacks small scale complexity inside. In parallel with the seismic model, a composition-dependent compressibility model with a high bulk modulus is developed to explain the African Superdome. To validate this dynamic model, we map the modeled chemistry and temperature into P and S velocity models. Synthetic seismogram sections generated for this 2D model are then compared directly with the corresponding seismic observations. These results explain the anti-correlation between the bulk velocity and shear velocity, as well as the sharpness of the edge.</p>
<p>A lower mantle S-wave triplication with a Scd branch occurring between S and ScS has been recognized for many years and has been interpreted in a variety of ways. The triplication is particularly strong when sampling regions beneath the circum-Pacific lower mantle fast velocity belt seen in global tomographic models, where it has been modeled with a 2–3% jump in S-velocity. The D" discontinuity may arise from a phase change for Perovskite to Post-Perovskite. We model the phase boundary height by mapping S-wave tomography into temperature. A few adjustable parameters involving reference phase boundary height and velocity jump are determined from comparing synthetic seismogram predictions with densely sampled observations. Adding 3D propagational effects caused by these structures through Perovskite to Post-Perovskite velocity jump predicted from mineral physics appears to generate compatible results with Scd waveform observations.</p>
<p>In the last chapter, we develop a new tool based on a decomposition referred to as a multi-path detector which can be used to distinguish between horizontal structure (in-plane multi-pathing) vs. vertical (out-of-plane multi-pathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in Southern Africa. The results further validate the case for distinct chemistry inside the African Superdome. We also present evidence of a narrow plume-like feature coming off the top of the large African low-velocity structure in the lower mantle. The plume's diameter is less than 150 km and consistent with an iso-chemical, low-viscosity plume conduit.</p>
https://thesis.library.caltech.edu/id/eprint/2411Defining the Relationship between Seismicity and Deformation at Regional and Local Scales
https://resolver.caltech.edu/CaltechTHESIS:09052012-110119932
Authors: Williams, Nneka Njeri Akosua
Year: 2013
DOI: 10.7907/CN29-YV40
<p>In this thesis, I use source inversion methods to improve understanding of crustal deformation along the Nyainquentanglha (NQTL) Detachment in Southern Tibet and the Piceance Basin in northwestern Colorado. Broadband station coverage in both regions is sparse, necessitating the development of innovative approaches to source inversion for the purpose of studying local earthquakes.</p>
<p>In an effort to study the 2002-2003 earthquake swarm and the 2008 M<sub>w</sub> 6.3 Damxung earthquake and aftershocks that occurred in the NQTL region, we developed a single station earthquake location inversion method called the SP Envelope method, to be used with data from LHSA at Lhasa, a broadband seismometer located 75 km away. A location is calculated by first rotating the seismogram until the azimuth at which the envelope of the P-wave arrival on the T-component is smallest (its great circle path) is found. The distance at which to place the location along this azimuth is measured by calculating the S-P distance from arrivals on the seismogram. When used in conjunction with an existing waveform modeling based source inversion method called Cut and Paste (CAP), a catalog of 40 regional earthquakes was generated.</p>
<p>From these 40 earthquakes, a catalog of 30 earthquakes with the most certain locations was generated to study the relationship of seismicity and NQTL region faults mapped in Google Earth and in Armijo et al., 1986 and Kapp et al., 2005. Using these faults and focal mechanisms, a fault model of the NQTL Region was generated using GOCAD, a 3D modeling suite. By studying the relationship of modeled faults to mapped fault traces at the surface, the most likely fault slip plane was chosen. These fault planes were then used to calculate slip vectors and a regional bulk stress tensor, with respect to which the low-angle NQTL Detachment was found to be badly misoriented. The formation of low-angle normal faults is inconsistent with the Anderson Theory of faulting, and the presence of the NQTL Detachment in a region with such an incongruous stress field supports the notion that such faults are real.</p>
<p>The timing and locations of the earthquakes in this catalog with respect to an anomalous increase in the eastward component of velocity readings at the single cGPS station in Lhasa (LHAS) were analyzed to determine the relationship between plastic and brittle deformation in the region. The fact that cGPS velocities slow significantly after the 2002-2003 earthquake swarm suggests that this motion is tectonic in nature, and it has been interpreted as only the second continental slow slip event (SSE) ever to be observed. The observation of slow slip followed by an earthquake swarm within a Tibetan rift suggests that other swarms observed within similar rifts in the region are related to SSEs.</p>
