Earthquakes release centuries of accumulated elastic strain in seconds, yet where and how this strain builds up across fault systems remain only partly understood. The contrast between seismogenic asperities\u2014regions that lock and rupture in earthquakes\u2014and aseismic barriers that creep or remain stable defines the scale, frequency, and segmentation of major ruptures. Constraining the spatial extent and temporal persistence of these regions requires geodetic observations that resolve deformation from local fault zones to plate-wide strain fields.
\r\n\r\nIn this thesis, I use spaceborne geodesy, specifically Interferometric Synthetic Aperture Radar (InSAR), to complement sparse Global Navigation Satellite System (GNSS) networks in quantifying crustal deformation and inferring tectonic processes across a continuum of spatial and temporal scales. We strive to understand how strain accumulates, transfers, and is released along major plate boundary systems within a probabilistic framework that explicitly incorporates uncertainty and prior assumptions.
\r\n\r\nI begin with the central San Andreas Fault (Chapter 1), where locked, transitional, and creeping segments coexist within a narrow zone of complex mechanical interaction. By jointly analyzing geodetic coupling and seismicity, I find that the fraction of background earthquakes scales with the aseismic slip rate normalized by the plate rate, suggesting a phenomenological link between fault coupling and micro-earthquake clustering.
\r\n\r\nI then address a broader methodological challenge in InSAR geodesy\u2014the treatment of reference-frame motion and its influence on long-wavelength deformation gradients (Chapter 2). After applying refined tropospheric and ionospheric corrections and inversion of large-scale InSAR velocity fields, I show that the absolute Euler rotation of a tectonic plate can be directly estimated from InSAR data, offering an independent constraint on plate kinematics. This capability extends InSAR beyond local fault studies, enabling continental and even global mapping of tectonic deformation, which is a prospect that becomes increasingly viable with the advent of modern L-band radar missions capable of penetrating vegetation and maintaining coherence over wide spatial and temporal baselines.
\r\n\r\nNext, I examine the southern Dead Sea Transform and Gulf of Aqaba, where the Arabian and Nubian plates undergo transtensional motion as the transform transitions into the Red Sea rift (Chapter 3). By compiling a decade of Sentinel-1 data, I map along-strike variations in fault coupling and localized extension toward the rift triple junction, delineating the gradual shift from strike-slip shear to crustal rifting.
\r\n\r\nFinally, I combine InSAR and GNSS time series to construct a spatially continuous model of interseismic coupling along the Nazca-South America margin (Chapter 4). The results reveal strong spatial correlations between heterogeneity in fault coupling and subducted bathymetric highs, underscoring the mechanical role of subducted topography in segmenting megathrust behavior and strain accumulation.
\r\n\r\nThrough these studies, I demonstrate how integrating long-wavelength geodetic imaging allows one to map crustal deformation continuously from fault to plate scales with quantified confidence, while explicitly identifying regions where the data remain uninformative or ambiguous. In doing so, we aim to bridge observation and theory to connect the patterns we see from orbit to the physics that govern the accumulation of strain and seismic release of elastic strain within the Earth\u2019s dynamic lithosphere.
" }, { "name": "Li, Yida", "degree": "PhD", "year": "2024", "title": "I. Dynamics of Subduction Initiation and II. Constraining Sedimentary Basin Structure with Seismic Ambient Noise", "advisor": "Gurnis, Michael C.; Clayton, Robert W.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302024-182513713", "creators": [ { "name": { "family": "Li", "given": "Yida" }, "id": "Li-Yida", "orcid": "0000-0003-0664-0247", "display_name": "Li, Yida" } ], "advisors": [ { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "orcid": "0000-0003-1704-597X", "role": "advisor", "display_name": "Gurnis, Michael C." }, { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "orcid": "0000-0003-3323-3508", "role": "co-advisor", "display_name": "Clayton, Robert W." } ], "committee": [ { "name": { "family": "Stock", "given": "Joann M." }, "id": "Stock-J-M", "orcid": "0000-0003-4816-7865", "role": "chair", "display_name": "Stock, Joann M." }, { "name": { "family": "Ross", "given": "Zachary E." }, "id": "Ross-Z-E", "orcid": "0000-0002-6343-8400", "role": "member", "display_name": "Ross, Zachary E." }, { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "orcid": "0000-0003-1704-597X", "role": "member", "display_name": "Gurnis, Michael C." }, { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "orcid": "0000-0003-3323-3508", "role": "member", "display_name": "Clayton, Robert W." } ], "option_major": [ "geophys" ], "doi": "10.7907/y3z2-z538", "abstract": "Subduction initiation, the inception of a subduction zone, heralds dramatic changes in tectonic plate kinematics and dynamics. In the first half of the thesis, I focus on understanding the dynamics of the subduction initiation process through a synthesis of numerical computations and theoretical frameworks. In Chapter 2, we employ force balance analysis and 2D geodynamic models to yield an analytical solution on the force evolution of the subducting plate. This formulation illuminates a pivotal phase in subduction initiation \u2014- the compression-to-extension transition of plate forces -\u2014 as a defining milestone. In Chapter 3, we extend this analytical framework into a sliced 3D context (2.5D) while incorporating the influence of strike-slip motion. Modified from Chapter 2, the analytical solution validates that strike-slip motion facilitates subduction initiation by accelerating the process of weakening. Chapter 4 ventures into 3D geodynamic modeling, focusing on the Puysegur trench -\u2014 a living example of subduction initiation. The models demonstrate a capability to match multiple geophysical and geological observations quantitatively with mechanical models. With a parametric search, we discover the best-fitting models require a relatively fast strain weakening rate, which can be explained by pore-pressure weakening at shallow depths and grain-size reduction at greater depths.
\r\n \r\n \r\nThe second part of this thesis transitions to ambient seismic noise correlation. In Chapter 5, we conduct an ambient noise tomography in northern Los Angeles basins with a newly obtained, dense seismic data set. The new shear wave velocity model exhibits a lower velocity in the basins than previous community models, which can potentially resolve the inconsistency between observed and calculated ground motions. In Chapter 6, we introduce a new method to identify the near-field noise sources from the spurious arrivals in ambient noise correlations. The correlation between the inverted noise sources and geological features in northern LA basins suggests the viability of this technique as a novel means of identifying geological structures, including faults.
