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Updated: Oct 13, 2010

7th International Symposium on Deep Seismic Profiling of the Continents, I: General Results and New Methods

Elsevier logo

Asilomar, CA, USA, 12 September - 15 September 1996

from TECTONOPHYSICS Volume 286, Issues 1-4, Pages 1-298 (10 March 1998)
Edited by Simon L. Klemperer and Walter D. Mooney

Abstracts of papers presented for publication in a special issue of Tectonophysics

(Full copies of these papers may be purchased for about US$30 each from Elsevier through their web site and follow the "journals" link to Tectonophysics)

Deep seismic sounding of the continental crust and mantle - a long-term view

Pages 1-4
Jack Oliver

Elsevier online abstract

Imaging and interpreting the shallow crystalline crust

Pages 5-18
B. Milkereit and D. Eaton

In the past, many tests of high-resolution seismic imaging methods, utilizing 'off-the-shelf' technology from hydrocarbon exploration have had only limited success. Some problems associated with the application of seismic profiling in the shallow crystalline crust differ from those encountered in sedimentary basins. Perhaps the most fundamental distinction stems from differences in statistical properties of the reflection coefficient time series. Analysis of power spectra derived from borehole data indicates that the reflection coefficients in the crystalline crust are depleted in the seismic bandwidth relative to sedimentary basins. This fundamental difference must be considered in the design of shallow crustal seismic surveys in order to achieve success. To a first approximation, acoustic impedance variations in sedimentary basins are controlled by velocity changes, whereas velocity and density can vary independently in the crystalline crust and changes in density may be relatively more important in controlling reflectivity. In certain instances, density fluctuations outweigh velocity fluctuations. For example, physical rock property data from mining districts in the Canadian Shield suggest that massive Fe-oxide or sulphide bodies will generate strong seismic reflections.

Elsevier online abstract

Seismic reflection profiling across the seismogenic fault of the 1995 Kobe earthquake, southwestern Japan

Pages 19-30
H. Sato, H. Hirata, T. Ito, N. Tsumura and T. Ikawa

The 1995 Kobe (Hyogo-ken Nanbu) earthquake (Mw 6.9) surface rupture appeared along the Nojima fault on the northwest coast of Awaji Island. The aftershock epicentres in northern Awaji Island are not aligned with the surface faulting. suggesting a complicated active fault structure. To reveal the structure of the seismogenic fault and its associated active faults, a perpendicular 41.6-km-long seismic reflection survey was undertaken across northern Awaji Island. The Trans-Awaji seismic reflection profile reveals the fault geometry beneath the area. Awaji Island was uplifted by movement on the Kariya fault along its east coast, which produced an asymmetry in Neogene basin that is shallower in the western part than in the eastern part. The faults beneath Awaji Island show a 'pop-up' or a 'positive flower structure' and the basement is bounded by two active faults, the Nojima along the west coast and the Kariya along the east coast. Both are high-angle, reverse faults with a right-lateral, strike-slip movement. The cluster of aftershocks in the northern part of Awaji Island has a Y-shaped hypocentral distribution. The Nojima and Kariya faults thus appear to connect in the middle of the upper crust at about 7 km depth, forming a seismogenic master fault. Three major seismic events have been reported on the active faults in northern Awaji Island: a prehistoric event dated 2000 yr BP, the 1596 Keicho-Fushimi earthquake, and the 1995 Kobe earthquake. Based on the structural relationship of the active faults that we observed in this study, we suggest that these events occurred on the proposed master fault at mid-crustal depth.

Elsevier online abstract

The San Gabriel Mountains bright reflective zone: possible evidence of young mid-crustal thrust faulting in southern California

