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Estimates of the onset times of P phases from active source experiments can effectively be used in developing wavespeed models, and the large number of recordings typical of such experiments incentivizes the development of automated approaches to generate these estimates. The simplicity and repeatability of an airgun source such as that used in the 2016 Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source (PICTURES) project in northern Chile suggested that a straightforward application of waveform cross-correlation would suffice for arrivals recorded by a network of inland seismic stations, but did not work well due to significant variations in waveform morphology. Application of an alternative algorithm typically used in passive source investigations, the Regressive ESTimator (REST) autopicking package, also proved unsatisfactory, largely because the limited spectral bandwidth of the airgun source and the frequent occurrence of local seismicity led to numerous false picks. This motivated the development of a new approach, named CORREL, that is a hybrid of REST and cross-correlation, with the addition of a constraint in the form of a polynomial function based on the REST picks that provides a reasonable prediction of an onset time. Compared to the results obtained originally from REST and simple waveform correlation, the application of CORREL to the PICTURES data both significantly increased the number of arrivals detected and greatly reduced the number of outliers. The predictive polynomial also provides CORREL a better means to discriminate true shots from the abundant natural seismicity.

Double differencing of body-wave arrival times has proved to be a useful technique for increasing the resolution of earthquake locations and elastic wavespeed images, primarily because (1) differences in arrival times often can be determined with much greater precision than absolute onset times and (2) differencing reduces the effects of unknown, unmodeled, or otherwise unconstrained variables on the arrival times, at least to the extent that those effects are common to the observations in question. A disadvantage of double differencing is that the system of linearized equations that must be iteratively solved generally is much larger than the undifferenced set of equations, in terms of both the number of rows and the number of nonzero elements. In this article, a procedure based on demeaning subsets of the system of equations for hypocenters and wavespeeds that preserves the advantages of double differencing is described; it is significantly more efficient for both wavespeed-only tomography and joint hypocenter location-wavespeed tomography. Tests suggest that such demeaning is more efficient than double differencing for hypocenter location as well, despite double-differencing kernels having fewer nonzeros. When these subsets of the demeaned system are appropriately scaled and simplified estimates of observational uncertainty are used, the least-squares estimate of the perturbations to hypocenters and wavespeeds from demeaning are identical to those obtained by double differencing. This equivalence breaks down in the case of general, observation-specific weighting, but tests suggest that the resulting differences in least-squares estimates are likely to be inconsequential. Hence, demeaning offers clear advantages in efficiency and tractability over double differencing, particularly for wavespeed tomography.

Aftershocks of the 16 September 2015 M8.3 Illapel earthquake in central Chile were recorded for a period of one year by the Chile-Illapel Aftershock Experiment (CHILLAX) seismic network. An initial catalog of about 100,000 events was generated by a novel automated picking algorithm that combines an auto-regressive detection/onset estimation method with some recently developed windowing techniques. We combine arrival times of P and S waves from about 9,000 of the best recorded of these events with relative arrival times of teleseismic P waves and phase delays of Rayleigh waves recovered from ambient noise to generate a three-dimensional image of P and S wavespeeds in that part of the Andean margin located beneath the network. Hypocenters of aftershocks located in the final model appear to be grouped into four distinct zones: two lower parallel zones that dip about 20° to the east, an upper, highly active zone that dips about 30° to the east, and a diffuse zone of smaller events located between this upper zone and the surface. Nearly all of the events with M > 4 are located at the bottom edge of the highly active zone and define a plane dipping about 26° to the east. Most of these events have almost identical thrust mechanisms and are separated from the lower zones by a steep gradient in increasing wavespeeds. The lower end of the upper zone, at about 55 km depth, is marked by another velocity gradient dipping to the west that bounds the diffuse shallow activity and projects to the surface west of the high Andes. We postulate that the triangular region delimited by these two high wavespeed gradients is a subduction wedge, generated by the removal of material from the leading edge of the upper plate by subduction erosion, and subsequently underplated back on to the overriding plate at depth. This model explains many of the features we observe in the seismicity and wavespeeds and is consistent with geologic observations of uplift and extension in this part of the Chilean coastal margin. Characteristics of this wedge, such as its shape and the lack of correlation with the coeval back-arc tectonic setting, suggest that high Andes are not directly coupled with the behavior of the subduction interface.

We investigate the fate of the Indian lithosphere following its descent beneath western Tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011. We use a non-linear iterative algorithm that simultaneously models absolute, regional, and relative teleseismic arrival times to obtain a 3-D velocity structure in a spherical segment that extends from 26°N to 37°N, from 76°E to 89°E, and from the surface to 430 km depth. We find that variations in P and S wave speeds in the upper mantle are similar, and identify a number of prominent fast anomalies beneath western Tibet and the adjacent Himalayas. We associate these fast anomalies with the mantle lithosphere of India that is likely colder and hence faster than the ambient mantle. Resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300 km, and the lack of significant downward smearing of these features. We interpret the presence of faster material below 300 km as being consistent with former Indian lithosphere having reached these depths. There are two main fast anomalies in our model. One resembles a ∼100 km wide sub-vertical column located directly beneath the India-Asia plate boundary. The other anomaly is thinner, and has the shape of a dipping slab that spans the north-south width of the Lhasa block. It dips towards the NE, starting near the Indus-Yarlung suture and ending north of the Bangong-Nujiang Suture at depths in excess of 300 km. Another finding of our study is the absence of major fast anomalies west of ∼80°E, which our resolution tests show to be significant. Our results do not support the notion of a continuous body of formerly Indian lithosphere being presently underthrust northward, and extending all the way to the northern boundary of the plateau. Rather, shapes of fast anomalies in western Tibet suggest colder material beneath the northernmost Himalaya and the southernmost part of the plateau. The presence of multiple relatively small-scale anomalies, rather than a single large fast body aligned with the India-Asia boundary, rules out subduction of the intact Indian lithosphere. Our preferred scenario for the process that currently removes the mantle lithosphere of India from the collision zone envisages viscous behavior, with gravity-driven instabilities leading to the formation of multiple drips of different size. To reconcile our findings with evidence of past underthrusting of Indian lithosphere beneath Tibet we postulate a geologically recent change in the mode of lithosphere removal. Our scenario includes initial underthrusting, subsequent gravitational destabilization, and removal of the mantle lithosphere of Indian plate. At present the mantle lithosphere of India that continues to underthrust southern Tibet develops a drip-like instability that is seen as a fast columnar feature in our model.

