[ { "id": "authors:k1rjb-2z941", "collection": "authors", "collection_id": "k1rjb-2z941", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121129-104248537", "type": "article", "title": "Further structural constraints and uncertainties of a thin\n laterally varying ultralow-velocity layer at the base of the mantle", "author": [ { "family_name": "Garnero", "given_name": "Edward J.", "clpid": "Garnero-E-J" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "abstract": "Constraints and uncertainties are presented for modeling of an ultralow-velocity zone layer (ULVZ) at the base of Earth's mantle using an SKS wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SP_dKS. Source or receiver effects are ruled out as causes for the SP_dKS anomalies used to map ULVZ structure, since systematic SP_dKS-SKS travel time moveout behavior is present in profiles of recordings of a given earthquake at many seismographic stations and also for many events recorded at one station. The southwest Pacific region produces strong variability in observed SP_dKS/SKS amplitude ratios (compared to synthetic seismograms), which geographically corresponds to an anomalous ULVZ region. Accurate determination of absolute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (V_p) reduction in the layer, the magnitude of S wave velocity (V_S) reduction and density (\u03c1) perturbation (if any). Synthetic seismogram experiments demonstrate several key points regarding uncertainties and constraints in modeling ULVZ structure: (1) thicker layers (up to 300 km thick) with mild reductions (e.g., \u22122.5 to \u22125.0%) cannot reproduce the anomalous SP_dKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade-offs exist between discontinuous velocity reductions and linear gradient reductions over a thicker zone; (3) uncertainties preclude precise determination of magnitude of \u03b4V_P and \u03b4V_S reductions, as well as the \u03b4V_S:\u03b4V_P ratio; (4) large density increases within the ULVZ (e.g., up to 60% and more) can efficiently broaden and delay the peak of the energy that we identify as SP_dKS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SP_dKS waveforms, and dampen spurious ringing energy present in Sd waveshapes due to the ULVZ; and (6) the minimum required V_p reduction for the most anomalous data (around \u221210%) trades off with thinner ULVZ structures containing larger velocity reductions (with possible density increases as well).", "doi": "10.1029/98JB00700", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1998-06-10", "series_number": "B6", "volume": "103", "issue": "B6", "pages": "12,495-12,509" }, { "id": "authors:f8a8t-7ww28", "collection": "authors", "collection_id": "f8a8t-7ww28", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121128-103352821", "type": "article", "title": "Modeling two-dimensional structure at the core-mantle\n boundary", "author": [ { "family_name": "Helmberger", "given_name": "D. V.", "clpid": "Helmberger-D-V" }, { "family_name": "Garnero", "given_name": "E. J.", "clpid": "Garnero-E-J" }, { "family_name": "Ding", "given_name": "X.", "clpid": "Ding-X" } ], "abstract": "Recent studies of SKS waveform modeling emphasize the strong variation of seismic properties at the core-mantle boundary (CMB) and the need for two-dimensional and three-dimensional waveform modeling capabilities. In particular, the bifurcation of SKS into SP _dKS and SKP _dS near 110\u00b0 shows strong regional variations. The first of these phases has a P wave diffraction along the bottom of the mantle near the source, while the latter phase occurs at the receiver end. Generalized ray theory proves effective in generating theoretical seismograms in this type of problem because each of these diffractions is associated with a particular transmission coefficient: T_(sp) which transmits shear waves into primary waves when crossing the CMB and T_(sp) which transmits the primary waves back into shear waves at the receiver end. Each region can then be isolated and have its separate fine structure, sharp or gradational. Two classes of boundaries are explored: the CMB as a simple, sharp interface and the CMB with a very low velocity transition layer (10% slower than reference models). The two diffractions produced by these structures have diagnostic arrival times and wave shapes and when combined with the geometric SKS produce distinct waveform characteristics not easily generated by other means. Since the ray paths associated with these three phases are virtually identical in the mantle and only differ along a short sample of CMB and in the one-dimensional fluid core, we can isolate the small localized CMB region sampled. Thus the waveform character of the extended SKS in the range of 105\u00b0 to 120\u00b0 becomes an excellent CMB probe which we demonstrate on a small sample of observations from the Fiji-Tonga region as recorded in North America.", "doi": "10.1029/96JB00534", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1996-06-10", "series_number": "B6", "volume": "101", "issue": "B6", "pages": "13,963-13,972" }, { "id": "authors:pdg5a-9mj46", "collection": "authors", "collection_id": "pdg5a-9mj46", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121127-152932227", "type": "article", "title": "Seismic detection of a thin laterally varying boundary layer\n at the base of the mantle beneath the central-Pacific", "author": [ { "family_name": "Garnero", "given_name": "Edward J.", "clpid": "Garnero-E-J" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "abstract": "We