" }, { "name": "Shen, Zhichao", "degree": "PhD", "year": "2022", "title": "Probing Water Below the Surface: Insights from Seismic Interferometry with Conventional and DAS Array", "advisor": "Zhan, Zhongwen", "url": "https://resolver.caltech.edu/CaltechTHESIS:05022022-204308721", "creators": [ { "name": { "family": "Shen", "given": "Zhichao" }, "id": "Shen-Zhichao", "orcid": "0000-0003-0458-5264", "display_name": "Shen, Zhichao" } ], "advisors": [ { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "orcid": "0000-0002-5586-2607", "role": "advisor", "display_name": "Zhan, Zhongwen" } ], "committee": [ { "name": { "family": "Jackson", "given": "Jennifer M." }, "id": "Jackson-J-M", "orcid": "0000-0002-8256-6336", "role": "chair", "display_name": "Jackson, Jennifer M." }, { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "orcid": "0000-0003-3323-3508", "role": "member", "display_name": "Clayton, Robert W." }, { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "orcid": "0000-0003-1704-597X", "role": "member", "display_name": "Gurnis, Michael C." }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "orcid": "0000-0002-5586-2607", "role": "member", "display_name": "Zhan, Zhongwen" } ], "option_major": [ "geophys" ], "doi": "10.7907/5vtn-1c34", "abstract": "Water is essential to our daily lives, yet its subsurface behavior remains challenging to track using remote observations. By extracting seismic waves traveling through the Earth, seismic interferometry is a powerful tool to image the Earth\u2019s interior, in particular the long-term and short-term behavior of water circulation. With conventional dense seismic networks and emerging distributed acoustic sensing (DAS), I demonstrate how seismic interferometry brings new insights on water below the surface ranging from the depths of the mantle transition zone (MTZ) to the subsurface aquifers of our planet.
\r\n\r\nBy applying a novel inter-source interferometry method that turns deep earthquakes into virtual seismometers, I not only present evidence for an intermediate-scale metastable olivine wedge and small-scale intra-slab scatterers in the MTZ beneath the Japan Sea, but also reveal their dimensions and velocity perturbations more accurately than before. Beyond the relative independent scales of slab structures, these results point toward a consistent picture of transformational faulting of metastable olivine as the initiation mechanism of deep earthquakes, petrologic processes associated with dehydration of subducting slabs, and an extremely dry slab core below 410-km. Borrowing the idea from inter-source interferometry, I develop a slab operator method by utilizing the waveform broadening due to the high-velocity anomaly. With synthetic tests and real data, I illustrate the feasibility of this method for accurately determining large-scale slab velocity perturbations.
\r\n\r\nShifting from the Earth\u2019s interior to the subsurface, I investigate the feasibility of vadose zone water monitoring with DAS. DAS provides an affordable and scalable solution for deploying ultra-dense seismic arrays by transforming existing optic-fiber cables into thousands of seismic sensors. With two years of ambient noise recorded on the Ridgecrest DAS array, the time-lapse images of seismic changes (dv/v) reveal an unprecedented high-resolution spatiotemporal evolution of water saturation in vadose zone. A striking correlation between the dv/v amplitude and the sedimentary thickness is observed, while the frequency analysis of dv/v measurements suggests an uppermost 10 m hydrologic source as the cause for dv/v temporal variability. The results demonstrate the great potential of DAS for long term subsurface water monitoring.
" }, { "name": "Lai, Voon Hui", "degree": "PhD", "year": "2020", "title": "Seismic Waveform Modeling of Natural Hazards and Sharp Structural Boundaries", "advisor": "Helmberger, Donald V.; Zhan, Zhongwen", "url": "https://resolver.caltech.edu/CaltechTHESIS:01102020-152646406", "creators": [ { "name": { "family": "Lai", "given": "Voon Hui" }, "id": "Lai-Voon-Hui", "orcid": "0000-0002-0738-0187", "display_name": "Lai, Voon Hui" } ], "advisors": [ { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "co-advisor", "display_name": "Helmberger, Donald V." }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "role": "co-advisor", "display_name": "Zhan, Zhongwen" } ], "committee": [ { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "role": "chair", "display_name": "Clayton, Robert W." }, { "name": { "family": "Jackson", "given": "Jennifer M." }, "id": "Jackson-J-M", "role": "member", "display_name": "Jackson, Jennifer M." }, { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "role": "member", "display_name": "Gurnis, Michael C." }, { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "member", "display_name": "Helmberger, Donald V." }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "role": "member", "display_name": "Zhan, Zhongwen" } ], "option_major": [ "geophys" ], "doi": "10.7907/5PQG-ST75", "abstract": "Seismic waveform modeling is a powerful tool for seismologists to learn about the Earth\u2019s 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.
\r\n\r\nPart 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.
\r\n\r\nPart 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 \u2013 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.