<p>In the Piceance Basin, CAP was used to determine source mechanisms of microearthquakes triggered as a result of fracture stimulation within a tight gas reservoir. The expense of drilling monitor wells and installing borehole geophones reduces the azimuthal station coverage, thus making it difficult to determine source mechanisms of microearthquakes using more traditional methods. For high signal to noise ratio records, CAP produced results on par with those obtained in studies of regional earthquakes. This finding suggests that CAP could successfully be applied in studies of microseismicity when data quality is high.</p>https://thesis.library.caltech.edu/id/eprint/7197Exploiting Seismic Waveforms of Ambient Noise and Earthquakes
https://resolver.caltech.edu/CaltechTHESIS:09272013-134450665
Authors: Zhan, Zhongwen
Year: 2014
DOI: 10.7907/NQHT-NA96
<p>In this thesis, I apply detailed waveform modeling to study noise correlations in different environments, and earthquake waveforms for source parameters and velocity structure.</p>
<p>Green's functions from ambient noise correlations have primarily been used for travel-time measurement. In Part I of this thesis, by detailed waveform modeling of noise correlation functions, I retrieve both surface waves and crustal body waves from noise, and use them in improving earthquake centroid locations and regional crustal structures. I also present examples in which the noise correlations do not yield Green's functions, yet the results are still interesting and useful after case-by-case analyses, including non-uniform distribution of noise sources, spurious velocity changes, and noise correlations on the Amery Ice Shelf.</p>
<p>In Part II of this thesis, I study teleseismic body waves of earthquakes for source parameters or near-source structure. With the dense modern global network and improved methodologies, I obtain high-resolution earthquake locations, focal mechanisms and rupture processes, which provide critical insights to earthquake faulting processes in shallow and deep parts of subduction zones. Waveform modeling of relatively simple subduction zone events also displays new constraints on the structure of subducted slabs.</p>
<p>In summary, behind my approaches to the relatively independent problems, the philosophy is to bring observational insights from seismic waveforms in critical and simple ways.</p>https://thesis.library.caltech.edu/id/eprint/7965Seismic Structure along Transitions from Flat to Normal Subduction: Central Mexico, Southern Peru, and Southwest Japan
https://resolver.caltech.edu/CaltechTHESIS:05292014-181733885
Authors: Dougherty, Sara Lyn
Year: 2014
DOI: 10.7907/WHSR-VY75
<p>The fine-scale seismic structure of the central Mexico, southern Peru, and southwest Japan subduction zones is studied using intraslab earthquakes recorded by temporary and permanent regional seismic arrays. The morphology of the transition from flat to normal subduction is explored in central Mexico and southern Peru, while in southwest Japan the spatial coincidence of a thin ultra-slow velocity layer (USL) atop the flat slab with locations of slow slip events (SSEs) is explored. This USL is also observed in central Mexico and southern Peru, where its lateral extent is used as one constraint on the nature of the flat-to-normal transitions.</p>
<p>In western central Mexico, I find an edge to this USL which is coincident with the western boundary of the projected Orozco Fracture Zone (OFZ) region. Forward modeling of the 2D structure of the subducted Cocos plate using a finite-difference algorithm provides constraints on the velocity and geometry of the slab’s seismic structure in this region and confirms the location of the USL edge. I propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, by a process analogous to that which occurred when the Rivera plate separated from the proto-Cocos plate 10 Ma.</p>
<p>In eastern central Mexico, observations of a sharp transition in slab dip near the abrupt end of the Trans Mexican Volcanic Belt (TMVB) suggest a possible slab tear located within the subducted South Cocos plate. The eastern lateral extent of the USL is found to be coincident with these features and with the western boundary of a zone of decreased seismicity, indicating a change in structure which I interpret as evidence of a possible tear. Analysis of intraslab seismicity patterns and focal mechanism orientations and faulting types provides further support for a possible tear in the South Cocos slab. This potential tear, together with the tear along the projection of the OFZ to the northwest, indicates a slab rollback mechanism in which separate slab segments move independently, allowing for mantle flow between the segments.</p>
<p>In southern Peru, observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest a smooth contortion of the slab. Concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry. The lateral extent of the USL atop the horizontal Nazca slab is found to be coincident with the margin of the projected linear continuation of the subducting Nazca Ridge, implying a causal relationship, but not a slab tear. Waveform modeling of the 2D structure in southern Peru provides constraints on the velocity and geometry of the slab’s seismic structure and confirms the absence of any tears in the slab.</p>