We studied the seismic velocity structure beneath the Krafla central volcano, NE Iceland, by performing 3-D tomographic inversions of 1453 earthquakes recorded by a temporary local seismic network between 2009 and 2012. The seismicity is concentrated primarily around the Leirhnjukur geothermal field near the center of the Krafla caldera. To obtain robust velocity models, we incorporated active seismic data from previous surveys. The Krafla central volcano has a relatively complex velocity structure with higher P wave velocities (V_p) underneath regions of higher topographic relief and two distinct low-V_p anomalies beneath the Leirhnjukur geothermal field. The latter match well with two attenuating bodies inferred from S wave shadows during the Krafla rifting episode of 1974–1985. Within the Leirhnjukur geothermalreservoir, we resolved a shallow (−0.5 to 0.5 km below sea level; bsl) region with low-V_p/V_s values and a deeper (0.5–1.5 km bsl) high-V_p/V_s zone. We interpret the difference in the velocity ratios of the two zones to be caused by higher rock porosities and crack densities in the shallow region and lower porosities and crack densities in the deeper region. A strong low-V_p/V_s anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt.

We present new, high-resolution, shear velocity models for the western Himalayas and West Tibet from the joint inversion of P receiver functions recorded using seismic stations from four arrays in this region and fundamental mode Rayleigh wave group velocity maps from 5–70 s covering Central and Southern Asia. The Tibetan Plateau is a key locality in understanding large-scale continental dynamics. A large number of investigations has examined the structure and processes in eastern Tibet; however, western Tibet remains relatively understudied. Previous studies in this region indicate that the western part of the Tibetan Plateau is not a simple extension of the eastern part. The areas covered by these arrays include the Karakoram and Altan-Tagh faults, and major terrane boundaries in West Tibet and the Himalayas. The arrays used include broadband data collected by the West Tibet Array, a U.S.-China deployment on the western side of the Tibetan Plateau between 2007 and 2011. We use the shear wave velocity models to obtain estimates of Moho depth. The Moho is deep (68–84 km) throughout West Tibet. We do not observe significant steps within the Moho beneath West Tibet. A large step in Moho depth is observed at the Altyn-Tagh fault, where Moho depths are 20–30 km shallower to the north of the fault compared to those to the south. Beneath the Lhasa Terrane and Tethyan Himalayas, we observe a low-velocity zone in the midcrust. This feature is not interrupted by the Karakoram Fault, suggesting that the Karakoram Fault does not cut through the entire crust.

The joint inversion of different geophysical datasets is an effective means to reduce the non-uniqueness and improve the reliability of geophysical inversion. In this study, seismic arrival time and Bouguer gravity datasets are jointly inverted to obtain an image of 3-D velocity structures in the Taiwan orogen. The model obtained from joint inversion fits the arrival time observations at least as well as when inverted individually, and the gravity observations are much better fit when included in the inversion, implying a reduction in ambiguity by simultaneously modeling the disparate datasets. Moho depth variations estimated by the 3-D P wave velocity model suggest a maximum Moho depth of 56 km located beneath the Backbone Central Range, and the trend of the Moho is largely consistent with the topography of the Central Range with eastward, asymmetric crustal thickening. The root beneath the Central Range appears to be smaller in lateral extent than previously imaged, and the velocity gradients into the uppermost mantle are significantly higher. The lack of evidence for a significant amount of Eurasian crust in the mantle supports geodynamical models of accreted, rather than consumed, continental crust in a collisional environment.

We study the elastic wave speed structure of the crust and the uppermost mantle in western Tibet using P wave and S wave arrival times from regional earthquakes recorded by a temporary seismic network. We relocate the earthquakes, and subsequently invert travel time residuals for 3-D distributions of wave speed. Resolution tests with a variety of input structures are used to verify the reliability of our results. The crust beneath western Tibet has low P wave speed (5.9–6.3 km/s) throughout its nearly 80 km thickness, with lower values in this range concentrated within the Lhasa block. Beneath the Himalaya wave speeds are higher. Southern and western limits of the slow material beneath the Tibetan Plateau correlate with the Karakoram fault, and dip beneath the plateau at ∼40° angle. We find no evidence of a subhorizontal low velocity zone in the crust. In the uppermost mantle, we find a long and narrow region of fast (up to 8.4 km/s) P wave speed extending from the Karakoram fault in NE direction, and crossing the Bangong-Nujiang suture. In a north-south cross section, the distribution of relatively fast P wave speed suggests a ramp-flat geometry consistent with India underthrusting the Tibetan Plateau at least as far as 32.5°N. A plausible interpretation of the upper mantle fast feature is the formation of eclogite from the mafic lower-crustal material of India after it is underthrust beneath Tibet. Notably, in western Tibet this process only takes place in a narrow region.