Pages 31-46
Trond Ryberg and Gary S. Fuis

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a line extending northeastward from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles basin and San Gabriel Mountains. Shots and receivers were spaced most densely through the San Gabriel Mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area. A stack of common-midpoint (CMP) data reveals a bright reflective zone, 1-s thick, that dominates the stack and extends throughout most of the mid-crust of the San Gabriel Mountains. The top of this zone ranges in depth from 6 s ( 18-km depth) in the southern San Gabriel Mountains to 7.5 s ( 23-km depth) in the northern San Gabriel Mountains. The zone bends downward beneath the surface traces of the San Gabriel and San Andreas faults. It is brightest between these two faults, where it is given the name San Gabriel Mountains 'bright spot' (SGMBS), and becomes more poorly defined south of the San Gabriel fault and north of the San Andreas fault. The polarity of the seismic signal at the top of this zone is clearly negative, and our analysis suggests it represents a negative velocity step. The magnitude of the velocity step is approximately 1.7 km/s. In at least one location, an event with positive polarity can be observed 0.2 s beneath the top of this zone, indicating a thickness of the order of 500 m for the low-velocity zone at this location. Several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone, possibly a 'master' decollement. (1) It represents a significant velocity reduction. If the rocks in this zone contain fluids, such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the SGMBS; near-lithostatic fluid pressure is required in the SGMBS. Such differential changes are believed to occur in the neighborhood of active fault zones, where 'fault-value' action has been postulated. Less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke. (2) It occurs at or near the brittle-ductile transition, at least in the southern San Gabriel Mountains, a possible zone of concentrated shear. (3) A thin reflection rising from its top in the southern San Gabriel Mountains projects to the hypocenter of the 1987 M 5.9 Whittier Narrows earthquake, a blind thrust-fault earthquake with one focal plane subparallel to the reflection. Alternatively, one could argue that the bends or disruptions in the reflective zone seen at the San Gabriel and San Andreas faults are actually offsets and that the reflective zone is therefore an older feature, possibly an older fault zone.

Elsevier online abstract

Orogenic mass transfer and reflection seismic patterns - evidence from DEKORP sections across the European Variscides (central Germany)

Pages 47-61
Onno Oncken

Orogenic wedges evolve by different mass transfer modes which control different styles of deformation, metamorphic pattern, and bulk reflectivity. Structural patterns and reflectivity strongly vary depending mainly on the length/thickness ratio of thrust sheets and on their displacement/length ratio. Reevaluation of DEKORP seismic sections across the central European Variscides (lines DEKORP 1A-C, 9N, 2N-2S, 3B) underlines the usefulness of the integrated interpretation strategy in establishing orogenic growth mode and evolution. The results are in contrast with reflectivity patterns predicted from critical taper theory. The northern flank of the Mid-European Variscides is formed by a doubly vergent orogenic wedge system which evolved from collision of a passive margin (Rhenohercynian zone) with a continental arc (Mid-German Crystalline Rise, MGCR, of the Saxothuringian zone). The two fold and thrust wedges show inward-dipping shear zone-related reflections above subhorizontal mid-crustal reflection bands, evidence of dominantly frontal accretion of upper crustal material in both wedges. In contrast, the sections crossing the axial zone (MGCR) either show arched reflection stacks related to underplated amphibolite-facies rocks in exposed antiformal nappe stacks, or a transparent wedge-shaped unit related to low-grade calcalkaline magmatics and steeply dipping Devono-Carboniferous sediments of the former upper-plate magmatic arc. Analysis suggests that part of this arc was tectonically eroded prior to collision. In combination with isograd and structural patterns the observed features indicate that heterogeneous basal accretion was the dominant accretion mode in the axial zone during collision. The discussed patterns are suggested to reflect the material transfer site into the orogenic wedge and therefore also growth mode and material provenance in collisional belts.

Elsevier online abstract

Seismic exploration of continental strike-slip zones

Pages 63-78
T. A. Stern and J. H. McBride

Seismic exploration of both Palaeozoic and active strike-slip zones shows strike-slip faults that penetrate all or most of the crust. Offsets on the Moho are evident, particularly at young and active zones with a component of compression, such as the Alpine Fault of New Zealand where a change in crustal thickness of about 20 km is observed. Moho offsets for the old Palaeozoic strike-slip zones are usually much less prominent. Careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets, the Moho structure consists of a localized keel-type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide. The large Moho offsets of young strike-slip zones may in some cases partially decay with time. Active strike-slip zones are becoming an important focus of study, but seismic exploration is hampered by complex near-surface geology, 3D structure and the difficulty of imaging steeply dipping structure in the subsurface. In order to meet this challenge, a wide range of seismic techniques is now being deployed. These include wide-angle seismic reflection, refraction, P-wave delays and the study of guided S-waves. Results from California give geophysical images of vertical strike-slip faults that penetrate to the Moho. In contrast, the Alpine Fault of New Zealand appears to be a surface manifestation of an inclined ( 40°) ramp, extending down to the lower crust and along which uplift and exhumation of the continental crust, and possibly strike-slip motion, is taking place.