" }, { "name": "Sousa, Francis Joseph", "degree": "PhD", "year": "2016", "title": "Tectonics of Central and Eastern California, Late Cretaceous to Modern", "advisor": "Saleeby, Jason B.; Farley, Kenneth A.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05252016-090108907", "creators": [ { "name": { "family": "Sousa", "given": "Francis Joseph" }, "id": "Sousa-Francis-Joseph", "orcid": "0000-0003-1623-4023", "display_name": "Sousa, Francis Joseph" } ], "advisors": [ { "name": { "family": "Saleeby", "given": "Jason B." }, "id": "Saleeby-J-B", "role": "advisor", "display_name": "Saleeby, Jason B." }, { "name": { "family": "Farley", "given": "Kenneth A." }, "id": "Farley-K-A", "orcid": "0000-0002-7846-7546", "role": "advisor", "display_name": "Farley, Kenneth A." } ], "committee": [ { "name": { "family": "Avouac", "given": "Jean-Philippe" }, "id": "Avouac-J-P", "orcid": "0000-0002-3060-8442", "role": "chair", "display_name": "Avouac, Jean-Philippe" }, { "name": { "family": "Saleeby", "given": "Jason B." }, "id": "Saleeby-J-B", "role": "member", "display_name": "Saleeby, Jason B." }, { "name": { "family": "Farley", "given": "Kenneth A." }, "id": "Farley-K-A", "orcid": "0000-0002-7846-7546", "role": "member", "display_name": "Farley, Kenneth A." }, { "name": { "family": "Stock", "given": "Joann M." }, "id": "Stock-J-M", "orcid": "0000-0003-4816-7865", "role": "member", "display_name": "Stock, Joann M." }, { "name": { "family": "Kirschvink", "given": "Joseph L." }, "id": "Kirschvink-J-L", "orcid": "0000-0001-9486-6689", "role": "member", "display_name": "Kirschvink, Joseph L." } ], "option_major": [ "geol" ], "doi": "10.7907/Z9H41PD8", "abstract": "The Late Cretaceous to Modern tectonic evolution of central and eastern California has been studied for many decades, with published work generally focusing on specific geographic areas and time periods. The resulting literature leaves the reader, whether graduate student, faculty member, or layperson, wondering what a coherently integrated tectonic evolution might look like, or if it would be at all possible to undertake such a task. This question is the common thread weaving together the four studies presented in this work. Each of the individual chapters is targeted at a specific location and time period which I have identified as a critical yet missing link in piecing together a coherent regional tectonic story. In the first chapter, we re-discover a set of major west down normal faults running along the western slope of the southern Sierra, the western Sierra fault system (WSFS). We show that one of these faults was offset by roughly a kilometer in Eocene time, and that this activity directly resulted in the incision of much of the relief present in modern Kings Canyon. The second chapter is a basement landscape and thermochronometric study of the hanging wall of the WSFS. New data from this study area provide a significant westward expansion of basement thermochronometric data from the southern Sierra Nevada batholith. Thermal modeling results of these data provide critical new constraints on the early exhumation of the Sierra Nevada batholith, and in the context of the results from Chapter I, allow us to piece together a coherent chronology of tectonic forcings and landscape evolution for the southern Sierra Nevada. In the third chapter, I present a study of the surface rupture of the 1999 Hector Mine earthquake, a dextral strike slip event on a fault in the Eastern California Shear Zone (ECSZ). New constraints on the active tectonics in ECSZ will help future studies better resolve the enigmatic mismatch between geologic slip rates and geodetically determined regional rates. Chapter IV is a magnetostratigraphic pilot study of the Paleocene Goler Formation. This study provides strong evidence that continued investigation will yield new constraints on the depositional age of the only fossil-bearing Paleocene terrestrial deposit on the west coast of North America. Each of these studies aims to provide important new data at critical missing links in the tectonic evolution of central and eastern California." }, { "name": "Given, Jeffrey Wayne", "degree": "PhD", "year": "1984", "title": "Inversion of Body-Wave Seismograms for Upper Mantle Structure", "advisor": "Harkrider, David G.", "url": "https://resolver.caltech.edu/CaltechTHESIS:10242018-091909982", "creators": [ { "name": { "family": "Given", "given": "Jeffrey Wayne" }, "id": "Given-Jeffrey-Wayne", "display_name": "Given, Jeffrey Wayne" } ], "advisors": [ { "name": { "family": "Harkrider", "given": "David G." }, "id": "Harkrider-D-G", "role": "advisor", "display_name": "Harkrider, David G." } ], "committee": [ { "name": { "family": "Harkrider", "given": "David G." }, "id": "Harkrider-D-G", "role": "chair", "display_name": "Harkrider, David G." }, { "name": { "family": "Anderson", "given": "Donald L." }, "id": "Anderson-D-L", "role": "member", "display_name": "Anderson, Donald L." }, { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "member", "display_name": "Helmberger, Donald V." }, { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "orcid": "0000-0001-8219-9428", "role": "member", "display_name": "Kanamori, Hiroo" }, { "name": { "family": "Stolper", "given": "Edward M." }, "id": "Stolper-E-M", "orcid": "0000-0001-8008-8804", "role": "member", "display_name": "Stolper, Edward M." } ], "option_major": [ "geophys" ], "doi": "10.7907/cgdh-0e19", "abstract": "We invert observed long- and short-period body-wave seismograms, travel times, and apparent velocity data to further constrain the compressional velocity structure in the upper mantle beneath northwestern Eurasia and the shear-wave velocity structure beneath western North America.
\r\n\r\nLong- and short-period WWSSN seismograms from nuclear explosions in the Union of Soviet Socialist Republics are incorporated with apparent velocity observations to derive an upper mantle model for northwestern Eurasia. The compressional waves from these explosions have several distinctive features that provide important new information about the character of the upper mantle in the region. The seismograms from 9\u00b0 to 13\u00b0 exhibit impulsive first arrivals, Pn, implying a smooth positive velocity gradient between depths of 60 and 150 km. There is a consistent pulse arriving about 2s after Pn at the distances of 13\u00b0 to 17\u00b0, and at larger ranges there are distinct reflections from two major discontinuities in the mantle. Synthetic seismograms displaying these features indicate a velocity model that correlates with other models from around the world, with a distinctive lid and low-velocity zone. The arrival following Pn is modeled by positioning the low-velocity zone between 150 and 200 km. The model is relatively smooth from a depth of 200 km down to 420 km, where a 5% jump in velocity produces a triplication in the travel time curve from 15\u00b0 to 23\u00b0. The observations from 21\u00b0 to 26\u00b0 clearly show another discontinuity at a depth of 675 km with a 4% change in velocity. These results suggest that stable continental regions may have a shadow zone that extends beyond 17\u00b0. Below 250 km there is no distinguishable difference between the model proposed for northwest Eurasia and models derived for the United States.
\r\n\r\nA systematic inversion technique is proposed to extract the maximum amount of information from these data. We use the WKBJ method to compute approximate synthetic seismograms in a radially heterogeneous earth. Where the WKBJ method breaks down, in low-velocity zones and near discontinuities, a generalized ray expansion is used in a layered model approximation to the velocity structure to isolate the energy that has reflected from these regions. Synthetic seismograms computed using these approximations compare very well to those computed by the more accurate method of summing primary reflections in a generalized ray sum yet require 1/20 the computation time. With this efficiency it is feasible to compute the differential seismograms necessary to pose an inverse problem.