<p>In southwest Japan, I estimate the location of a possible USL along the Philippine Sea slab surface and find this region of low velocity to be coincident with locations of SSEs that have occurred in this region. I interpret the source of the possible USL in this region as fluids dehydrated from the subducting plate, forming a high pore-fluid pressure layer, which would be expected to decrease the coupling on the plate interface and promote SSEs.</p>https://thesis.library.caltech.edu/id/eprint/8429Some Advances in Computational Geophysics: Seismic Wave and Inverse Geodynamic Modeling
https://resolver.caltech.edu/CaltechTHESIS:05262016-150442765
Authors: Li, Dunzhu
Year: 2016
DOI: 10.7907/Z9X63JW2
<p>In this thesis, I develop computational methods that link theory with geophysical observations, with one part devoted to the development of forward models of seismic wave propagation through the mantle and core, and a second part devoted to the inversion of viscous flow in the mantle.</p>
<p>First order seismic structure of the earth has been well described radially since the PREM model was introduced. With the help of seismic tomography methods, many large-scale heterogeneous structures have become well imaged. Based on this progress, the information in seismic waveforms, which provides extra constraints, is becoming more important in determination of the detailed structure within the earth's interior. However, 3-D modeling of seismic wave propagation remains computationally expensive, especially at high frequency, because the computing cost scales with fourth power of frequency. Thus 2-D modeling is often used, and in many cases is sufficient for the problem. To use 2-D modeling in global seismology, several issues need to be considered: how to handle the differences in geometric spreading between 2-D and 3-D modeling, how to incorporate earthquake sources into a 2-D code, and how to handle the spherical geometry of the earth. In the first part of my thesis, we solve all three problems, using a 2-D staggered finite difference method with a post-processing step. The post-processing automatically corrects the geometric spreading difference between 2-D and 3-D wave propagation; the earthquake sources are added to the 2-D finite difference simulation using a momentum source and transparent box approaches; the earth-flattening is discussed, especially for the density transformation. Benchmarks of the new method against with 1-D and 3-D code demonstrates the the accuracy of the method.</p>
<p>We then use the new code in a study of the interface between outer and inner core. Inner Core Boundary (ICB) is thought to be crucial in estimating the energy released in generating the geomagnetic field. One direct constraint on ICB properties is using reflected P wave from ICB, the PKiKP phase. Due to its small amplitude, near distance PKiKP is seldom observed. However, we find several events beneath Central American as having good set of PKiKP recordings from the USArray seismic network, as well as other core phases like P wave reflection with Core Mantle Boundary (CMB). The amplitude of the phases display large scatters across the stations, which are potentially caused by many factors, including site effects of the stations, upper mantle inhomogeneity, or a bumpy structure along either the CMB or ICB. After comparing amplitude ratio of between PKiKP and PcP phase, analyzing how this ratio changes for different nearby events, and computing forward models using our new method that investigate different factors influence the PKiKP phases, we attribute a stacked amplitude pattern as caused by ICB structure, in which PKiKP phase amplitude rapidly changes within a small range. Finally, we model this observed seismic pattern as a small dome-like anomalies above ICB, where the material changes from that of the outer core to that of inner core gradually.</p>
<p>The final part of my thesis is on a geodynamic inversion problem for mantle convection. Mantle convection is an important process that determines plate motions and subduction. Numerous forward models indicate that the constitute relation (viscosity law) is of key importance for mantle convection. Despite substantial effort attempting to determine the viscosity structure of the mantle, either through forward and inverse geophysical models or through laboratory work, many first order questions remain. We assume the realistic viscosity structure, which is temperature and strain-rate dependent, can be parameterized using a set of scalar parameters. Given this set of viscosity parameters and an initial temperature, the mantle evolves following a set of partial differential equations (PDEs). Our goal with the inverse problem is to recover the viscosity parameters and initial temperature by fitting the observational data, which here includes plate motion history and the present day temperature distribution of the mantle. We formulate this inversion problem following a PDE constrained optimization framework. We first define the cost function we want to minimize; then, the