Elsevier online abstract

Study of the crystalline crust from a two-ship normal-incidence and wide-angle experiment

Pages 79-91
Satish C. Singh, Philip J. Hague and Michael McCaughey

In 1992, British Institutions' Reflection Profiling Syndicate conducted a two-ship coincident near-vertical and wide-angle experiment to determine the detailed velocity structure of the continental crust in the Central North Sea. The results show a near-surface high velocity gradient layer (P-wave velocity reaches 6.0 km/s at 5 km depth), a high-velocity zone (6.5 km/s) at 10-15 km depth, a 19-km-thick upper crust and the Moho at a depth of 32-34 km along the 135-km profile. The travel-time and amplitude of wide-angle data require the presence of a dipping reflector originating at the Moho (32 km depth) and continuing down to 38 km in the mantle. The velocity between 19 and 24 km depth is slightly lower (6.1 km/s) than that in the upper crust. The waveform modelling of the combined normal-incidence and wide-angle data requires a number of alternating high- and low-velocity layers in the lower crust (19-33 km). The velocity contrast in the lower part of the lower crust (29-33 km) is large, of the order of 1 km/s (6.5-7.5 km/s). There are a few alternating high- (8.2 km/s) and low-velocity (7.5 km/s) layers in the upper part of the mantle (33-36 km). The high velocity (6.5 km/s) in the upper crust may be due to a high grade of metamorphism, and it may have been formed during the subduction process. The low-velocity (6.1 km/s) in the upper part of the lower crust might be caused by the presence of meta-sediments. The large velocity variations (6.5-7.5 km/s) in the lower part of the lower crust suggest the presence of mafic materials, and may result from under-plating of mafic materials from the mantle. The fractionation of melt can also produce the layering in the upper part of the mantle that is observed from seismic analyses.

Elsevier online abstract

Skeletons and fractals - a statistical approach to deep crustal seismic data processing and interpretation

Pages 93-109
Kris Vasudevan and Frederick A. Cook

Coherent seismic reflection events are identified using seismic skeletonization, an automatic pattern recognition technique in which waveforms are parameterized such that the attributes of each event are identified and stored in a relational database, or 'event file'. This new approach to seismic interpretation allows statistical analyses of the attributes including estimation of scaling laws for reflectivity observed on deep seismic data, and development of new filtering techniques based on characteristics such as dip and reflection event length. Applications of the technique to deep crustal reflection data sets from the Alberta Basement Transect of the LITHOPROBE project assist interpretation in areas that lack surface outcrop or other means of relating reflections from the deep crust to the surface.

Elsevier online abstract

Super-deep reflection profiling: exploring the continental mantle lid

Pages 111-121
David N. Steer, James H. Knapp and Larry D. Brown

Reflections from the continental mantle lithosphere have now been reported from common mid-point (CMP) profiles at numerous sites around the world. These reflections include: (1) distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle; (2) kilometer scale, discontinuous, diffusely distributed, or isolated reflections at varying depths; (3) continuous, sub-horizontal reflections tens of kilometers in extent, often interpreted as the 'base of the lithosphere' in spite of a wide variation in depth and; (4) very deep reflections in the mantle lithosphere (> 125 km). The first category of mantle reflections is most commonly observed in proximity to crustal sutures, lending credence to their interpretation as fossil subduction zones. Isolated, discontinuous mantle reflections are by far the most common and, though observed in both ancient and modern collisional environments, are difficult to interpret due to both their limited extent and a corresponding lack of three-dimensional control. Sub-horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide. Of special note are the various reports of specific reflections at 80-100 km (22-24 s twt) in the North Sea, eastern Europe, and the Urals which suggest a more ubiquitous mantle horizon. The deepest events imaged thus far in a controlled source CMP survey are the 'super-deep' mantle reflections of the URSEIS project that have been identified at 135-165 km (35-43 s) and 225 km (56 s). Of the many reported mantle reflections, these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo-mechanical transition should be reflective at high frequencies (10-20 Hz).

Elsevier online abstract

Some recent developments in the acquisition and processing of seismic data

Pages 123-142
Manik Talwani and Barry Zelt

Industry-style 3-D surveys are different in the volume of data, processing techniques and expected results from 'academic' 3-D surveys. Industry has developed new acquisition techniques for marine 3-D surveys: multi-streamer seismic ships, vertical cables, and dual-sensor bottom cables. As opposed to 2-D post-stack time migration, new techniques involving 2-D and 3-D pre-stack depth migration are becoming important. Examples are given. Migration of wide-angle data to obtain subsurface imaging is discussed. Attribute analyses, including the use of wavelets to detect azimuthal anisotropy and lithologic contrasts are described. The use of the recent developments in wavelets in seismic analysis is introduced.

