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Strain in magmatic rifts is accommodated by both faulting and dike intrusion, but little is known of the frequency of dike intrusions in early-stage rifts. We use a new earthquake data set from a dense temporary seismic array (2013–2014) in the ~7-Myr-old Magadi-Natron-Manyara section of the East African Rift, which includes the carbonatitic Oldoinyo Lengai volcano that erupted explosively in 2007–2008. Full moment tensor analyses were performed on M>3.4 earthquakes (0.03- to 0.10-Hz band) that occurred during the intereruptive cycle. We find two opening crack-type and various non-double-couple earthquake source mechanisms and interpret these as fluid-involved fault rupture. From waveform analysis on the nearest permanent seismic station, we conclude that similar rupture processes probably occur over eruptive and intereruptive cycles. The repeated and dynamically similar fluid-involved seismicity, along with intrabasinal localization of active deformation, suggests that significant and persistent strain is accommodated by magmatic processes, modulated by tectonic cycles.

We employ an empirical approach to study the phenomenon of surface wave mode conversion due to lateral heterogeneity, and, as an example, assess its impact on a specific waveform inversion methodology used for surface wave tomography. Finite difference modelling in 2-D media, using a method that allows modelling of a single surface wave mode at a time, is combined with frequency domain decomposition of the wavefield onto a basis of local mode eigenfunctions, to illuminate mode conversion as a function of frequency and heterogeneity parameters. Synthetic waveforms generated by the modelling are inverted to study the effects of mode conversion on the inversion process. For heterogeneities in the upper mantle depth range of ∼40–300 km, we find that heterogeneity strengths of about 5 per cent (with sharp lateral boundaries), or lateral boundary length scales of 10–15 times the seismic wavelength (with 10 per cent maximum strength) produce significant mode conversion at periods of 30 s and shorter. These are significant in the sense that, depending on source strength, converted mode amplitudes can be well above typical noise levels in seismology. Correspondingly, waveform inversion with higher modes reveals the inadequacy of the path average approximation at these periods, with the potential for errors as large as 7 per cent in inferred group velocities, which will translate into errors in the inverted shear-velocity structure.

The Deep Fault Drilling Project DFDP) on the central Alpine Fault, South Island, NewZealand, has motivated a broad range of geophysical and geological studies intended to characterize the fault system in the locality of the drill site at various scales. In order to better understand the structural features of the central Alpine Fault, we have developed 3-D P-and S-wave velocity VP and VS) models of the region by double-difference tomography using data sets from multiple seismic networks. In previous work, the quality of the S-wave model has been poor due to the small number of available S-wave picks. We have utilized a new high-accuracy automatic S-wave picker to increase the number of usable S-wave arrivals by more than a factor of two, thereby substantially improving the VS model. Compared to previous studies, our new higher-resolution VP model based on more observations shows a clear VP contrast higher VP on the southeast hanging wall side) at depths of 5-10 km near the DFDP drill sites. With our better resolved VS model, in the same region, we detect a sharply defined high VS body VS > 3.7 km s(-1)) within the hanging wall. Our earthquake relocations reveal the presence of clusters within and around low-velocity zones in the hanging wall southeast of the Alpine Fault. Together with the improved earthquake locations, the P-and S-wave tomography results reveal the Alpine Fault to be marked by a velocity contrast throughout most of the study region. The fault dips southeastwards at about 50. from 5 to 15 km depth, as inferred from the velocity structure, seismicity and observations of fault zone guided waves.

The magmatic plumbing system beneath Askja, a volcano in the central Icelandic highlands, is imaged using local earthquake tomography. We use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the P wave velocity (Vp) and the Vp/Vs ratio. Extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is ~4 km in the upper crust (depth

Observations of travel time anomalies of inner core-sensitive PKPdf seismic body waves, as a function of path orientation with respect to the earth's rotation axis, have been interpreted as evidence of anisotropy in the inner core. Paths from earthquakes in the South Sandwich Islands to stations in Alaska show strongly anomalous travel times, with a large spread that is not compatible with simple models of anisotropy. Here we assess the impact of strong velocity heterogeneity under Alaska on the travel times, directions of arrival and amplitudes of PKPdf. We use 3D ray-tracing and 2.5D waveform modelling through a new, high-resolution tomography model of the upper mantle beneath Alaska. We find that the structure beneath Alaska, notably the subducting slab, is reflected in the patterns of these PKPdf observations, and this can be replicated by our model. We also find similar patterns in observed teleseismic P waves that can likewise be explained by our slab model. We conclude that at least 2 s of the travel time anomaly often attributed to inner core anisotropy is due to slab effects in the upper mantle beneath Alaska.