\r\n\r\nWith a fast means of computing synthetic seismograms, an inverse problem can be posed to relate the differences between observed and synthetic seismograms to perturbations in the velocity structure. The problem is nonlinear, especially at high frequencies, but at long periods an iterative technique based on a linearized relation between perturbations in the velocity structure and the seismograms is effective if a reasonable initial model is assumed. Some simple tests of the method indicate that convergence to a satisfactory final model is possible even when starting with a model that predicts substantially different seismograms than those observed.
\r\n\r\nWe invert long-period SH waves recorded on WWSSN seismographs at distances from 15\u00b0 to 31\u00b0 in the western United States and East Pacific Rise to determine the upper mantle shear velocity structure beneath these regions. A high velocity gradient near 400 km produces clear later arrivals from 15\u00b0 to 17\u00b0. We interpret large later phases observed al distances from 23\u00b0 to 27\u00b0 as another large velocity gradient at between 600 and 720 km depth. Inversion of these seismograms suggests that the velocity gradient in the upper 200 km of the mantle is small; there is an increase in the velocity gradient around 250 km resulting in a 4% velocity increase by 360 km. The large velocity gradient near 400 km results in a velocity increase of around 8\u00bd% between 360 km and 420 km depth. The velocity gradient becomes smaller between 420 and 600 km with a cumulative increase of 5% over these depths. The total increase in velocity from 600 to 750 km is about 14%. Below 750 km the velocity gradient is assumed to be similar to those predicted by global studies of travel times.
\r\n\r\nThere are differences in published travel time data and models that have been derived to fit the SS phases and SS-S differential times observed in this region. The discrepancies amount to about 5s in the direct S-wave travel time at distances of 15\u00b0 to 18\u00b0. The discrepancy appears to be on the order of 3 s from 19\u00b0 to 23\u00b0 and is not resolvable beyond. These disagreements are probably the manifestation of large velocity heterogeneities in the uppermost mantle; either assumption concerning absolute travel times can be fit by models that are virtually identical below 270 km. Absolute travel times can constrain absolute velocities and, thus, are necessary to constrain the depth to discontinuities. Waveform data can constrain the structural details better. A joint waveform and travel time inversion method is a very useful tool for interpreting seismograms for earth structure.
\r\n" }, { "name": "Cole, David Martin", "degree": "PhD", "year": "1980", "title": "A Numerical Boundary Integral Equation Method for Transient Motions", "advisor": "Harkrider, David G.; Minster, Jean-Bernard H.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02142024-183355928", "creators": [ { "name": { "family": "Cole", "given": "David Martin" }, "id": "Cole-David-Martin", "display_name": "Cole, David Martin" } ], "advisors": [ { "name": { "family": "Harkrider", "given": "David G." }, "id": "Harkrider-D-G", "role": "advisor", "display_name": "Harkrider, David G." }, { "name": { "family": "Minster", "given": "Jean-Bernard H." }, "id": "Minster-Jean-Bernard-H", "role": "advisor", "display_name": "Minster, Jean-Bernard H." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "abstract": "This thesis presents the results of a study of a numerical technique for the solution of initial-boundary value problems of linear elastodynamics. The numerical method is based on a boundary integral equation (BIE) formulation of the mechanics of bodies of arbitrary shape. These integral equations are discretized and a time stepping technique is used to so1ve the resulting system of linear algebraic equations.
\r\n\r\nThe theoretical basis of the continuous problem and the general interpolation and discretization scheme are described in Chapter 1. The problem is then specialized to the two-dimensional case of antiplane strain and most subsequent calculations and discussions take place in this context. The performance of the numerical method depends entirely on the interpolation scheme used, and on the manner in which boundary shapes are approximated.
\r\n\r\nThe consequences of particular interpolation schemes for boundary value problems on a half-plane are discussed in Chapter 2. The results of several numerical calculations are compared with exact, or much more accurate solutions. This chapter also presents a compari\u00adson of the performance of the numerical BIE method with the performance of other specialized numerical procedures which have been used previously for problems of this nature. The BIE method yields results which are as accurate, or more accurate than the other methods for given discretization parameters.
\r\n\r\nThe method is applied to basic boundary value problems for\r\ncurved symmetric and nonsymmetric boundaries in Chapter 3. The solutions obtained there are again compared to more accurate or exact solutions produced by independent methods. The general dependence of errors on discretization parameters is discussed.
\r\n\r\nChapter 4 gives the solution of a problem in which a Love wave propagates through a limited region of laterally varying structure.\r\nThe time stepping nature of the BIE method makes feasible certain rearrangements of the numerical equations which yield a representation of the mechanical system in which the incident, unperturbed Love wave arises as an inhomogeneous term. Solution of this localized numerical equation then yields an intermediate variable, the change in the traction boundary value of the layered space surface, which is used to evaluate the scattered displacement wave.
\r\n\r\nThe performance characteristics and unusual properties of the time stepping BIE method are summarized in the General Summary. The appendices deal with several subjects. Appendix A gives an evaluation of singular integrals arising in the general continuous integral equation formulation. Appendix B gives a body force equivalent of nonequilibrium static initial values. Appendix C discusses the con\u00ad vergence and stability of solutions obtained using a particular inter\u00ad-polation scheme. Appendix D contains FORTRAN subroutines used in evaluating discrete kernels for the antiplane strain case. Appendix E gives the solution to a diffraction problem which is used to evaluate\r\nerrors in a BIE solution of the same problem which is given in Chapter 3.