Interstation Pg and Sg differential traveltime tomography is performed in the northeastern margin of the Tibetan plateau, providing reliable mid-lower crustal velocity images while minimizing errors from earthquake mislocation and origin time. Prominent low P and S velocities ( P and S ) in the northeastern Tibetan plateau are bound to the east by the Minjiang fault and the southwest segment of the Longmenshan fault zone (LMSFZ). In contrast, relatively high P and S velocities (>6.1 km/s for V P and >3.7 km/s for V S ) are found beneath the Sichuan basin and in the triangular region bound to the west by the Minjiang fault and to the east of the northeast segment of the LMSFZ. Significant low velocity anomalies suggest a weak mid-lower crust that flows beneath the eastern Tibetan plateau, while the high velocities beneath the Sichuan basin indicate a rigid, cold and stable crust. The strong lithospheric mantle beneath the Sichuan basin inferred from previous studies may act as a barrier to the eastward escape of crustal flow from the eastern Tibetan plateau. The segmentation of the LMSFZ is reconfirmed by the distinct mid-lower crustal velocities in the southwest and northeast segments. High velocity and low conductivity anomalies in the mid-lower crust beneath the eastern Minshan uplift and the western Qinling orogen suggest that no crustal flow reaches this area.

The geometry of the entire crust from the northern part of the Tarim Basin to the southwestern Tian Shan east of Kashi is imaged on a N-S–directed explosive-source deep seismic-reflection profile. The profile reflects the sedimentary formations in the northern part of the Tarim Basin and the fold-and-thrust belt of the southern Tian Shan. N-dipping reflectors of the lower crust, as well as fluctuations in Moho depth, below which several mantle reflectors were observed, reveal the fine crustal structure beneath the junction of the southwest Tian Shan and the Tarim Basin. Mesozoic–Cenozoic shortening of the southwestern Tian Shan occurred at a crustal scale involving detachment-related folding in the basin directed northward toward the mountains and reverse faulting in the mountains directed toward the basin. In addition, a crocodile fabric developed within the lower crust beneath the basin area. The lithospheric structure revealed by the seismic-reflection section between the Tarim Basin and the Tian Shan Mountains reflects a process of intracontinental collision.

The Tarim basin is distinct from the Tibetan plateau by the apparently low degree of internal deformation. Relative motion between the lithospheres of Tibet and Tarim likely results in coherent mantle deformation, with a diagnostic signature of anisotropic seismic wave speed. Birefringence (splitting) in core-refracted shear phases (SKS, PKS) observed along the Tibet–Tarim border is indicative of seismic anisotropy along their path. The mantle deformation direction inferred from shear wave splitting under the assumption of a single source of anisotropy is approximately E–W. The size of the splitting delays inferred in our analysis suggests that most of the anisotropic signature is coming from the mantle. Furthermore, our data suggest vertical stratification of rock fabric. Focal mechanisms of regional earthquakes, both published and derived from new data, provide evidence for ~ NNE–SSW compression within the Tarim basin north of the border with Tibet. This type of internal deformation in the lithosphere is consistent with its underthrusting (subduction?) beneath Tibet. Within the crust of the Tibetan plateau we find a more complex pattern suggestive of the ~ E–W extension, and generally consistent with the sense of motion on the Altyn Tagh fault that extends along the Tibet–Tarim boundary. Therefore, at the Tibet–Tarim border we find close similarity between deformation directions within the crust of the plateau and within the upper mantle on both sides of the boundary. A plausible explanation of such similarity would be the coupling of the crust and the upper mantle, with no weak zone being present in the lower crust. One scenario for such coupling would involve an extension of a deformation zone associated with the Altyn Tagh fault into the uppermost mantle, making this fault zone very similar to a plate boundary.

Tomographic inversion with interstation Pn differential traveltime data provides a reliable velocity image of the uppermost mantle across the Longmenshan Fault Belt, which borders the eastern Tibetan plateau and Sichuan Basin. Low Pn velocities ( 8.1 km/s) appear beneath the Sichuan Basin to the east, suggesting a relatively weak upper mantle beneath eastern Tibetan plateau and a strong upper mantle beneath the Sichuan Basin. The Pn velocities are consistent with the proposition that the rigid and stable lithosphere of the Sichuan Basin may act as a barrier to the eastward escape flow of the ductile crust of the eastern Tibetan plateau. Consistency of Pn velocities with the smoothed topography, crustal thickness and Bouguer gravity implies that the dynamic processes of the lithospheric mantle should be responsible for the crustal thickening and relief of uplift in the eastern Tibetan plateau. However, the Longmeshan Fault Belt appears to be separated into two segments by the Huya and Leidong faults as manifested by significant discrepancies in topography gradient, seismic activity, rupture process and aftershock distribution of Wenchuan earthquake. This segmentation is also supported by difference in Pn velocities along the strike of the fault belt, suggesting different tectonic environments in the crust and lithospheric mantle. Understanding of the mechanism for the segmentation requires further investigation.

SUMMARY We present a method for imaging quasi-vertically dipping faults with surface records of reflected P waves from small earthquakes. Faults are boundaries between geological structures, such as tectonic plates, and are located in earthquake active regions such as Parkfield, California. The high degree of seismic activity enables the use of multiple seismic recordings in our fault identification algorithm. Major challenges occur because of the quasi-vertical orientation of the fault and the fact that the wave reflected by the fault and recorded by the surface receivers is not well modelled by the direct arrival of the propagating wave generated by the earthquake source. Our method uses the 2-D acoustic wave equation as the model for P-wave propagation. We assume that an approximate wave speed map on the reflection side of the fault is available and the source locations are known, for example, from traveltime tomography. We also assume that the source time function is known. The new features of our method arise because earthquake sources are located very close to the fault. This has two implications: (1) the direct arrival and the reflected wave arrive almost simultaneously, so that it is impossible to separate them on a seismogram using standard techniques, and (2) most of the reflections occur above the critical angle which introduces a distortion in the reflected wave. To overcome these difficulties we use a modelled incident wave to (1) remove the direct arrival from the data, and (2) remove the post-critical distortion from the reflected wave. We justify the distortion removal using the leading-order term of an asymptotic expansion, and an optimization procedure. To complete our algorithm we utilize some features of reverse time migration: (1) the use of full acoustic wave equation for modelling and backpropagation, and (2) zero-lag correlation of the backpropagated time reversed reflected and incident fields. We present numerical examples of fault reconstructions with synthetic data.