Elsevier online abstract

AVO analysis by offset-limited prestack migrations of crustal seismic data

Pages 143-153
M. Simon

The physical causes of reflections from the crystalline crust are still a matter of debate. The polarity of the wavelet and the amplitude-versus-offset (AVO) dependence of seismic reflection data can help to reveal the nature of a reflector. In contrast to seismic data from sedimentary environments, however, it is very difficult to get polarity and AVO information from crystalline basement data. Reflections in prestack data are typically weak, incoherent and often obscured by numerous diffractions. There is, however, a better chance of obtaining reliable AVO information from prestack-migrated data after true-amplitude processing, as the migration process unravels the jumble of reflections and diffractions and improves the signal/noise ratio. Important steps in this processing sequence are careful editing followed by surface-consistent amplitude corrections for source and receiver coupling to the ground. Migration-velocity models can be obtained from refraction seismics and/or from migration-velocity analyses. The migration itself is of Kirchhoff-type and applied to COF gathers (COF = Common OFfset) to get one migrated section for each COF. These COF-migrated images can then be used to get AVO functions for specific reflections. Examples from the DEKORP near-vertical reflection line KTB8502 show some strong reflections, from beneath a granite body, at about 9 to 11 km depth. Both the positive and negative reflection coefficients of the reflectors are consistent with an interval velocity model obtained from migration-velocity analyses. AVO functions from these reflections include offsets from zero to 12 km and reflection angles from zero to about 30°. Absolute reflection coefficients are surprisingly small with maximum values of 0.015. Two-layer modelling has been done to match the AVO dependence of these reflections.

Elsevier online abstract

Attenuation of near-surface diffracted energy in deep seismic data by DMO correction

Pages 155-159
Lone Klinkby and Morten Wendell Pedersen

Seismic data are often contaminated by scattered waves from shallow diffractors such as offshore installations and structural irregularities. As the waves travel in the water layer and shallow sub-bottom they are damped considerably less than near-vertical reflected waves. Far from the diffractors the stacking velocity of the noise will be nearly identical to the stacking velocities of the primary reflections. This implies that CMP stacking of normal-moveout corrected data does not suppress the noise, and the necessary attenuation will typically be done separately by prestack 2D velocity-filtering or array simulation in the shot and receiver domains. However, by changing the moveout of the diffraction curves through DMO correction, the stacking velocity of the noise will be close to the true velocity of the diffracted waves, and suppression through CMP stacking is possible. As DMO is related to CDP smearing, which is a marginal problem for deep seismic data, it is normally not used as a part of the standard processing schemes. The noise suppression features of the DMO processor are demonstrated with a data example from the North Sea.

Elsevier online abstract

Effects of the shallow subsurface on upper crustal seismic reflection images

Pages 161-169
K. Holliger and J. O. A. Robertsson

Seismic imaging of the upper crust is strongly affected by the interactions of the seismic wavefield with the shallow subsurface. In order to better understand these effects, we have evaluated the seismic responses of a suite of canonical models of the upper crystalline crust and the near-surface region. Our modelling is restricted to two dimensions, but otherwise includes the full seismic wavefield, notably the effects of attenuation and topographic variations along the free-surface. We find that S-wave scattering is important in the upper crust and contributes significantly to the vertical component of the seismic reflection response. The backscattered wavefield undergoes mode conversions when it interacts with the free-surface. Even moderate topographic variations or velocity variations in the near-surface region enhance these mode conversions and cause additional scattering. Much of the Earth's surface is covered by thin layers of unconsolidated material or weathered bedrock with high attenuation and high velocity contrasts at the layer boundaries. Scattered and mode-converted seismic energy gets trapped in these layers and thus interacts repeatedly with the free-surface and its topography. In analogy to large-scale seismic resonance effects of sedimentary valley fills, the level and duration of source-generated noise depends heavily on the degree of attenuation in the shallow layers: low attenuation causes high noise levels and vice versa. In contrast, the level of source-generated noise is less sensitive to near-surface layer thicknesses and velocity contrasts.