explore lowermost mantle structure beneath the Pacific with long\u2010period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle\u2010side boundary layer at the CMB, having P\u2010velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large\u2010scale low velocity feature in the lower mantle beneath the central\u2010Pacific. SKS, SP_dKS, and SKKS data for the generally faster\u2010than\u2010average circum\u2010Pacific lower mantle are well\u2010fit by models lacking any such low\u2010velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions.", "doi": "10.1029/95GL03603", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "1996-05-01", "series_number": "9", "volume": "23", "issue": "9", "pages": "977-980" }, { "id": "authors:gggvr-7zp40", "collection": "authors", "collection_id": "gggvr-7zp40", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121205-105547056", "type": "article", "title": "A very slow basal layer underlying large-scale low-velocity\n anomalies in the lower mantle beneath the Pacific:\n evidence from core phases", "author": [ { "family_name": "Garnero", "given_name": "Edward J.", "clpid": "Garnero-E-J" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "abstract": "A multi-phase analysis using long-period World Wide Standardized Seismograph Network and Canadian Network data has been conducted using core-phases for deep focus events from the southwest Pacific. These include SKS, S2KS, SVd_(iff), and SP_dKS. The last phase emerges from SKS near 106\u00b0 and is associated with a P-wave diffracting along the bottom of the mantle. Patterns in S2KS - SKS differential travel times (T_(S2KS-SKS)) correlate with those in SP_dKS - SKS (T_(SPdKS-SKS)). T_(S2KS-SKS) values strongly depend on variations in V_S structure in the lower third of the mantle, whereas T_(SPdKS-SKS) values mainly depend on V_P structure and variations in a thin zone (100 km or less) at the very base of the mantle. Anomalously large T_(S2KS-SKS) and T_(SPdKS-SKS) values (relative to the Preliminary Reference Earth Model (PREM)) are present for Fiji-Tonga and Kermadec events (recorded in North and South America), along with anomalously large SV_(diff) amplitudes well into the core's shadow. More northerly paths beneath the Pacific to North America for Indonesian and Solomon events display both PREM-like and anomalous times. A model compatible with the observations is presented, and contains a thin very-low-velocity layer at the base of the mantle that underlies the large volumetric lower-mantle low-velocity regions in the southwest Pacific. A low-velocity layer of 20\u2013100 km thickness with reductions of up to 5\u201310% (relative to PREM) can reproduce T_(SPdKS-SKS) as well as SV_(diff) amplitudes. Large-scale (more than 1000 km) lower-mantle V_S heterogeneity (2\u20134%) can explain long-wavelength trends in T_(S2KS-SKS). The exact thickness and velocity reduction in the basal layer is uncertain, owing to difficulties in resolving whether anomalous structure occurs on the source- and/or receiver-side of wavepaths (at the CMB).", "doi": "10.1016/0031-9201(95)03039-Y", "issn": "0031-9201", "publisher": "Elsevier", "publication": "Physics of the Earth and Planetary Interiors", "publication_date": "1995-09", "series_number": "1-3", "volume": "91", "issue": "1-3", "pages": "161-176" }, { "id": "authors:vehzs-tt773", "collection": "authors", "collection_id": "vehzs-tt773", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20121205-104754253", "type": "article", "title": "On seismic resolution of lateral heterogeneity in the Earth's outermost core", "author": [ { "family_name": "Garnero", "given_name": "Edward J.", "clpid": "Garnero-E-J" }, { "family_name": "Helmberger", "given_name": "Donald V.", "clpid": "Helmberger-D-V" } ], "abstract": "Issues concerning resolution of seismically determined outermost core properties are presented with an example from three earthquakes in the Fiji-Tonga region. Travel time behavior of the commonly used family of SmKS waves, which travel as S in the mantle, P in the core, reflecting m \u2212 1 times at the underside of the core-mantle boundary (CMB), are analyzed over a large distance range (125\u2013165\u00b0). Data having wavepaths through an area of known D\u2033 heterogeneity (\u00b12%) exhibit systematic anomalies in SmKS differential times. Two-dimensional wave propagation experiments demonstrate how large-scale lower-mantle velocity perturbations can explain long-wavelength behavior of such anomalous SmKS times, though heterogeneity on smaller scales may be responsible for the observed scatter about these trends. If lower-mantle heterogeneity is not properly accounted for in deriving a core model, misfit of the mantle model maps directly into core structure. The existence of outermost core heterogeneity is difficult to resolve at present, owing to uncertainties in global lower-mantle structure. Resolving a one-dimensional chemically stratified outermost core also remains difficult, owing to the same uncertainties. Inclusion of the slowly accruing broadband data should help in this regard. Restricting study to higher multiples of SmKS (m = 2, 3, 4) can help reduce the effect of mantle heterogeneity, because of the closeness of the mantle legs of the wavepaths. SmKS waves are ideal in providing additional information on the details of lower-mantle heterogeneity.", "doi": "10.1016/0031-9201(94)02976-I", "issn": "0031-9201", "publisher": "Elsevier", "publication": "Physics of