SUMMARY We consider the problem of using scattered waves to recover an image of the medium in which the waves propagate. We address the case of scalar waves when the sources and receivers are sparse and irregularly spaced. Our approach is based on the single-scattering (Born) approximation and the generalized Radon transform. The key to handling sparse sources and receivers is the development of a data-weighting scheme that compensates for non-uniform sampling. To determine the appropriate weights, we formulate a criterion for measuring the optimality of the point-spread function, and solve the resulting optimization problem using regularized least squares. Once the weights are determined, they can be used to compute the point-spread function and thus determine resolution, and they can also be applied to the measured data to form an image. Tests of our minimization scheme with different regularization parameters show that, with appropriate weighting, individual scatterers can be resolved at subwavelength scales even when data is noisy and the locations of both sources and receivers are uncertain. We show an example in which the source–receiver geometry and frequency bandwidths correspond to seismic imaging from multiple local earthquakes (passive seismic imaging). The example shows that the weights determined by our method improve the resolution relative to reconstructions with constant weights.

The High Himalaya are one of the most spectacular consequences of the ongoing collision between India and Eurasia, yet the role that the lithosphere beneath them play in this collision is not well understood. To illuminate the influence of the Himalaya on collisional processes, we produced tomographic images of the subsurface beneath them from arrival times of P- and S-waves generated by 551 teleseisms and 1051 local/regional events and recorded by the Himalayan–Nepal–Tibet Broadband Seismic Experiment (HIMNT) from 2001 to 2002. Compared to those in initial 1D models, the RMS P- and S-arrival time misfits in our final 3D model were reduced by as much as 46% (from 0.45 s to 0.24 s) and 38% (from 0.97 s to 0.6 s), respectively. Our results show strong crustal variations perpendicular to, but no significant lateral variation along, the trend of the High Himalaya within our study area. In the NS direction the crust thickens from about 60 km under the Lesser Himalaya to 70–80 km under the Tethys Himalaya. There is no localized crustal thickening beneath the High Himalaya. Wavespeeds and seismicity patterns beneath the Tethys Himalaya suggest that the middle crust (at 30–50 km depth) is ductile and are consistent with the presence of a mid-crustal reflector associated with the stronger gradients in the lower crust. Correlation between relocated earthquakes and the Xainza-Dinggye rift indicates that the rift is not cutting through the crust at the present time. Shear wave splitting from core phases indicates that the upper mantle beneath the Tethys Himalaya is nearly isotropic. The combination of isotropy and higher seismic wavespeed leads us to suggest that eclogites included in the lowermost crust and the uppermost mantle are responsible for the mid-crustal reflector under the Tethys Himalaya, as well as for a slightly high wavespeed anomaly just beneath the Moho. This interpretation is supported by the existence of granulitic eclogite in the Ama Drime Massif ∼ 50 km northeast of Mt. Everest.

Using recently collected data from western Tibet we find significant variation in the strength, vertical distribution and attributes of seismic wave speed anisotropy, constrained through a joint application of teleseismic shear wave splitting techniques and a study of P-S mode-converted waves (receiver functions). We find that the crust of Tibet is characterized by anisotropy on the order of 5%–15% concentrated in layers 10–20 km in thickness, and with relatively steep (30°–45° from the vertical) slow symmetry axes of anisotropy. These layers contribute no more than 0.3 s to the birefringence in teleseismic shear waves, significantly smaller than splitting in many of the observations, and much smaller than birefringence predicted by models developed through group inversions of shear-wave recordings. Consequently, we interpret models constrained with shear-wave observations in terms of structures in the upper mantle. Near the Altyn–Tagh fault our data favor a two-layer model, with the upper layer fast polarization approximately aligned with the strike of the fault. Near the Karakorum fault our data are well fit with a single layer of relatively modest (~ 0.5 s delay) anisotropy. Fast polarization in this layer is ~ 60°NE, similar to that of the lower layer in the model for the Altyn Tagh fault site. Assuming that layers of similar anisotropic properties at these two sites reflect a common cause, our finding favors a scenario where Indian lithosphere under-thrusts a significant fraction of the plateau. Data from a site at the southern edge of the Tarim basin appear to be inconsistent with a common model of seismic anisotropy distribution. We suspect that thick sediments underlying the site significantly distort observed waveforms. Our ability to resolve features of anisotropic structure in the crust and the upper mantle of western Tibet is limited by the small amount of data collected in a 6 month observing period. We stress the importance of future teleseismic data collection in Tibet over extended periods, to insure better directional distribution of observed seismic sources.

To obtain an image of the deep structure of the Tien Shan in central Asia, we invert P and S receiver functions jointly for almost 40 local broad-band seismograph stations. The inversion is performed using a simulated annealing technique. The combined inversion is an improvement on earlier studies, where P and S receiver functions were inverted separately. Using this approach, we deal with structural imaging problems that are usually investigated with teleseismic body wave and surface wave tomography techniques. We demonstrate that the uppermost mantle in the north of the central Tien Shan is composed of a high-velocity lid a few tens of kilometers thick above a pronounced low-velocity zone. The crustal structure in this region provides evidence of magmatic underplating. These features are likely related to a small plume that is manifested by basaltic eruptions of Cretaceous–Paleogene age. The low-velocity layer is also found in a southeast trending corridor, which may correspond to the Bachu uplift in the Tarim basin. Crustal thickness beneath the orogen varies from about 45 to about 70 km. The smallest values, most likely inherited from the pre-orogenic era, are found in a neighborhood of the Talas–Fergana fault. Similar values are characteristic of the Kazakh shield in the north and the Tarim basin in the south. The largest values are found beneath the bounding ranges. We infer that uplift of the central Tien Shan is unlikely to be caused by crustal shortening alone.