" }, { "name": "Butler, Rhett Giffen", "degree": "PhD", "year": "1979", "title": "Seismological Studies Using Observed and Synthetic Waveforms", "advisor": "Helmberger, Donald V.; Anderson, Donald L.; Kanamori, Hiroo", "url": "https://resolver.caltech.edu/CaltechTHESIS:07112024-201950214", "creators": [ { "name": { "family": "Butler", "given": "Rhett Giffen" }, "id": "Butler-Rhett-Giffen", "display_name": "Butler, Rhett Giffen" } ], "advisors": [ { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "advisor", "display_name": "Helmberger, Donald V." }, { "name": { "family": "Anderson", "given": "Donald L." }, "id": "Anderson-D-L", "role": "advisor", "display_name": "Anderson, Donald L." }, { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "orcid": "0000-0001-8219-9428", "role": "advisor", "display_name": "Kanamori, Hiroo" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/bfn4-vz23", "abstract": "Application of waveforms to four topics in seismology is pre\u00adsented. Detailed waveform analyses of three earthquakes are reported in Chapter I. The Oroville, California earthquake of 8/1/75 has a north-south striking, westward dipping normal fault mechanism with a small component of left-lateral motion. A surface wave seismic moment\r\nof 1.9 x 10\u00b2\u2075 dyne-cm. is a factor of 3 greater than the teleseismic\r\nbody wave determination. Slow deformations on the Oroville fault may explain the enhanced excitation of the surface waves. The Tangshan, China earthquake of 7/27/76 and its principal aftershock represent a complex intraplate event sequence with strike-slip, normal, and thrust\r\nfaulting. The main shock was a bilateral strike-slip event, striking\r\nN40\u00b0E, with a seismic moment of 1.8 x 10\u00b2\u2077 dyne-cm. Associated thrusting occurred concurrently with the main shock. The principal aftershock was an oblique, normal double event, striking approximately perpendicular\r\nto the main event, with a seismic moment of 8 x 10\u00b2\u2076 dyne-cm. The 4/26/73\r\nHawaii earthquake is a subcrustal, double event. The events are consistent with left-lateral strike-slip motion on en echelon southward dipping faults. Evidence of lateral heterogeneity in the Hawaii source region\r\nis suggested by incompatibility between and azimuthal amplitude anomalies associated with the P and SH data.
\r\n\r\nIn Chapter II shear travel times are obtained by a waveform correla\u00adtion technique. A total of 87 SH travel-times are measured from the 1968 Borrego Mountain, California and 1973 Hawaii earthquakes. The Bor\u00adrego data have a trend toward faster travel times at 40\u00b0, but show an overall 6 second slow baseline with respect to the Jeffreys-Bullen Table. The Hawaii data contain large azimuthal scatter suggesting lateral heterogeneity in the near source region. The shear phase SS is modeled using a Hilbert transform to mimic distortion incurred at an internal caustic in its propagation. Significant variation is found in SS travel time residuals for paths reflected under the Cana\u00addian shield. A correlation of the variation with tectonic sub-pro\u00advince is suggested. Differential travel times of multiple ScS deter\u00ad mined by waveform cross-correlation are shown to contain a systematic bias late with respect to conventional visual onset timing methods.\r\nThe timing bias for Scs\u2082-ScS differential times ranges between 2.2 and\r\n3.8 seconds late, and depends upon the average Q_\u03b2 of the mantle.
\r\n\r\nIn Chapter III direct body waves and fundamental surface waves are calculated for a credible, hypothetical great earthquake on the San Andreas fault. Amplitudes and durations of long period ground motion (T > 1 second) are found for a receiver in downtown Los Angeles.\r\nCalculations are carried out for various epicenters, dislocation profiles, and time functions. Ground motion from Love radiation is found to be most important with peak-to-peak amplitudes up to 14 cm. and durations up to 5 minutes.
\r\n\r\nChapter IV presents a study of short period P wave amplitudes from nuclear explosions in the Soviet Union recorded by WWSSN stations in the United States. Thirty-four events in five test sites are analyzed. A well-defined amplitude pattern is obtained for each source region. A pattern of lateral variation of amplitude in the United States is obtained for a northern azimuth of approach. Stations in the western United States do not show systematically lower amplitudes than eastern stations, in contrast to previous studies. A preliminary data set of earthquakes in the Kurile Islands and South America indicate the amplitude pattern in the U.S. varies azimuthally.
" }, { "name": "Heaton, Thomas Harrison", "degree": "PhD", "year": "1979", "title": "Generalized Ray Models of Strong Ground Motion", "advisor": "Helmberger, Donald V.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04302007-150812", "creators": [ { "name": { "family": "Heaton", "given": "Thomas Harrison" }, "id": "Heaton-Thomas-Harrison", "orcid": "0000-0003-3363-2197", "display_name": "Heaton, Thomas Harrison" } ], "advisors": [ { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "advisor", "display_name": "Helmberger, Donald V." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/4VEQ-8942", "abstract": "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.
\r\n\r\nThe 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.
\r\n\r\nSeveral 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 10\u00b2\u2075 dyne-cm which is somewhat smaller than that found from teleseismic body waves by Burdick and Mellman (1976).
\r\n\r\nBecause 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 10\u00b2\u2076 ergs.
" }, { "name": "Johnson, Carl Edward", "degree": "PhD", "year": "1979", "title": "I. CEDAR - An Approach to the Computer Automation of Short Period Local Seismic Networks. II. Seismotectonics of the Imperial Valley of Southern California", "advisor": "Anderson, Donald L.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04232010-074317474", "creators": [ { "name": { "family": "Johnson", "given": "Carl Edward" }, "id": "Johnson-Carl-Edward", "display_name": "Johnson, Carl Edward" } ], "advisors": [ { "name": { "family": "Anderson", "given": "Donald L." }, "id": "Anderson-D-L", "role": "advisor", "display_name": "Anderson, Donald L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/DBG9-3260", "abstract": "A real-time detection and recording system (CEDAR) is developed as a means of automating the acquisition and processing of data from short- period local networks. This system has been used for the past two years for the analysis of data from 150 stations in Southern California with an annual workload of about 7500 local events. Two minicomputers are used with one dedicated to the real-time detection and digital recording of local earthquakes while the other is used for timing, location, and data archiving based on interactive graphical techniques. The use of this system has substantially reduced the effort required for the routine analysis of local data. The discussion is kept at a general level so as to be useful to those setting up similar systems with somewhat different requirements. In support of the CEDAR system a magnitude scale, M_(CA), is developed that is particularly adapted to the needs of local digital seismic networks. The supporting algorithm is based on median absolute amplitudes of any on-scale portion of the post-S seismic coda. The use of a power law coda shape function in the form a(t) = a_ot^(-q) makes the proposed method directly commensurable with the already widely used and highly successful duration method. The MCA magnitude scale is predicated on the same short-term averages used by the event detection algorithm on the real- time system, permitting a direct stochastic analysis of the spatial magnitude thresholds of a particular configuration of the detection logic. Such an a priori evaluation of detection capability is necessary since detection failure results in considerable extraneous effort. The use of these techniques has permitted the compilation of a local earthquake catalog and attendant phase data base that are substantially more uniform and accurate than what is generally obtained using manual methods.