the Earth and Planetary Interiors", "publication_date": "1995-03", "series_number": "2", "volume": "88", "issue": "2", "pages": "117-130" }, { "id": "authors:4aw2g-5j123", "collection": "authors", "collection_id": "4aw2g-5j123", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20180329-152142655", "type": "article", "title": "Preliminary observations from the use of US-Soviet Joint Seismic Program data to model upper mantle triplications beneath Asia", "author": [ { "family_name": "Garnero", "given_name": "E. J.", "clpid": "Garnero-E-J" }, { "family_name": "Helmberger", "given_name": "D. V.", "clpid": "Helmberger-D-V" }, { "family_name": "Burdick", "given_name": "L. J.", "clpid": "Burdick-L-J" } ], "abstract": "New short-period waveform data from the US-Soviet Joint Seismic Program (JSP) make possible investigations of Asian upper mantle structure. the goal of this paper is to explore the potential use of the newly available JSP data to gain a qualitative view of upper mantle structure beneath Asia, and to facilitate more detailed future detailed future upper mantle studies. In a reconnaissance approach, waveform upper mantle studies. In a reconnaissance approach, waveform predictions from upper mantle P-wave velocity models of previous studies are compared to the JSP data to investigate regional differences in the central Asian upper mantle. Data coverage brackets the upper mantle triplications with excellent multi-source-to-stations sections. the abundance of data for controlled source-receiver geometries and the impulsive nature of the arrivals enable us to stack seismograms to improve signal-to-noise ratio. Arrivals from the 400 and 670 km discontinuities are apparent in the data and are compared to predictions of the mantle models. the principal result is that, for the regions studied, paths through cratonic regions of Asia are compatible with shield-type models, while paths through highly deformed regions of Asia are compatible with models derived for tectonically active regions, suggesting large lateral variations beneath the Eurasian continent. Use of the JSP data in a comparative approach is fast and simple, and proves effective in obtaining a first-order understanding of the Asian upper mantle. This result also presents the potential for qualitative studies elsewhere with digital portable stations.", "doi": "10.1111/j.1365-246X.1993.tb02544.x", "issn": "0956-540X", "publisher": "Royal Astronomical Society", "publication": "Geophysical Journal International", "publication_date": "1993-04-01", "series_number": "1", "volume": "113", "issue": "1", "pages": "252-259" }, { "id": "authors:fqsf4-gpr38", "collection": "authors", "collection_id": "fqsf4-gpr38", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140806-092337671", "type": "article", "title": "The Location and Source Parameters of the Lompoc, California, Earthquake of 4 November 1927", "author": [ { "family_name": "Helmberger", "given_name": "D. V.", "clpid": "Helmberger-D-V" }, { "family_name": "Somerville", "given_name": "P. G.", "clpid": "Somerville-P-G" }, { "family_name": "Garnero", "given_name": "E. J.", "clpid": "Garnero-E-J" } ], "abstract": "In this paper, we address the relocation, magnitudes, and the style of faulting of the Lompoc earthquake from a sparse assortment of teleseismic and regional seismograms. The highest quality teleseismic waveform data come from a station at De Bilt (Netherlands) that remains in operation. Thus, recordings of numerous modern events in central coastal California (i.e., the 1969 Santa Lucia Banks, 1983 Coalinga, 1978 Santa Barbara, and 1989 Loma Prieta earthquakes) have been used for comparison with the 1927 records. Location constraints for the Lompoc event were established from the De Bilt recording by comparing S-P and SSS-S waveform matches against the above master events to avoid the effect of unknown clock errors on locations that use absolute times. These same seismograms were modeled to estimate the depth, faulting parameters, and source strength. A similar approach using observational comparisons and numerical modeling was applied to the regional waveform data obtained from the stations at Berkeley, Tucson, and Pasadena.\n\nOur results indicate a north-northwesterly striking reverse event located about 40 km west of Point Conception, which is in excellent agreement with the recent tsunami modeling results by Satake and Somerville (1992). This location is 25 km south of that proposed by Hanks (1979) and well within his error bars. We obtain a body-wave moment of 1 \u00d7 10^(26) dyne-cm, a trapezoidal time history of (2, 2, 2) sec. and a source depth of 10 km. The weak beginning of the Pnl wavetrain at Berkeley indicates some source complexity, which is characteristic of many large events. The fault parameters are strike = N20\u00b0W, dip = 66\u00b0NE, and rake = 95\u00b0. Most seismicity catalogs report a M_s = 7.3 for this event, after Gutenberg and Richter (1956), but this was a long-period body-wave magnitude and not a surface-wave result. Their original worksheets indicate a smaller Ms = 7.0. The body waves of the Loma Prieta event (M_s = 7.1) appear distinctly larger than those of the Lompoc event at De Bilt, in agreement with our lower estimate of source strength.", "issn": "0037-1106", "publisher": "Seismological Society of America", "publication": "Bulletin of the Seismological Society of America", "publication_date": "1992-08", "series_number": "4", "volume": "82", "issue": "4", "pages": "1678-1709" } ]