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU. See also: http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml; http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

We present and discuss estimates of shear-wave splitting in the upper mantle of eastern Europe, based on SKS techniques. Estimates of the direction of polarization of the fast split wave α and the delay of the slow wave δt are obtained at seven stations: UZH, KOV and LVV, within and near the Carpathians, SIM, SEV and YAL in the Crimean peninsula, and KIEV in the Russian plate. Fast directions at all stations are around 130°, close to the values previously found in central Europe. In the Carpathians, similar values of δt are observed within the Pannonian depression, where the subcrustal lithosphere, if present, is very thin, and in the foredeep with a thick subcrustal lithosphere. This similarity suggests that the observed anisotropy is not inherited from earlier orogenies and preserved in the lithosphere. In the Crimea, the fast direction of anisotropy in the mantle is perpendicular to the strike of the Crimean mountains of late Jurassic-Cretaceous age, contrary to the expected alignment of the fast direction with the strike of a collisional belt. In general, the data are consistent with a present-day or recent large-scale mantle flow in central and eastern Europe, parallel to the Alpine belt.

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

The northeastern margin of the Tibetan Plateau, which includes the Qiangtang and Songpan-Ganzi terranes as well as the Kunlun Shan and the Qaidam Basin, continues to deform in response to the ongoing India–Eurasia collision. To test competing hypotheses concerning the mechanisms for this deformation, we assembled a high-quality data set of approximately 14 000 P- and 4000 S-wave arrival times from earthquakes at teleseismic distances from the International Deep Profiling of Tibet and the Himalaya, Phase IV broad-band seismometer deployments. We analyse these arrival times to determine tomographic images of P- and S-wave velocities in the upper mantle beneath this part of the plateau. To account for the effects of major heterogeneity in crustal and uppermost mantle wave velocities in Tibet, we use recent surface wave models to construct a starting model for our teleseismic body wave inversion. We compare the results from our model with those from simpler starting models, and find that while the reduction in residuals and results for deep structure are similar between models, the results for shallow structure are different. Checkerboard tests indicate that features of ∼125 km length scale are reliably imaged throughout the study region. Using synthetic tests, we show that the best recovery is below ∼300 km, and that broad variations in shallow structure can also be recovered. We also find that significant smearing can occur, especially at the edges of the model. We observe a shallow dipping seismically fast structure at depths of ∼140–240 km, which dies out gradually between 33°N and 35°N. Based on the lateral continuity of this structure (from the surface waves) we interpret it as Indian lithosphere. Alternatively, the entire area could be thickened by pure shear, or the northern part could be an underthrust Lhasa Terrane lithospheric slab with only the southern part from India. We see a deep fast wave velocity anomaly (below 300 km), that is consistent with receiver function observations of a thickened transition zone and could be a fragment of oceanic lithosphere. In NE Tibet, it appears to be disconnected from faster wave velocities above (i.e. it is not downwelling or subducting here). Our models corroborate results of previous work which imaged a relatively slow wave velocity region below the Kunlun Shan and northern Songpan-Ganzi Terrane, which is difficult to reconcile with the hypothesis of southward-directed continental subduction at the northern margin. Wave velocities in the shallow mantle beneath the Qaidam Basin are faster than normal, and more so in the east than the west.

We carry out a joint inversion of surface wave dispersion curves and teleseismic shear wave arrival times across the Tibetan collision zone, from just south of the Himalaya to the Qaidam Basin at the northeastern margin of the plateau, and from the surface to 600 km depth. The surface wave data consist of Rayleigh-wave group dispersion curves, mainly in the period range from 10 to 70 s, with a maximum of 2877 source–receiver pairs. The body wave data consist of more than 8000 S-wave arrival times recorded from 356 telesesmic events. The tomographic images show a ‘wedge’ of fast seismic velocities beneath central Tibet that starts underneath the Himalaya and reaches as far as the Bangong–Nujiang Suture (BNS). In our preferred interpretation, in central Tibet the Indian lithosphere underthrusts the plateau to approximately the BNS, and then subducts steeply. Further east, Indian lithosphere appears to be subducting at an angle of ∼45°. We see fast seismic velocities under much of the plateau, as far as the BNS in central Tibet, and as far as the Xiangshuihe-Xiaojiang Fault in the east. At 150 km depth, the fast region is broken by an area ∼300 km wide that stretches from the northern edge of central Tibet southeastwards as far as the Himalaya. We suggest that this gap, which has been observed previously by other investigators, represents the northernmost edge of the Indian lithosphere, and is a consequence of the steepening of the subduction zone from central to eastern Tibet. This also implies that the fast velocities in the northeast have a different origin, and are likely to be caused by lithospheric thickening or small-scale subduction of Asian lithosphere. Slow velocities observed to the south of the Qaidam suggest that the basin is not subducting. Finally, we interpret fast velocities below 400 km as subducted material from an earlier stage of the collision that has stalled in the transition zone. Its position to the south of the present subduction is likely to be due to the relative motion of India to the northeast.