[1] We use P wave and S wave arrivals from microseismic earthquakes to construct 3-D tomographic Vp and Vs images of the magma storage region beneath Askja's central volcano in the Northern Volcanic Zone of Iceland. A distinctive ellipsoidal low-velocity anomaly, with both Vp and Vsvelocities 8-12% below the background, is imaged at 6-11 km depth beneath the caldera. The presence of a shallow magma chamber is corroborated by geodetic and gravity studies. The small Vp/Vs anomaly suggests a lack of pervasive melt. We interpret this anomaly as a region of multiple sills, some frozen but hot, others containing partial melt. A second, smaller low-velocity anomaly beneath the main magma storage region may represent a magma migration pathway. This interpretation is supported by the close proximity to the anomaly of clusters of deep, magmatically induced earthquakes. However, the location and shape of this deep anomaly are poorly constrained by the current data set.

A global optimizing approach is developed and implemented to retrieve one-dimensional crustal structure by waveform inversion of teleseismic receiver functions. The global optimization for the inversion is performed using a Differential Evolution (DE) algorithm. This modeling approach allows the user to perturb, within a preset range of reasonable bounds, multiple parameters such as V"p, V"p/V"s, thickness and anisotropy of each layer to fit the receiver function waveforms. Compared with linear modeling methods, the global optimal solution can be achieved with fewer model parameters (e.g., a small number of layers) and hence eliminate potential artifacts in the final model. Receiver function bins with small ray parameter intervals are used in the inversion, which can reduce distortion caused by modeling a single receiver function stacked from many recordings spread over a wide range of epicenter distance. The efficacy of this global optimizing approach is demonstrated with synthetic datasets and real receiver functions from the permanent seismic station BJT.

We combine teleseismic P arrival times from the recent MANAS deployment of broadband sensors with P and S arrival times from local events recorded by the GENGHIS deployment and analog observations from the Kyrgyz Institute of Seismology to generate a high resolution (~ 20 km) image of elastic wavespeeds in the crust and upper mantle beneath the western Tien Shan. The total data set consists of 29,006 P and 21,491 S arrivals from 2176 local events recorded at 144 stations along with 5202 P arrivals from 263 teleseismic events recorded at 40 stations. The most significant feature in our image of the mantle beneath the Tien Shan is a pair of large, elongated high wavespeed regions dipping in opposite directions from the near surface to depths of at least 400 km. These regions appear to be continuous and extend upwards to bounding range fronts where the Tarim Basin is being overthrust by the Kokshal range on the south side, and the Kazach shield underthrusts the Kyrgyz range on the north side. While it is tempting to interpret these high wavespeed anomalies as evidence for contemporary subduction of continental lithosphere, such a scenario is difficult to reconcile with both the timing of the orogen and the size of the wavespeed anomaly. We suggest instead that they represent downwelling side-limbs of a lithospheric delamination beneath the central part of the Tien Shan, possibly by siphoning of the bordering continental lithosphere as the central part descends.

We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s , for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P - and S -wave t * -values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t * , and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t * -values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low- Q area ( Q p and Q s about 50–75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high- Q feature ( Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low- Q feature ( Q p and Q s about 50–80) attributed to a wedge of sedimentary rocks; uniformly low Q p - and Q s -values suggest that the wedge is fluid rich. A low- Q basin feature ( Q p and Q s about 50–75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high- Q feature ( Q p and Q s about 220–300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q -values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.

SUMMARY We systematically analysed shear wave splitting (SWS) for seismic data observed at a temporary array and two permanent networks around the San Andreas Fault (SAF) Observatory at Depth. The purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the SAF in this segment and its temporal behaviour in relation to the occurrence of the 2004 Parkfield M 6.0 earthquake. The dense coverage of the networks, the accurate locations of earthquakes and the high-resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the SAF zone. The results show that the primary fast polarization directions (PDs) in the region including the SAF zone and the northeast side of the fault are NW–SE, nearly parallel or subparallel to the SAF strike. Some measurements on the southwest side of the fault are oriented to the NNE–SSW direction, approximately parallel to the direction of local maximum horizontal compressive stress. There are also a few areas in which the observed fast PDs do not fit into this general pattern. The strong spatial variations in both the measured fast PDs and time delays reveal the extreme complexity of shear wave anisotropy in the area. The top 2–3 km of the crust appears to contribute the most to the observed time delays; however substantial anisotropy could extend to as deep as 7–8 km in the region. The average time delay in the region is about 0.06 s. We also analysed temporal patterns of SWS parameters in a nearly 4-yr period around the 2004 Parkfield main shock based on similar events. The results show that there are no appreciable precursory, coseismic, or post-seismic temporal changes of SWS in a region near the rupture of an M 6.0 earthquake, about 15 km away from its epicentre.