derivative of the cost function with respect to viscosity parameters and initial temperature is calculated following the discrete adjoint equations; finally, a gradient-based optimization method, limited memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) approach is used to find the minimum. To accelerate the optimization process, we modified the traditional LBFGS by adding a preconditioner, and achieve a more rapid convergence. To test our method, we use two synthetic cases: a sinking cylinder within a viscous layer and a realistic subduction model. We find that in the initial temperature-only inversion, the initial temperature can be recovered well; in the joint inversion of initial temperature and viscosity parameters, the temperature, as well as effective viscosity, can also be recovered reasonably, but there are trade offs between viscosity parameters. Presumably, the trade off in viscosity parameters is related to the ill-posedness of the problem.</p>https://thesis.library.caltech.edu/id/eprint/9775Earthquake Source Characterization Through Seismic Observations and Numerical Modeling
https://resolver.caltech.edu/CaltechTHESIS:11232016-044435496
Authors: Lui, Semechah Ka Yan
Year: 2017
DOI: 10.7907/Z9QN64QM
<p>In this thesis, I present a series of works on the characterization of source properties and physical mechanisms of various small to moderate earthquakes through both observational and numerical approaches. From the results, we find implications on a broader scheme of topics relating to larger earthquakes, shear zone structure, frictional properties of faults, and seismic hazard assessment.</p>
<p>Part I consists of two studies using waveform modeling. In Chapter 2, we present an in-depth study of a series of intraslab earthquakes that occurred in a localized region near the downdip edge of the 2011 M<sub>w</sub> Tohoku-Oki megathrust earthquake. By refining source parameters of selected events, simulating their rupture properties and comparing their mechanisms to stress changes caused by the main shock in the region, we are able to identify the true rupture plane and the reactivation of a subducted normal fault, enhancing our understanding on the downdip shear zone. In Chapter 3, based on similar techniques, we further develop a systematic methodology to perform fast assessments on important source properties as an earthquake occurs. For two M<sub>w</sub> 4.4 earthquakes in Fontana, moment magnitude and focal mechanism can be accurately estimated with 3 to 6 s after the first P-wave arrival, while focal depth can be constrained upon the arrival of S waves. Rupture directivity can also be determined with as little as 3 seconds of P waves. This study opens the opportunity to predict ground motions ahead of time and can potentially be useful for Earthquake Early Warning.</p>
<p>Part II involves the modeling of seismic source properties and physical mechanisms of interacting earthquakes in dynamic rupture simulations. In particular, we focus on small repeating earthquake sequences that trigger one another. In Chapter 4, we quantify the relative importance of physical mechanisms that contribute to earthquake interaction and identify that the stress change caused by post seismic slip is the dominating factor. Our findings introduce the possibility to constrain frictional properties of the fault based on earthquake interactions. We further apply this working model in Chapter 5 to reproduce the actual interacting repeating sequences in Parkfield. We are able to identify possible physical mechanisms that cause the inferred high stress drops of these repeating events, as well as reproduce their synchronized seismic cycles. Results from our simulations are consistent with the observed scaling relation between the recurrence time interval and the seismic moment of these events. Our findings indicate that the difference between the observed and the theoretical scaling relations can be explained by the significant aseismic slip in the rupture area.</p>https://thesis.library.caltech.edu/id/eprint/9983Dynamic Earthquake Source Modeling and the Study of Slab Effects
https://resolver.caltech.edu/CaltechTHESIS:06142018-121935019
Authors: Bai, Kangchen
Year: 2019
DOI: 10.7907/JVVW-M348
<p>In this Thesis, I report my Ph.D. research on two major issues that are devoted towards constructing more realistic earthquake source model using computational tools: (1) constructing physically consistent dynamic rupture models that include complexities in fault geometry as well as heterogeneous stress and frictional properties inferred from observations; (2) study the effect of subducting slab structure on earthquakes that occur inside it with a special focus on the teleseismic waveforms.</p>
<p>Fault step over is one of the most important geometric complexities that control the propagation and arrest of earthquake ruptures. In Chapter 2, we study the role of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs. We conclude that the maximum step over distance that a rupture can jump is approximately proportional to seismogenic depth. We also conclude that the pre-stress conditions have a fundamental effect on step over jump distance while the critical nucleation size has a secondary effect.</p>