—The three-dimensional P-wave velocity structure of the Bear Valley region of central California is determined by applying a circular ray-tracing technique to 1735 P-wave arrivals from 108 locally recorded earthquakes. Comparison of the results obtained from one-dimensional and laterally varying starting models shows that many of the features in the structure determined are fairly insensitive to the choice of the starting model. Velocities associated with the Gabilan granites southwest of the San Andreas Fault are slightly higher than those in the Franciscan formation to the northeast, and these two features are separated in the southern part of the region by a narrow fault zone with very low velocities. In the southeastern part of the region, where the Gabilan granites do not abut the San Andreas Fault, the low velocities of the fault zone cross over to the southwestern side of the fault. They also appear to extend to depths of at least 15km, thus locally reversing the contrast across the San Andreas Fault that prevails farther to the northwest. In the northwestern part of the region, the low velocities of the fault zone split and follow the surface traces of the San Andreas and Calaveras Faults, but do not appear to extend to depths much deeper than about 6km. There also appears to be a well-defined contrast in structure in the middle of the Santa Clara Valley, suggesting the existence of a fault in the basement of the valley that may be a southern extension of the Sargent Fault into this region. Relocated hypocenters beneath the San Andreas Fault cluster in a zone that dips about 80° southwest and intersects the surface trace of the fault in the southern part of region.

A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process.

We analyzed P-SV converted phases recorded by the Kyrgyzstan Broadband Network (KNET) to investigate the nature of phase transitions in the upper mantle beneath the central Tien Shan and the Kazakh shield. We find that P-SV phases from the 410 km discontinuity recorded by several stations located along the range front are about 2 s earlier from beneath the Tien Shan than they are from beneath the Kazach shield. Because the delay times at stations located on the Kazakh shield are normal and seismic velocities in the upper mantle beneath this part of the Tien Shan apparently are low (about 4% less than the Kazakh shield to the north), the early arrival of the conversion from 410 km implies that this discontinuity is elevated by about 20 km. Because the 410 km discontinuity can be elevated by the introduction of cold material, our results are consistent with a model in which the lithosphere in this area developed a root during collision that later detached and is now residing near 410 km depth.

[1] The convergence of the Philippine Sea and the Eurasian plates in the Taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities. The geological processes underlying the collision and its relation to subduction are the primary targets of TAIGER (Taiwan Integrated Geodynamic Research) project. Newly acquired passive and active sources data on land, supplemented by ocean bottom as well as permanent seismic network data, are used to derive a new 3-D tomographic velocity model. Using the 7.5 km/sec contour as a marker for crustal deformation, two trend-parallel “roots,” one under the Central Range with a maximum depth of 55 km and the other under the Coastal Range at 40 km, are found to extend from southern to central (∼24°N) Taiwan. Between the two roots and lying approximately beneath the Longitudinal Valley, the 7.5 km/sec contour rises to 25 km depth. In the upper mantle, a high velocity zone, east-dipping and its upper surface coinciding with a Wadati-Benioff zone in the south (∼22.8°N), becomes near vertical and ill-defined in the north (∼23.9°N). The crustal deformation as defined by the “roots” and the along-trend variations of the upper mantle structures under Taiwan provide key information for the orogeny. With the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play.

[1] The 27 February 2010 Maule, Chile (Mw=8.8) earthquake is one of the best instrumentally observed subduction zone megathrust events. Here we present locations, magnitudes and cumulative equivalent moment of the first ∼2 months of aftershocks, recorded on a temporary network deployed within 2 weeks of the occurrence of the mainshock. Using automatically-determined onset times and a back projection approach for event association, we are able to detect over 30,000 events in the time period analyzed. To further increase the location accuracy, we systematically searched for potential S-wave arrivals and events were located in a regional 2D velocity model. Additionally, we calculated regional moment tensors to gain insight into the deformation history of the aftershock sequence. We find that the aftershock seismicity is concentrated between 40 and 140 km distance from the trench over a depth range of 10 to 35 km. Focal mechanisms indicate a predominance of thrust faulting, with occasional normal faulting events. Increased activity is seen in the outer-rise region of the Nazca plate, predominantly in the northern part of the rupture area. Further down-dip, a second band of clustered seismicity, showing mainly thrust motion, is located at depths of 40–45 km. By comparing recent published mainshock source inversions with our aftershock distribution, we discriminate slip models based on the assumption that aftershocks occur in areas of rapid transition between high and low slip, surrounding high-slip regions of the mainshock.

SUMMARY 2-D P -wave velocity models were determined for the subduction zones near Iquique and Antofagasta in northern Chile, simultaneously inverting P - and S-wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations. A 2-D parametrization was used because of the paucity of data, but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3-D inversions. The crust and upper mantle are parameterized by constant velocity regions of irregular shape, with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve P-wave velocities. Beneath the Antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting Nazca plate. This crust has a P-wave velocity of 7.3 f 0.1 km s-' and is observed down to a depth of 60 f 10 km. This depth also corresponds to the maximum depth of seismogenic coupling in the Chilean subduction zone. The subducted oceanic crust overlies an oceanic upper mantle with a P-wave velocity of 8.0 f 0.1 km s-'. Apparently, oceanic crust is being subducted beneath Iquique as well. However, this feature is less constrained by the data available from this region.