\r\n\r\nThe nature of earthquake swarms in the Imperial Valley is investigated with the goal of placing specific constraints on the physical mechanisms governing their behavior. Within the Imperial Valley most earthquakes occur as swarms concentrated within a narrow, sharply bounded, spindle-shaped zone joining the northern terminus of the Imperial Fault with the southern end of the San Andreas Fault. Although over the past five years the seismicity within this zone, designated the Brawley Seismic Zone, is surprisingly uniform, on time scales of a few weeks activity is highly clustered in both space and time. Seismicity is not confined to a few \"hot spots\", as might be expected, but rather seems to move around, seldom if ever reactivating the site of a previous swarm. Seven sequences of swarms are analyzed in detail using a master event approach in order to provide some insight into supporting tectonic structures. It is generally observed that swarm sequences comprise discrete bursts of activity, each of which appears to \"illuminate\" a single planar fracture transverse to the major tectonic elements in the Imperial Valley such as the Imperial Fault and the Brawley Fault. Development of activity during a sequence of swarms generally begins with high clustered activity followed by continuous, progressive involvement of the transverse structures, and progressive but discontinuous development in the form of spatially and temporally isolated clusters along the major fault elements. Observed migration rates range from .5 km/hr to .5 km/day. The consistency observed with respect to the pattern of development of independent sequences strongly suggests that a deterministic, physical model can be obtained. One possible model is suggested that relates the swarms on transverse structures with propagating, episodic creep on the major transforms. In this model both the creep rate and the triggering of earthquake swarms is governed by perturbations of pore-pressure in a fluid-infiltrated elastic matrix.\r\n" }, { "name": "Rial M., Jos\u00e9 Antonio", "degree": "PhD", "year": "1979", "title": "I. The Caracas, Venezuela Earthquake of 1967: A Multiple Source Event. II. Seismic Waves at the Epicenter's Antipode", "advisor": "Helmberger, Donald V.", "url": "https://resolver.caltech.edu/CaltechTHESIS:11112019-154510111", "creators": [ { "name": { "family": "Rial M.", "given": "Jos\u00e9 Antonio" }, "id": "Rial-M-Jos\u00e9-Antonio", "display_name": "Rial M., Jos\u00e9 Antonio" } ], "advisors": [ { "name": { "family": "Helmberger", "given": "Donald V." }, "id": "Helmberger-D-V", "role": "advisor", "display_name": "Helmberger, Donald V." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/725f-v881", "abstract": "This dissertation consists of two unrelated subjects. Although both deal with the construction of synthetic seismograms, the first (Chapter I) is the detailed study of a complicated earthquake source, whereas the second (Chapter II) deals with the structure of the earth's interior as seen from the antipode of a seismic wave source. The two can be classified as problems involving forward inversion of seismological data. There is, however, a significant common ground in the spirit of the approach in the sense that seismic wave interference phenomena, present in both, are used as allies in reducing the inherent non-uniqueness of the inversion process.
" }, { "name": "Hadley, David Milton", "degree": "PhD", "year": "1978", "title": "Geophysical Investigations of the Structure and Tectonics of Southern California", "advisor": "Kanamori, Hiroo; Silver, Leon T.", "url": "https://resolver.caltech.edu/CaltechTHESIS:01302012-140641318", "creators": [ { "name": { "family": "Hadley", "given": "David Milton" }, "id": "Hadley-David-Milton", "display_name": "Hadley, David Milton" } ], "advisors": [ { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "role": "advisor", "display_name": "Kanamori, Hiroo" }, { "name": { "family": "Silver", "given": "Leon T." }, "id": "Silver-L-T", "role": "advisor", "display_name": "Silver, Leon T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/1GTN-2W64", "abstract": "Regional variations in the crustal structure of southern California are defined by travel-time data from natural and artificial events. We show that the crust of the Mojave, northeastern Peninsular Ranges, eastern Transverse Ranges and Colorado Desert is dominated by a velocity of 6.2\u00b10.1 km/sec. The western Transverse Ranges and the western portion of the Peninsular Ranges are typified by a crustal\r\nvelocity of 6.7 km/sec. The data indicate that the Transverse Ranges do not have a distinct crustal root. As the topography is not supported isostatically, the Range must be sustained by major north-south compression. A composite profile extending north from the southern end of the Salton Sea defines a crustal thickness for the\r\nCoachella Valley of less than 20 km. Through the inversion of Rayleigh wave dispersion data obtained from the analysis of teleseismic surface waves recorded across southern California, we have obtained average S-wave models for the southern Mojave-central Transverse Ranges and the Peninsular Ranges. The observed P-wave velocities and the calculated Poisson's ratio from both P- and S-wave data require a quartz rich crust for the Mojave and a more mafic crust for the Peninsular Ranges. All S-wave models suggest a slight mid- crustal velocity reversal that is approximately coincident with the bottom of the seismic\r\nzone.
\r\n\r\nRegional variations in P_n velocities are obtained from several reversed refraction profiles . These data show that P_n varies from 7.7 to 8.2 km/sec. The high P_n values, 8.2 km/sec, are observed in the eastern Mojave, the western Transverse Ranges and the Coast Ranges.\r\nThe 7.8 km/sec P_n velocity extends from the Imperial Valley, through the central Transverse Ranges, and across the western Mojave. P_n profiles indicate that the Moho beneath the eastern Transverse Ranges and the southeastern Mojave dips 2-3\u00b0 west.
\r\n\r\nP-delay studies of a vertically incident PKP phase indicate that a high velocity, 8.3 km/sec structure exists within the shallow upper mantle beneath much of the geomorphic Transverse Ranges. This feature is not offset by the San Andreas fault. We suggest that the continuity of this anomaly across the plate boundary indicates that if the upper mantle participates in plate motion, the mantle plate boundary must be laterally displaced from the crustal boundary. We suggest that the mantle boundary may extend northwest from the Salton Trough and across the eastern end of the velocity anomaly, in the vicinity of the active Helendale-Lenwood-Camprock faults. We propose that the\r\nhorizontal decoupling between the crust and mantle, required by the lateral displacement at depth of the plate boundary, is accommodated, in part, within the 7.8 km/sec layer.