SUMMARY The Woodlark rift system is one of the few places where active ocean basin formation can be studied. Within this rift system, continental extension rates are some of the fastest on the planet, and extension progresses eastwards to full seafloor spreading. We use results from a recent passive seismic experiment to address the role of magmatism prior to the onset of seafloor spreading. We invert local earthquake P and S traveltimes for 3-D structure around and ahead of the active spreading tip. From the local earthquake tomography we observe three main structures. Seismic velocities in the crust show a sharp contrast between regions that resemble continental crust ahead of the spreading tip and oceanic crust to the east. In the continental portion, P velocities increase from 5.6 km s −1 to 6.9 km s −1 between 10 and 25 km depth, indicating a bulk felsic to intermediate composition, similar to other continental regions. Beneath the seismic Moho (23‐28 km depth, as defined by receiver functions) a 10‐15-kmthick gradient zone exist locally with velocities from 7.0 to 7.9 km s −1 , which may reflect an underplated mafic, granulite facies lowermost crust, or perhaps magmatic intrusion into the lithospheric mantle. In contrast, farther east and closer to the active spreading tip, velocities rapidly increase from 6.5 to 7.2 km s −1 between 8 and 18 km depth. These fast crustal velocities appear in a narrow zone roughly 60 km wide and indicate a mafic crust, similar to oceanic crust. Our velocity results suggest that the transition from diffuse continental rifting to localized seafloor spreading likely occurs across a narrow zone. Magmatism prior to the onset of seafloor spreading may not play a significant role in altering crust until the onset of seafloor spreading, except through underplating at the base of the crust.

Using P - and S -wave arrivals from local earthquakes recorded by a temporary broadband seismic network (ghengis) and a Kyrgyzstan broadband seismic network (knet), we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central Tien Shan and adjacent areas, based on one- dimensional and three-dimensional velocity models. The results indicate that seismic activity is not restricted to only central Tien Shan but also penetrates into the edges of the Kazakh platform and the Tarim basin to the north and south, respectively. It implies that most range-bounding faults in the central Tien Shan are active and they play a significant role in the tectonic activity of the mountain belt. However, seismic activity of the Talas-Fergana fault in the study area is different; its central segment seems more active than its eastern segment. We infer that this part of the fault may be activated by neighboring faults, whereas its eastern segment could be temporally locked. The earthquakes at the depths of 30–40 km beneath the east-central Tien Shan reveal an interaction between the upper and mid-lower crust. This suggests that brittle failure is possible to greater depth within the crust. Another possibility is that the seismic activity in the mid-lower crust is affected by a positive buoyancy force created by mantle upwelling of this area. These observations indicate that dynamic processes such as small-scale mantle convection or mantle upwelling, which resulted in lithospheric thinning or removal, still occur beneath the central Tien Shan.

The Altai–Sayan region of central Asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between India and Eurasia. To add constraints to the role of the upper mantle in this type of tectonic environment, we analyzed intermediate-period, analogue records of SKS and SKKS to determine the state of mantle anisotropy in this region. We find that for all stations located in actively deforming areas, the fast direction of anisotropy correlates well with indicators of Cenozoic to the present-day deformation, and correlates with older structures only when they happen to align with younger ones. The general east-northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors. At one station (NVS), located outside of any actively deforming region, the N–S fast direction may correspond to the long-wavelength strain field of the mantle beneath much of Siberia. These correlations, along with the size of the delay times (1–2 s), suggest that the anisotropy in the upper mantle beneath the Altai–Sayan is responding to recent tectonic activity.