Elsevier online abstract

Estimates of upper-crustal heterogeneity in the Baltic Shield from seismic scattering and borehole logs

Pages 171-183
C. E. R. Line, R. W. Hobbs and D. B. Snyder

Constraints on small-scale heterogeneity in the upper crust of the Baltic Shield were obtained from stochastic modelling of a variety of seismic data from the BABEL and Siljan Ring surveys. A non-linear least-squares inversion scheme was applied to wavefield fluctuations in seismic reflections to obtain statistical parameters of the medium through which the wave energy propagated. The reflections are from dolerite sills intruded at depths of 6-12 km into the Palaeoproterozoic upper crust of the central Baltic Shield. Analysis of the BABEL data set reveals that the upper crust is a weakly scattering stochastic medium, with velocities characterised by an exponential autocorrelation function with a correlation length of 150 ± 50 m and rms velocity perturbation of 1.5 ± 0.5%. Further inversions show that most of the scattering occurs within the upper few kilometres of the crust. Analysis of the Sijan seismic data set reveals heterogeneity with length scales of 210 ± 50 m and a velocity perturbation of about 1.0%. The autocovariance and power spectrum of the sonic log, and studies of frequency-dependent Q in VSP data, from the Gravberg-1 borehole, give consistent estimates of basement heterogeneity. The upper crust in this region is characterised by a correlation distance of 35 ± 5 m and rms velocity perturbation of 4-6% in the uppermost 1-2 km of crust, associated with near-surface fracturing, and a correlation distance of 140 ± 50 m and rms velocity perturbation of 1.5 ± 0.5% in the uppermost 6-8 km, associated with compositional heterogeneity.

Elsevier online abstract

Imaging of crustal structures from vertical array measurements

Pages 185-192
H. -P. Harjes, M. Janik, J. Müller and M. Bliznetsov

Walk-away or offset Vertical Seismic Profiling (VSP), a technique widely used in oil and gas exploration, was carried out in crystalline rock at the KTB (Kontinentales Tief-Bohrprogramm) drill site in Germany. The walk-away VSP experiment comprised a shot profile of 10 km length at the surface across the well and a vertical receiver array in the borehole between 3210 m and 3560 m depth with geophone intervals of 25 m. To identify reflected/refracted seismic energy from crustal structures the 'Controlled Directional Reception' (CDR) method was used. The key element is the vertical geophone array which allows the determination of the vertical slowness with great accuracy due to the excellent coherence of waveforms. Slant stack processing and subsequent directional migration produced a clear image of mid-crustal reflectors at 10 to 12 km depth. Additionally, a number of dipping faults were detected in the upper crust, that can be connected to surface geology. As the walk-away VSP experiment was part of an integrated seismic measurement program, the results were compared with those from 2D and 3D surface surveys and borehole measurements. This led us to conclude that hydraulic fracture zones play an important role in the reflectivity of crystalline rocks at KTB.

Elsevier online abstract

Prestack depth migration of dense wide-angle seismic data

Pages 193-208
B. C. Zelt, M. Talwani and C. A. Zelt

Prestack depth migration of wide-angle seismic data represents an extension of traditional imaging with near-vertical incidence data because it includes a larger component of the recorded wave field. To date, however, studies that have employed wide-angle migration have suffered because only widely spaced data were available and because only very simple synthetic tests were performed. Although wide-angle migration has the potential to increase our ability to image deep-crustal structures, particularly when closely spaced data are collected, a thorough study of this technique has been lacking. To address this, we present a case study of prestack depth migration of relatively dense synthetic wide-angle marine data. The objectives are to identify potential benefits and limitations of this approach and answer such fundamental questions as how close the receiver spacing must be for a typical survey to image effectively with wide-angle data. This will facilitate the design of better seismic experiments. Our study employs Kirchhoff prestack depth migration of variably spaced full wave-field synthetic wide-angle ocean-bottom hydrophone (OBH) data generated using a realistic velocity model based on the passive eastern margin of the United States. We show how an increase in OBH density improves the migration by increasing the lateral resolution and signal-to-noise ratio. We also investigate the contribution of various offset ranges to the migrated image and show how the wider-angle components contribute primarily to the deepest parts of the image with relatively low spatial frequency compared to the near-vertical incidence components. To investigate how errors in the velocity model affect imaging as a function of offset range we migrate the data using a velocity model derived from refraction and reflection traveltime inversion. This example demonstrates the need to obtain an increasingly accurate model as increasingly wider-angle data are migrated. To effectively image structures in our 200 × 40 km synthetic velocity model, an OBH spacing of approximately 2 km is required.