\r\n" }, { "name": "Hileman, James Alan", "degree": "PhD", "year": "1978", "title": "Part I: A Contribution to the Study of the Seismicity of Southern California. Part II: Inversion of Phase Times for Hypocenters and Shallow Crustal Velocities", "advisor": "Allen, Clarence R.; Minster, Jean-Bernard H.", "url": "https://resolver.caltech.edu/CaltechTHESIS:09202024-225628727", "creators": [ { "name": { "family": "Hileman", "given": "James Alan" }, "id": "Hileman-James-Alan", "display_name": "Hileman, James Alan" } ], "advisors": [ { "name": { "family": "Allen", "given": "Clarence R." }, "id": "Allen-C-R", "role": "advisor", "display_name": "Allen, Clarence R." }, { "name": { "family": "Minster", "given": "Jean-Bernard H." }, "id": "Minster-Jean-Bernard-H", "role": "advisor", "display_name": "Minster, Jean-Bernard H." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/6805-xg34", "abstract": "Southern California seismicity data for the period 1932 through 1975 are summarized in a series of epicenter maps. These maps show the seismic activity for one-year periods, for five-year periods, and as summaries for all earthquakes with magnitudes equal to or greater than 4, 5, and 6 respectively. Enlarged epicenter maps are given for the Los Angeles area and for the more significant aftershock sequences in the region.
\r\n\r\nA regional value of b = 1.00 \u00b1 .02 (log N = a - bM) is found for forty years of data. For several smaller areas in the region, b-values have a range of 0.76 - 1.00. Temporal variations of b-values (maximum likelihood estimates) for the various areas studied do not show any strong correlation with the occurrence of large earthquakes. A slight increase of the regional b-value after the 1952 Kern County earthquake is suggested.
\r\n\r\nSeismicity in the Imperial Valley indicates that several faults\r\nthere are susceptible to triggering, i.e. they are loosely coupled to motions of neighboring faults. Some earthquake swarms indicate this ease of triggering. A survey of the high level of swarm activity in the Imperial Valley is given. Two unusual aftershock sequences with periodic activity are described because the periodicity suggests sensi\u00adtivity to some triggering phenomenon.
\r\n\r\nSome seismicity in Southern California seems to be aligned in weakly defined zones that are transverse to the general tectonic fabric. These zones are thought to reflect conditions in the lower crust or uppermost mantle. The cause of these zones is unknown, but their trends are similar to those for the early Paleozoic continental boundary and to a recently discovered upper-mantle velocity anomaly.
\r\n\r\nDepth is usually the least-certain hypocenter parameter because it depends critically upon the accuracy of the velocity model. With enough arrival-time data, velocity estimation is feasible in addition to the usual hypocenter determinations. Linear least-squares inversion theory is adapted for the simultaneous determination of hypocenters and local velocity structure. A maximum likelihood formulation is used so that the data are weighted according to their estimated vari\u00adances. A tradeoff parameter controls the relative importance of the RMS error and the amount by which the model is changed at each itera\u00adtion. The inversion is also stabilized by specifying the allowed variances of each of the model parameters.
\r\n\r\nArrival times for a set of 20 earthquakes in the central Mojave Desert were inverted to improve the local velocity model. Each of the trials indicated that shallow crustal velocities in the vicinity of Galway Lake are somewhat lower than those of the usual velocity models. The velocities were not strongly constrained by this data set. This study points out the need for several seismographic stations placed within an aftershock area for best control of velocity estimates.
" }, { "name": "Okal, Emile Andr\u00e9", "degree": "PhD", "year": "1978", "title": "I. Application of Normal Mode Theory to Seismic Source and Structure Problems. II. Seismic Investigations of Upper Mantle Lateral Heterogeneity", "advisor": "Harkrider, David G.", "url": "https://resolver.caltech.edu/CaltechTHESIS:08262011-121733057", "creators": [ { "name": { "family": "Okal", "given": "Emile Andr\u00e9" }, "id": "Okal-Emile-Andr\u00e9", "display_name": "Okal, Emile Andr\u00e9" } ], "advisors": [ { "name": { "family": "Harkrider", "given": "David G." }, "id": "Harkrider-D-G", "role": "advisor", "display_name": "Harkrider, David G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/H2Q8-X139", "abstract": "In Part I, the theory of the normal modes of the Earth is investigated and used to build synthetic seismograms in order to solve source and structural problems. After a study of the physical properties of spheroidal modes leading to a rational classification, two specific\r\nproblems are addressed: the observability of deep isotropic seismic sources and the investigation of the physical properties of the Earth in the neighborhood of the Core-Mantle boundary, using SH waves diffracted at the core's surface.\r\n\r\nIn Chapter 1, it is shown that five different families of spheroidal modes can be isolated on the basis of their physical properties, including group velocities, attenuation and excitation functions. Except for a few hybrid modes, these families are arranged in \"pseudoovertone\" branches, along which physical properties vary smoothly. The simplified model of a spherical, non-gravitating Earth is used to give a theoretical description of the properties of modes with low angular orders. Their group velocity is shown to be consistent with the physical concept of dispersion along a pseudo-overt one branch, thereby justifying the use of asymptotic expansions along them in generating synthetic seismograms. An interpretation of the existence of the various families in terms of an increase in modecoupling\r\nwith angular order is presented. A formal classification of\r\nthe spheroidal modes into the five families is made, and a new nomenclature reflecting the physical properties of the modes is proposed.\r\n\r\nIn Chapter 2, the relative excitation of body and surface waves by isotropic and deviatoric sources is studied as a function of depth and frequency. Since the fundamental Rayleigh wave excitation dies off faster as a function of frequency and depth for isotropic than for deviatoric sources, an ultra-long period record at Pasadena of\r\nthe Colombian deep shock of 1970 (for which a compressional precursor was proposed), is studied and compared to synthetic seismograms calculated for several source models. The best agreement is obtained for a pure double-couple source. Linear combinations of synthetics for deviatoric and isotropic sources are tested for a wide range of\r\nrelative amplitudes, showing the data to be little sensitive to the presence of a reasonably large isotropic component.