We determined the shear-wave velocity structure of the crust and upper mantle beneath the central part of the Kola peninsula from the analysis of P-wave receiver functions and mantle P-SV converted phases recorded at stations Apatity (APA) and Lovozero (LVZ). The times of P-SV converted phases from the 410 and 660 km discontinuities are close to those predicted by the IASP91 model. Phase conversions at the crust-mantle boundary beneath the Baltic shield northeast of LVZ and southwest of APA are consistent with a sharp transition from crust to mantle at a depth of 40 km, while conversions from the intervening Khibina plutonic region are consistent with a gradual transition between depths of 20 and 40 km. We infer that short (∼50 km) wavelength lateral variations in the crust-mantle transition persist in this region, despite the inactivity of the Kola peninsula since Devonian times.

Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

[1] Knowledge of the rock types and pressure-temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation, erosion and earthquakes. Seismic-wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the P-T conditions at which they exist. But laboratory-measured ratios of velocities of P to S waves (Vp/Vs) have been shown to be effective. Results of 3-D Vp and Vp/Vs tomographic imaging based on dense seismic arrays in the highly seismic environment of Taiwan provides the first detailed Vp/Vs structures of the orogen. The sharp reduction in the observed Vp/Vs ratio in the felsic core of the mountain belts implies that theα-β quartz transition temperature is reached at a mean depth of 24 ± 3 km. The transition temperature is estimated to be 750 ± 25°C at this depth, yielding an average thermal gradient of 30 ± 3°C/km.

A shallow double seismic zone (SDSZ) has been found in the descending Australian plate beneath the central part of the New Hebrides island arc, directly above a large gap in intermediate depth seismicity and between two seismic boundaries. Ambient seismicity occurs mostly in the upper part of the SDSZ, while earthquakes in the lower part occur in clusters (swarms or aftershocks of large earthquakes). The distance between the upper and lower levels of the SDSZ is 50–70 km, and they are joined at 80 km depth by a near-horizontal band of seismicity. Thrust-faulting mechanisms predominate for earthquakes in the upper level of the SDSZ. Those in the lower level, however, appear to be normal faulting, despite their being aftershocks of large thrust events. We suggest that with the absence of a pull from the detached lithosphere the upper part of the Australian plate in the region of the SDSZ is resistant to subduction, and thus the downward displacements caused by large earthquakes in the adjoining regions result in a localized rebound. The location of the aftershocks within the plate suggests that a new plate boundary is forming, which will eventually replace that outlined by the residual seismicity in the upper level. Thus the leading edge is decoupling, and the boundary will eventually shift back to the lower level of the SDSZ.

Teleseismic S, SKS and SKKS data, collected from a temporary broadband array across the Himalayan front in Pakistan, are analyzed for shear-wave splitting parameters. The SKS and SKKS phases have ray paths originating from both the South Pacific and Colombia which have azimuths approximately 40° apart with respect to the Pakistan array. If significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths. No evidence was seen for any shear-wave splitting beneath any of the stations in the array. Teleseismic S waves were also used in order to provide better azimuthal coverage for the shear-wave splitting measurements. We were able to correct for any source-side anisotropy when needed. No receiver-side splitting was observed in any of the S wave data. The lack of shear-wave splitting beneath the Pakistan array indicates that there is no appreciable large-scale azimuthal anisotropy beneath this part of the Himalayas. Therefore, if there is any significant strain in the upper mantle beneath this area, it must either be vertically oriented, or, if horizontal, vertically vary in such a way that the integrated effect on S wave splitting is null.

The island of Taiwan owes its existence to the collision of an island arc on the Philippine Sea Plate with the Eurasian continental shelf. Most of the earthquakes in the mantle beneath Taiwan clearly are related to one of these plates subducting beneath the other, but beneath the central part of the island there is a zone of subcrustal seismicity that is not obviously related to any subduction zone. We investigated the uncertainties in the locations of these anomalous earthquakes by rereading P and S wave arrival times and computing information density functions for several of these events. The seismic zone itself is then defined through a joint probability function incorporating the entire data set. Individual information density functions show that several events are likely to be located at depths exceeding 60 km, with two events exceeding 80 km depth. The joint probability function shows a narrow zone of seismicity, about 20 km wide along strike, plunging to the east. Combining these results with evidence from surface geology, focal mechanisms and subsurface tomography, we suggest that these events are caused by an eastward displacement of the Eurasian mantle north of 24°N which juxtaposes it against colder subducted crust to the south. Moreover, the heat produced by this type of shearing in the mantle must be insignificant for brittle failure to exist at these depths.