Elsevier online abstract

Study of out-of-plane effects in the inversion of refraction/wide-angle reflection traveltimes

Pages 209-221
C. A. Zelt and B. C. Zelt

In the presence of three-dimensional (3D) inhomogeneous structure, the results of 2D traveltime inversion will be in error since the effects of 'out-of-plane' or 3D ray sampling are ignored. We have inverted synthetic data using 2D and 3D algorithms to examine the errors caused by 3D inhomogeneities which produce significant out-of-plane ray bending. The results of inverting data from 2D experiments are compared with vertical slices through 3D models obtained by inverting data using a number of recently employed 3D recording geometries. Our results show that, even for strong 3D inhomogeneities, out-of-plane effects are relatively small, with crustal velocity errors of less than 0.15 km/s, and intra-crustal boundary depth errors generally less than 2 km. These errors are approximately equal to the uncertainties commonly assigned to crustal models derived from traveltime inversion. The artifacts are also similar in magnitude to the lateral smearing that occurs in 3D models when using relatively coarse 3D geometries. Only for a dense network of profiles will a 3D inversion using off- and in-line data provide greater lateral resolution than a 2D independent or simultaneous inversion of in-line data along each profile. 2D and 2.5D inversion of crooked-line data in the presence of strong velocity variations produces erroneous small-scale velocity structure. We conclude that most 3D crustal experiments cannot be justified on the basis that the results from a 2D experiment or a network of 2D profiles will be significantly in error due to out-of-plane effects. 3D experiments can be justified when a dense grid of shots and receivers is used or if a volume image, as opposed to a cross-sectional image, is required.

Elsevier online abstract

Crustal imaging in southern California using earthquake sequences

Pages 223-236
Sergio Chávez-Pérez and John N. Louie

An inexpensive means to further understand the geometry of active faults in southern California arises from the use of aftershock recordings to image crustal structures. The advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active-source seismic data. We show that it is feasible to image fault structure using three-dimensional, wide-angle prestack Kirchhoff migration. We achieve this with the use of aftershock traces recorded on the short-period vertical stations of the Southern California Seismic Network. This work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross-sections rather than having to associate events with certain faults. Further, it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths. We demonstrate the validity of these images as showing reflective structures, and the ability to use clipped high-gain seismograms as sign-bit data to yield valid geometric imaging. Work with data from the 1991 Sierra Madre earthquake sequence images the prominent lower-crustal reflective zone observed beneath most of the San Gabriel Mountains by the Los Angeles Region Seismic Experiment Line 1. Aftershocks of the 1994 Northridge earthquake allow us to image a north-dipping structure that may represent the fault plane of a crustal-penetrating blind thrust. The images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced-section reconstructions of shallow-crustal profiles and borehole data. Our results are more consistent with a thick-skinned tectonic regime in the vicinity of the Northridge earthquake, rather than a thin-skinned model.

Elsevier online abstract

Thin crust and active upper mantle beneath the Southern Sierra Nevada in the western United States

Pages 237-252
Stanley Ruppert, Moritz M. Fliedner and George Zandt

Analysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevada Continental Dynamics (SSCD) project resolves a small crustal root (40-42 km thick) centered 80 km west of the Sierran topographic crest. The SSCD experiment consisted of a west-east profile across the Sierra Nevada at approximately 36.5°N and a north-south profile extending the length of Owens Valley, located eastward of the Sierran topographic crest. Two-dimensional finite-difference travel-time inversion of Pg and Pn arrivals resolves upper-crustal velocities of 6.0 to 6.4 km s-1 within the Sierran Batholith and Basin and Range, an upper-crustal west-dipping wedge of higher velocities (6.8-7.2 km s-1) consistent with ophiolitic material underlying the Great Valley sedimentary sequence, and higher velocities in the lower crust beneath the Basin and Range (6.8-7.0 km s-1) than those beneath the Sierran Batholith (6.6 km s-1). Low average Pn velocities (7.6-7.9 km s-1) and a laminated transitional Moho imaged beneath the Sierran Batholith also differ from the higher Pn velocities (7.9-8.0 km s-1) and sharp first-order Moho observed beneath the Basin and Range. The crust decreases in thickness both westward of the root to 28-34 km beneath the Great Valley and eastward to 35 km beneath the highest Sierran topography and decreases further to 27-30 km beneath the Basin and Range. Crustal thickness also appears to increase southeast to northwest from 29-30 km beneath the Garlock Fault in the south to 38-40 km beneath the north end of Chalfant Valley. Juxtaposition of the crustal model with previous P-wave tomography models of the south-ernmost Sierra Nevada upper-mantle reveal that the thickest Sierran crust on the west-east profile overlies a pronounced upper-mantle high-velocity anomaly (+5%), whereas the region of laminated Moho overlies a flanking upper-mantle low-velocity region (-3%). The upper-mantle velocity anomalies, relatively low Pn, and relatively flat Moho, observed beneath the Sierran crest suggest that the recent uplift of the Sierra Nevada is due to asthenospheric flow and/or lithospheric thinning beneath the southeastern Sierra Nevada and Basin and Range.