\r\n\r\nIn Chapter 3, profiles of seismic shear waves diffracted around the core (Sd) for three deep events recorded at stations across North America and the Atlantic Ocean are used to determine the properties of the lower mantle in the vicinity of the core-mantle boundary. The S wave velocity above the surface of the core is found to be 7.22 \u00b1\r\n0.1 km/s, in agreement with gross Earth models, but higher than previously reported values from direct measurements of Sd. No evidence for a low- velocity zone in the lower mantle is found. Synthetic seismograms for Sd are easily generated through normal mode summation. A comparison of the present data with a synthetic profile for Earth model 1066A gives excellent agreement at periods greater\r\nthan 45 seconds. Synthetics for other models confirm the absence of a strong low-velocity zone at the base of the mantle, and are used to strongly constrain any possible rigidity of the uppermost layers of the core.\r\n\r\nIn Part II, data sets of seismic body and surface waves are used in a search for possible deep lateral heterogeneities in the mantle. In both cases, it is found that seismic data do not require structural differences between oceans and continents to extend deeper than 250 km. In general, differences between oceans and continents are found\r\nto be on the same order of magnitude as the intrinsic lateral heterogeneity in the oceanic plate brought about by the aging of the oceanic lithosphere. A consistent similarity is inferred between stable shields and the oldest parts of the oceans.\r\n\r\nIn Chapter I, an analysis of records of multiply reflected ScS phases from ten deep focus earthquakes yields near-vertical one-way travel-time residuals ranging from -3.5 to +5.0 seconds. Continental and oceanic residuals overlap, and both indicate large lateral variations. Similar values are found for the older oceanic basins (Western Pacific, Brazil Basin) and continental shields. Most, if not all, of the variations can be attributed to differences in the\r\nlithosphere and asthenosphere, down to a depth of 200 km, and the present results are in good agreement with local models derived by independent means. Oceanic islands are found to be anomalous with respect to the neighboring ocean floor, the mantle beneath Hawaii, Iceland and Trindade (South Atlantic) being exceptionally slow.\r\n\r\nIn Chapter 2, Rayleigh wave phase velocities at very long\r\nperiods (185 to 290 seconds) are investigated and regionalized, taking into account the lateral heterogeneities in the oceanic plates revealed by earlier studies at shorter periods. The two-station method is applied to a few 'pure-age' oceanic paths, and is shown to\r\nbe compatible with an average gross Earth model below depths of 180 km. Under this assumed oceanic model, regionalized for age above 180 km, continental velocities are derived from a set of experimental great-circle values, both new or taken from previously published studies. The results basically agree with the earlier studies by\r\nKanamori or Dziewonski, and it is suggested that the assumption of a uniform oceanic model may have been responsible for some scatter in Kanamori's solution. The results of the present inversion are successfully checked against a set of values derived by the two-station\r\nmethod from a pure continental, tectonic, path. A recent event in Indonesia is then used as a further independent check, in what is believed to be the first experimental determination of Rayleigh wave phase velocities over a pure shield path at very long periods. The shield velocities fall within the range of variation of their oceanic\r\ncounterparts with the age of the plate, in agreement with the results of Chapter 1. This makes velocities derived theoretically from models involving deep continent vs. ocean lateral heterogeneities inconsistent with the present set of experimental data. Finally, it is shown that\r\nDziewonski's model S2 reconciles all experimental seismic data relative to shields without being significantly different from oceanic models below 240 km.\r\n" }, { "name": "Raikes, Susan Ann", "degree": "PhD", "year": "1978", "title": "I. Regional Variations in Upper Mantle Compressional Velocities beneath Southern California. II. Post-Shock Temperatures: Their Experimental Determination, Calculation, and Implications", "advisor": "Kanamori, Hiroo; Ahrens, Thomas J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04142020-164137815", "creators": [ { "name": { "family": "Raikes", "given": "Susan Ann" }, "id": "Raikes-Susan-Ann", "display_name": "Raikes, Susan Ann" } ], "advisors": [ { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "orcid": "0000-0001-8219-9428", "role": "advisor", "display_name": "Kanamori, Hiroo" }, { "name": { "family": "Ahrens", "given": "Thomas J." }, "id": "Ahrens-T-J", "role": "advisor", "display_name": "Ahrens, Thomas J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "geophys" ], "doi": "10.7907/3323-8080", "abstract": "The establishment in Southern California of a large seismographic network provides an unique opportunity for studying the seismic velocity variations within a tectonically active region that includes a major plate boundary, whose surface expression is the San Andreas Fault. In the first part of this thesis, the compressional velocity within the upper mantle beneath Southern California is investigated through observations of the dependence of teleseismic P-delays at all stations of the array on the distance and azimuth to the event. The variation of residuals with azimuth was found to be as large as 1.3 sec at a single station; the delays Here stable as a function of time, and no evidence was found for temporal velocity variations related to seismic activity in the area. These delays were used in the construction of models for the upper mantle P-velocity structure to depths of 150 km, both by ray tracing and inversion techniques. The models exhibit considerable lateral heterogeneity including a region of low velocity beneath the Imperial Valley, and regions of increased velocity beneath the Sierra Nevada and much of the Transverse \u00b7Ranges. These changes are attributed to variation in the degree of partial melting within the upper mantle; their relationship to, and implications for, regional tectonics are discussed in the final chapter of this section.
\r\n\r\nOne of the major uncertainties in the interpretation of shock wave data is the temperature reached under shock compression and subsequent release. The second half of this thesis describes the development of a technique for the experimental determination of post-shock temperatures, its application to several metals and silicates shocked to pressures in the range 5 to 30 CPa. The technique utilises an infra-red radiation detector to determine the brightness temperature of the free surface of the sample after the shock wave has passed through it, and has yielded highly reproducible results that are consistent for the wavelength ranges 4.5 to 5.75 and 7 to 14\u00b5. The comparison of these results with values calculated using conventional theories provides some insight into the thermal processes occurring in shock waves. In particular, the measured temperatures are generally higher than those calculated; this is attributed to elasto-plastic effects in metals, and is probably associated with strength effects in silicates, both of which are commonly ignored in the calculation of theoretical temperatures. The implications of these observations for the interpretation of shock-induced metamorphism and impact phenomena, and for the application of shock-wave data to the interpretation of the behaviour of silicates within the earth's mantle, are discussed in the final chapter.
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