Arrival times of compressional (P) and shear (S) waves generated by earthquakes located in the New Hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large-scale one- and three-dimensional elastic wave velocity structures of the subduction zone between 15° and 20°S and from the surface to about 250 km depth. The results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other, and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures. The results for one-dimensional structure indicate a gradual increase of velocity with depth until a 9% reversal appears between 60 and 100 km depth. The three-dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending Benioff zone. Taken with other observations such as the attenuation of high-frequency shear waves travelling across this gap and the locations of active volcanoes at the surface, we infer that the low-velocity region represents a thermal anomaly of about 750°C which alters the physical properties of the descending plate in this region. At shallower depths there is evidence of low-velocity structures included in the descending plate: one beneath north Malekula island and another between Malekula and Efate islands corresponding to the large embayment of the leading edge of the upper plate. The locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small-scale features such as seamounts and accretionary wedges.

The three-dimensional P and S wave velocity structures and hypocenters of 420 events beneath the western Hazara Arc are obtained simultaneously by inverting travel time data observed at fifteen Tarbela seismic stations. In general, the P and S wave velocity distribution of the top layer (0–6 km depth) correlates well with surface geology. Within this layer we find a low-velocity region beneath the Hazara Thrust Zone (HTZ) corresponding to the underthrusted Murree Formation, and there are high-velocity regions south of the Main Mantle Thrust (MMT) which are associated with the exposed Cambrian, late Paleozoic, and Tertiary granites. A low-velocity zone immediately to the west of the Hazara-Kashmir Syntaxis (HKS) indicates the existence of a Miocene foreland basin which is covered by late stage southeasterly directed thrusts along the Hazara Arc and is consistent with the idea that the HKS is detached from the lower crust. From the Salt Range to the HTZ, the Indian plate dips at a shallow angle, about 2°–3° to the northeast. North of the HTZ the underthrusting Indian plate dips gently to the northeast with an increased slope of 5° to 8° until it reaches the Indus-Kohistan Seismic Zone (IKSZ). Along the NW trending IKSZ the Indian plate bends more steeply to the northeast beneath a seismically active midcrustal wedge directed to the southwest. The larger events in the IKSZ are interpreted as occurring on a major thrust zone that can be followed to a depth of 24 km. The IKSZ appears to consist of an upper seismic zone (from the surface to about 8 km) and a lower seismic zone (12 km to 24 km) separated by an aseismic region about 4 km thick. The lower IKSZ may represent the leading edge of a southwestward directed slab which has not yet ruptured the surface. Hypocenters of relocated earthquakes indicate that the HTZ is about 30 km wide with most of the larger microearthquakes occurring at 12–14 km. Seismicity along the HTZ suggests that the Panjal, and Murree thrusts are active.

The vast majority of the microearthquakes recorded occurred to the east: on the Huaytapallana fault in the Eastern Cordillera or in the western margin of the sub-Andes. The sub-Andes appear to be the physiographic province subjected to the most intense seismic deformation. Focal depths for the crustal events here are as deep as 50 km, and the fault plane solutions, show thrust faulting on steep planes oriented roughly north-south. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a northeast-southwest trend that agrees with the fault scarp and the east dipping nodal plane of two large earthquakes that occurred on this fault in 1969. The recorded microearthquakes of intermediate depth show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with the suggestion that beneath Peru the slab first dips at an angle of 30 deg to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault plane solutions of intermediate depth microearthquakes have horizontal T axes oriented east-west.

Vitaly Khalturin, a pioneering seismologist with encyclopedic knowledge of regional seismic signals and a mentor and friend of seismologists around the world, died on 17 April 2007 in Stanford, California, four days after suffering a major stroke.⇓ ![][1] Vitaly Khalturin (second from left) in 1976, working at home with graduate students in Garm, Tajikistan. Photo by T. Rautian. Born on 15 August 1927 in Leningrad, U.S.S.R., Vitaly grew up in war-torn and politically tumultuous surroundings. His father was a writer well-known in Russian literary circles and, from childhood on, Vitaly knew many prominent Russian authors. While still a student at the University of Leningrad he married a fellow student, Tat'yana Glebovna Rautian. After receiving their masters' degrees in 1951, the two moved to Central Asia as part of the Complex Seismological Expedition (CSE) of the U.S.S.R.'s Academy of Sciences' Institute of Earth Physics, where they would work together for more than 40 years. Vitaly and Tat'yana spent their early years in Garm, Tajikistan, building CSE's network of seismographic stations. Far from any libraries or centers of science, they were forced to learn for themselves—from first principles in classical physics—how to design experiments and interpret seismograms. Under harsh economic circumstances, they installed and maintained an extended seismic network. They taught generations of students—from Russia, Armenia, Georgia, and each of the Central Asian republics—the rigors and joys of earthquake studies. It is remarkable that years later the quality of their work has turned out to be so informative to research groups operating with fewer obstacles and far greater … [1]: /embed/graphic-1.gif