<p>Seismic wave carries information of source as well as structures along the path it travels. It was found that seismic waves generated by shallow events in subduction zones whose ray path coincide with the down going slab structure display waveform complexities that feature multipathing. In Chapter 3, we study deep earthquakes whose depth phases sample the slab structure on their way up to the surface. Differential travel time sP-P analysis shows a systematic decrease of up to 5 seconds from Europe to Australia and then to Pacific which is indicative of a dipping high velocity layer above the source region. Finite-difference simulations showed that a slab shaped structure that follows the Benioff zone at shallow depth and steepens beyond 400 km produces a model that can account for the sP-P differential travel times of 5 seconds for oceanic paths. In Chapter 4, we design a slab operator that can be applied on the 1D synthetics to generate 2D synthetics with slab structure. We hope this operator can be used for generating more accurate Green's functions that could potentially serve earthquake source inversion.</p>
<p>In Chapter 5, we design a dynamic rupture model of the Mw 7.8 Gorkha, Nepal earthquake. We employ a novel approach of integrating kinematic inversion results which provide low frequency stress distribution and stochastic high frequency stress motivated by earthquake cycle models and observations. By doing this, we are able to reproduce the observed frequency dependent rupture processes, in particular the concentration of high-frequency radiation in the down-dip part of the rupture.</p>
<p>In Chapter 6, I report my on going work on the spectral element method based earthquake cycle simulator. Large scale earthquake cycle simulation with consideration of complicated velocity structure and fault geometry is a great challenge for numerical modeling. I tried to push forward this boundary by extending the existing spectral element earthquake cycle simulator to enable cycle simulations on bi-material faults. This chapter includes a benchmark test in 2D that demonstrates the correctness of this new algorithm and an application of this method on bi-material fault earthquake cycle modeling.</p>https://thesis.library.caltech.edu/id/eprint/11085Seismic Waveform Modeling of Natural Hazards and Sharp Structural Boundaries
https://resolver.caltech.edu/CaltechTHESIS:01102020-152646406
Authors: Lai, Voon Hui
Year: 2020
DOI: 10.7907/5PQG-ST75
<p>Seismic waveform modeling is a powerful tool for seismologists to learn about the Earth’s dynamics, either how a natural hazard evolves with time, or the long-term deformation process governed by fine-scale structures along boundaries inside the Earth. Knowing that the recorded seismograms reflect the cumulative effects of the source, the earth structure, and the instrument response, I carefully study the characters of the seismograms such as the arrival time, amplitude, frequency content, and multipathing, for several settings, with the goal of improving our description of either the source or the structure.</p>
<p>Part 1 focuses on source characterization for non-earthquake natural hazards. I perform moment tensor inversions for the large seismic events at the Kilauea summit to infer the triggering mechanisms for the explosive eruptions and caldera collapse during the 2018 eruption sequence. The addition of infrasound data is crucial to resolve the uncertainties in the moment tensor solutions, particularly the depth and the necessity of the isotropic component. I also present a new mechanistic model to describe the seismic signal from debris flow and apply to the 2018 Montecito debris flow in which key parameters such as boulder size and flow rate and their evolution during the event can be determined using a single seismic station.</p>
<p>Part 2 consists of three studies spanning from the crust to the core, where forward waveform modeling is used to improve our understanding of the sharp structural boundaries and their role in observed ground motion and long-term dynamics. Numerical simulation and dense array analysis are used to model the direct effect of shallow basin structures in Los Angeles on shaking duration and reveal the importance of basin edges and attenuation model for predicting ground motion during large shallow ruptures. I also identify a strong velocity contrast in the lower crust – upper mantle structure across the San Andreas plate boundary system and, given velocity is a proxy to lithospheric strength, the sharp contrast can have a significant role in modulating the long-term plate deformation. Lastly, we observe strong waveform anomalies at the edge of the Pacific Large Low Shear Velocity Province (LLSVP) which have great importance in governing deep mantle convection. To fit the observation, I propose a model of ultra-low velocity zone (ULVZ), plume and slab interacting at the edge of the LLSVP. The configuration and location of this ULVZ-plume-slab interaction is important in inferring the mechanism behind plume generation which gives rise to the Hawaii-Emperor Seamount chain.</p>https://thesis.library.caltech.edu/id/eprint/13615