Previous seismological studies have placed the source of the uplift of the Sierra either within the crust, suggesting a Mesozoic age for the source of the uplift, or in the upper mantle, consistent with late Cenozoic creation of the buoyant material producing the uplift of the range. We deployed 16 temporary seismometers in the high part of the southern Sierra Nevada to augment the permanent Southern California Seismic Network and record arrivals from regional and teleseismic earthquakes. Arrival times of P waves from 54 teleseisms recorded at these stations are advanced by over a second by a high-velocity body in the upper mantle west and northwest of Lake Isabella. Inversion of the arrival times indicates that this “Isabella anomaly” is of limited north-south extent (about 40–60 km), has compressional velocities about 4–5% higher than its surroundings, and probably extends from about 100 to 200 km depth. The limited north-south extent of the “Isabella anomaly” indicates that it is unrelated to the Sierra; we speculate that it is the downgoing part of a small scale convection system similar to that inferred beneath Southern California. This inversion does not clearly reveal either a large crustal root or a substantial low-velocity body in the upper mantle beneath the Sierra. Although the presence of either degrades the fit to the arrival times and requires high-velocity material beneath the low-velocity material of either root, Bouguer gravity anomalies require low-density material under the Sierra. Assuming that arrival times from earthquakes 150–350 km north and south from the southern Sierra come from a common refractor (the one-layer structure), the upper mantle P wave velocity (P_n) beneath the High Sierra is about 7.6–7.65 km/s; if the arrivals from north and south are from different refractors (the two-layer structure), material with a P wave velocity greater than ∼7.2 km/s (the “7 J.x” layer) would lie under a nearly flat interface from more normal crustal velocities and be separated by a north-dipping interface from underlying mantle with velocities about 7.9–8.1 km/s. The P_n velocity beneath the region immediately to the east is significantly greater (7.9–8.0 km/s) than that of material at equal depths under the Sierra. For the one-layer structure, further assuming that mean crustal velocities are uniform along north-south lines, we find little dip on the Moho in the area; using the arrival times from earthquakes to the south, we infer a depth of 33±5 km for the Moho beneath the southern High Sierra. This structure of a thin to normal crust over a low-velocity mantle can be reconciled with earlier observations that were used to infer a thick crust under the Sierra. By considering the Bouguer gravity anomaly, the surface geology, refraction profiles in this region, and our own observations, we suggest that 1/3 to 1/2 of the modern elevation in the range is supported by lateral (east-west) density contrasts in the crust; the remainder is supported by density contrasts in the uppermost mantle or lateral variations in the thickness of the “7.x” layer. Our interpretation is that the southern Sierra overlies mantle lithosphere that has been thinned and warmed in response to regional lithospheric extension in Neogene time. This part of the upper mantle might have provided the melt that migrated to the east and produced volcanics in the southwestern Great Basin; depletion of the upper mantle might have increased the seismic velocity and decreased the density of material about 60–100 km beneath the southern Sierra.

[1] We report a sequence of crustal quakes that began after the Mw = 8.8 thrust-subduction Maule earthquake that affected the Central Chile margin on 27 February 2010. This activity lasted by several months, having the most important events on 11 March 2010 (Mw = 6.9 and Mw = 7.0) with normal focal mechanisms. Seismicity shows a rupture oriented along a NW-striking and SW-dipping normal fault from the surface down to the interplate contact. Seismicity can be correlated with neotectonics extensional structures similarly oriented in the region, which have coexisted with NNE-SSW reverse faults since the late Pliocene, even though both have older periods of activity since the Paleozoic. This crustal rupture would have been triggered by the high Coulomb stress change produced by the Maule earthquake, enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high Vp/Vs ratio. The occurrence of relevant neotectonic activity in coincidence with short-term deformation suggests a relationship with long-term tectonic features of this region, which would have been acting as a barrier during the interseismic period, increasing the strain accumulation and triggering contractional faulting in the crust, as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting. Crustal faulting in Pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the Maule earthquake.

[1] Reviewing data from regional permanent seismic stations and a 2001–2002 deployment of portable broadband seismic stations along the Tibet-Tarim border, we identified five unusually deep earthquakes located beneath the West Kunlun range. Application of regional centroid moment tensor analysis puts these events at depths of 40–60 km, with overall uncertainty ranging between 5–8 km. Previous investigations of the region indicate a Moho depth of ∼62 km, implying that these earthquakes occurred near the bottom of the crust. Hence we infer that the lowermost crust beneath the range is brittle and seismically active. Two of these earthquakes are located directly beneath the trace of the Altyn-Tagh fault, with a nodal plane nearly parallel to the local strike of the fault, and a left-lateral sense of slip consistent with the long-term displacement across the fault. The two events suggest that the Altyn-Tagh fault extends through the entire crust in the West Kunlun.

We determined the structure of the crust beneath the central and western Tien Shan by analyzing broadband (1–20 s) analog seismograms of converted P-SV phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations. The one-dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the Talasso-Fergana fault. Specifically, the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault. Also, velocities at depths between 10 and 35 km are about 10% lower in the east, although the depth range of these lower velocities is not well resolved. The Talasso-Fergana fault is an important boundary for other observables; in addition to the structural discontinuities observed at the surface, the areas east of the Talasso-Fergana fault are associated with abnormally low mantle velocities, outcrops of basalt, low Q, and short-wavelength variations in anisotropy. Integrating these observations, we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust. Likewise, the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids. The gross contrast in structure east and west of the Talasso-Fergana fault could reflect a contrast in the dynamics of mountain building in these two regions. We postulate that while crustal shortening is the dominant mechanism controlling topography west of the Talasso-Fergana fault, vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault.

After a long quiet period for earthquake activity with magnitude greater than 8.5, several great subduction megathrust earthquakes occurred during the past decade: Sumatra in 2004 and 2005, Chile in 2010, and Japan in 2011. Each of these events caused loss of life and damage to critical infrastructure on an enormous scale. And, in April, a Mw 8.2 earthquake occurred off the Chilean coast.

Recordings in western Tibet of Rayleigh and Love waves at periods less than 70 s from aftershocks of the 2008 Sichuan earthquake cannot be matched by an isotropic velocity model beneath Tibet. These intermediate‐period Rayleigh and Love waves require marked radial anisotropy in the middle crust of Tibet, with the vertically polarized S‐waves propagating more slowly than S‐waves with horizontal polarization. The magnitude of anisotropy inferred using paths entirely within Tibet is slightly greater than that obtained previously from a tomographic inversion of a dataset covering a larger region. Anisotropy in the middle crust likely reflects deformation of the middle crust, and is consistent with the notion of mid‐crustal flow and thinning of the crust.