Elsevier online abstract

Interpretation of crustal seismic velocities in the San Gabriel-Mojave region, southern California

Pages 253-271
Care L. McCaffree Pellerin and Nikolas I. Christensen

The increased concern over seismic hazards in the Los Angeles region has created interest in the crustal structure and mechanical properties associated with faults, like the Sierra Madre and San Andreas. This study presents the seismic properties of major lithologies found in the vicinity of the San Andreas fault zone in order to correlate with the Los Angeles Region Seismic Experiment (LARSE) results. Scientists from the LARSE group have recently acquired high-quality seismic data long three transects. Thirty-nine rock samples were collected along the LARSE Line 1, which trends NE-SW across the central Mojave Desert and the San Gabriel Mountains. These rock samples represent the following lithologies: Precambrian gneisses, San Gabriel anorthosite, Mesozoic granitic intrusives, Pelona schist. Mendenhall granulite gneisses and San Andreas fault zone cataclasites. The seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure. Average compressional and shear wave velocities at 150 MPa show a wide range of values. The Mendenhall gneiss and the San Gabriel anorthosite complex have high velocities, 6.5 km/s and 6.4 km/s, respectively. The Pelona schist, the Mojave granitic intrusives, and the San Andreas cataclasitic rocks have average velocities of 5.8 km/s, 5.2 km/s, and 5.1 km/s, respectively. Average densities range from 2320 kg/m3 for fault zone cataclasites to 3100 kg/m3 for a Mendenhall granulite gneiss. Gneisses and schists show significant seismic anisotropy (5-20%) due to preferred mineral orientation, while most other rocks are nearly isotropic. Poisson's ratios range from a high of 0.29 in the San Gabriel anorthosite to a low of 0.22 in the San Gabriel gneiss. Comparison of the measured seismic properties with the LARSE velocity model allows interpretation of crustal structure in terms of rock compositions. The upper crust southwest of the San Andreas fault contains granitic intrusives, gneisses and schists, while under the Mojave Desert, the upper crust is dominated by gneisses. High velocities under the San Gabriel Mountains represent either lower crustal granulites or anorthosite. The velocity decrease beneath the San Andreas fault zone originates from minor amounts of cataclasite or the presence of low-velocity lithologies such as Pelona schist.

Elsevier online abstract

Waves of the future: Superior inferences from collocated seismic and electromagnetic experiments

Pages 273-298
Alan G. Jones

The advent of high-quality seismological studies of the Earth's continental lithosphere has been paralleled by an explosion in both the quality and quantity of concomitant high-resolution electromagnetic studies. The latter were inspired by technological and intellectual advances during the last decade in the acquisition, processing, modelling and inversion of particularly natural-source magnetotelluric (MT) data. The complementary nature of seismics and MT leads to rejection of hypotheses that may be tenable if only one of them is applied. Equally, inferences supported by both have stronger conviction. Perhaps most useful is when apparent incompatibilities must be reconciled by re-examination of both datasets. This is demonstrated through examples of magnetotelluric and seismic reflection studies undertaken in the last decade in many tectonic environs, from Palaeoproterozoic collision zones to passive margins to active collision zones. Some aspects of MT are explained, particularly the method's sensitivity and resolution of geoelectric directionality and dimensionality. New directions are proposed whereby greater utility of the joint datasets can occur, both at the outset during data acquisition, and in the interpretation phase in modelling and inversion. Also, laboratory measurements of seismic, electrical and rheological properties of the same rock sample will make integrated interpretation more tenable.

Elsevier online abstract