[1] We use SS-S differential travel times to map the upper mantle structure of the Tibetan plateau and its surroundings. New observations from analog stations in the former Soviet Union help to fill in gaps in the spatial coverage of previous studies, providing new data for eastern Tibet, the Kunlun range, and the Tarim basin. The effects of crustal thickness variations, anisotropy, and global heterogeneity are evaluated and explicitly accounted for by incorporating results of previous studies and by ray tracing through a global 3D model. The resulting residuals suggest that, with the exception of the eastern part of the plateau, Tibet is underlain almost entirely by lithosphere of normal to greater than normal thickness. In general, there appear to be strong lateral gradients in wave speed between eastern and western Tibet. We find little evidence for low wave speeds under northern Tibet to the west of about 87°E. Instead, the average shear wave speeds in the upper 300 km of the mantle beneath this part of Tibet are about 0.5 km/s faster than those directly to the north beneath the Tarim basin. These results support models of Tibetan dynamics in which the lithosphere underlying northern Tibet underplates the crust as opposed to those in which large portions of this lithosphere disappear by convection, subduction, or detachment. At the same time, the dominant E-W gradient in residuals implies that 2D models inadequately explain the dynamics of all of Tibet.

Mounting evidence suggests that fault zone heterogeneity may play a crucial role in the localization of rupture in earthquakes [Aki, 1995]. The heterogeneity can take several forms: spatial variations in physical properties (elastic properties, pore fluid pressure, etc.) or complexity in the fault surface (bends, offsets, etc.). High-resolution, three-dimensional models of the P and S wave velocity (Vp and Vs) structure and accurate hypocenters provide tools for studying the effects of fault zone heterogeneity on rupture localization. A dense, passive array of 48 seismic instruments was deployed south of Hollister, California, from mid-November 1994 to late May 1995. The array obtained local-earthquake seismograms for high-resolution, three-dimensional imaging of the (Vp and Vs) structure in the San Andreas fault zone. In mid-May, anactive seismic experiment was also carried out. Figure 1 shows a 7-km wide, 3- to 4-km deep zone of very low Vp imaged during the experiment. The zone is bounded on the southwest by the San Andreas fault and the adjacent Northern Gabilan Range. The San Andreas fault appears to be approximately vertical to at least 5 km depth (the limit of good model resolution), contrary to an existing model based on gravity data.

Relocations of earthquakes, recorded by a local network of stations in Afghanistan and Tadjikistan in 1966 and 1967, indicate a narrow seismic zone (width ≲30 km) dipping steeply into the mantle to a depth of 300 km beneath the Pamir and Hindu Kush ranges. Very low seismicity was observed at depths less than about 70 km, the approximate depth of the Moho. Clear gaps in activity exist also within the zone of intermediate depth seismicity. One gap, about 50 km wide near 37°N and at depths greater than 100 km, separates a steeply northward dipping zone to the southwest from a steeply southeastward dipping zone to the northeast. This gap probably marks either a tear in the downgoing slab or a gap between two oppositely dipping slabs. Fault plane solutions, determined by Soboleva for events between 1960 and 1967, generally show steeply plunging T axes approximately within the planar seismic zone. They therefore are grossly similar to those at island arcs where no deep earthquakes occur and presumably result from gravitational body forces acting on a relatively dense slab of lithosphere. At the same time there is a very large variation in the fault plane solutions, much larger than is common at island arcs. Appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Ave., N.W., Washington, DC 20009. Document J80-003; $1.00. Payment must accompany order.

Brief studies of microearthquakes in four separate parts of eastern Afghanistan reveal a high level of seismicity over a broad area. In general, the activity is not concentrated on well-defined faults, nor does it define new faults, but seismicity on or close to the Chaman and Sarubi faults attests to their activity. First motions of P waves are consistent with left- and right-lateral strike-slip motion, respectively, on these two faults. Fault plane solutions and composite solutions of events in different areas throughout the region differ from one another, but in general, the P axes are parallel to the north-northwest direction of relative motion between India and Eurasia. Several earthquakes beneath Kabul and its immediate surroundings emphasize a need for further study of its seismic hazard. The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J80-001; $1.00. Payment must accompany order.