SUMMARY We adapt a 2-D spectral domain finite difference waveform tomography algorithm previously used in active source seismological imaging to the case of a plane wave propagating through a 2.5-D viscoelastic medium in order to recover P and S wave speed variations from body waves recorded at teleseismic distances. A transferable efficacy that permits recovery of arbitrarily heterogenous models on moderately sized computers provides the primary motivation for choosing this algorithm. Synthetic waveforms can be generated either by specifying an analytic solution for a background plane wave in a 1-D model and solving for the source distribution that would produce it, or by solving for a scattered field excited by a plane wave source and then adding the background wavefield to it. Because the former approach typically involves a concentration of sources at the free surface, the latter tends to be more stable numerically. We adapt a gradient approach to solve the inverse problem to maintain tractability; calculating the gradient does not require much more computational effort than does the forward problem. The waveform tomography algorithm can be applied in a straightforward way to perform receiver function migration and traveltime inversion.

We have adapted a 2D spectral domain finite difference waveform tomography algorithm previously used in active source seismological imaging to the case of a plane wave propagating through a 2.5D viscoelastic medium in order to recover P and S wavespeed variations from body waves recorded at teleseismic distances. A transferable efficacy that permits recovery of arbitrarily heterogeneous models on moderately sized computers provides the primary motivation for choosing this algorithm. Synthetic waveforms can be generated either by specifying an analytic solution for a background plane wave in a 1D model and solving for the source distribution that would produce it, or by solving for a scattered field excited by a plane wave source and then adding the background wavefield to it. Because the former approach typically involves a concentration of sources at the free surface, the latter tends to be more stable numerically. We adapt a gradient approach to solve the inverse problem to maintain tractability; calculating the gradient does not require much more computational effort than does the forward problem. The waveform tomography algorithm can be applied in a straightforward way to perform receiver function migration and travel time inversion. We will discuss an application of this technique to imaging the crust and upper mantle beneath the Tien Shan range in central Asia.

THE deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt, but this deformation is difficult to observe. Seismic azimuthal anisotropy manifested by shear-wave splitting provides the best geophysical evidence of deformation in the upper mantle. Here we use observations of shear-wave splitting to investigate deformation in the upper mantle beneath the Tien Shan in Central Asia. We find that underneath most of the Tien Shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt, but it deviates by nearly 90° where the upper mantle is anomalously hot. The former relationship, previously observed in other collisional belts, seems to be usual for this environment. The anomalous directions, however, provide evidence of a rising plume beneath the range, suggesting more generally that the along-strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection.

P-wave residuals recorded at stations in Nepal from events to the northwest and closer than about 20 deg are consistently earlier than those from other directions by about 2.5 sec. These early arrivals are associated with paths confined to the upper 300 km of the earth and suggest that cold material occupies the uppermost mantle beneath the Karakorum, northwest Himalaya, and western Kunlun. Thus, these data suggest that convective downwelling occurs more vigorously in this region than beneath the rest of the Himalaya, Tibet, and their surroundings.

We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper-mantle velocities in the New Guinea arc-continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well-recorded earthquakes were chosen for use in three-dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre-Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well-defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125- to 250-km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate-depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100-km-deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south-dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate-depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well-located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125- and 175-km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one-dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35- to 171-km km depth beneath the Bismarck Sea back-arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north-dipping slabs beneath both northeast New Guinea and New Britain.

Well-constrained hypocenters (latitude, longitude, depth, and origin time) are required for nearly all studies that use earthquake data. We have examined the theoretical basis behind some of the widely accepted “rules of thumb” for obtaining accurate hypocenter estimates that pertain to the use of S phases and illustrate, in a variety of ways, why and when these “rules” are applicable. Results of experiments done for this study show that epicentral estimates (latitude and longitude) are typically far more robust with respect to data inadequacies; therefore, only examples illustrating the relationship between S phase arrival time data and focal depth and origin time estimates are presented. Most methods used to determine earthquake hypocenters are based on iterative, linearized, least-squares algorithms. Standard errors associated with hypocenter parameters are calculated assuming the data errors may be correctly described by a Gaussian distribution. We examine the influence of S-phase arrival time data on such algorithms by using the program HYPOINVERSE with synthetic datasets. Least-squares hypocenter determination algorithms have several shortcomings: solutions may be highly dependent on starting hypocenters, linearization and the assumption that data errors follow a Gaussian distribution may not be appropriate, and depth/origin time trade-offs are not readily apparent. These shortcomings can lead to biased hypocenter estimates and standard errors that do not always represent the true error. To illustrate the constraint provided by S-phase data on hypocenters determined without some of these potential problems, we also show examples of hypocenter estimates derived using a probabilistic approach that does not require linearization. We conclude that a correctly timed S phase recorded within about 1.4 focal depth's distance from the epicenter can be a powerful constraint on focal depth. Furthermore, we demonstrate that even a single incorrectly timed S phase can result in depth estimates and associated measures of uncertainty that are significantly incorrect.

[1] We developed a three-dimensional (3D) shear-wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shear-wave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks.

[1] Using arrival data from GHENGIS and Kyrgyz Network (KNET) stations, we determined the uppermost mantle structure of the central Tien Shan from Pn tomography. Pn velocities in the north-central Tien Shan are slower than normal for stable lithosphere. They are very slow in the south-central Tien Shan, where a hot mantle previously has been postulated. The Pn velocity variation correlates well with seismicity. Most earthquakes are concentrated in the north-central Tien Shan, but almost no earthquakes in the south-central Tien Shan, even though both geologic and GPS observations show active convergence there. This correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle. Specifically, we propose that the very slow Pn velocity beneath the south-central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has conducted well up into the crust, inducing a ductile rheology that prevents significant seismicity.

[1] Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best-fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault.