Travel Time Residuals Research Articles

Seismic anisotropy of the mantle lithosphere beneath the Swedish National Seismological Network (SNSN)

Body-wave analysis — shear-wave splitting and P travel time residuals — detect anisotropic structure of the upper mantle beneath the Swedish part of Fennoscandia. Geographic variations of both the splitting measurements and the P-residual spheres map regions of different fabrics of the mantle lithosphere. The fabric of individual mantle domains is internally consistent, usually with sudden changes at their boundaries. Distinct backazimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Based upon joint inversion of body-wave anisotropic parameters, we instead propose 3D self-consistent anisotropic models of well-defined mantle lithosphere domains with differently oriented fabrics approximated by hexagonal aggregates with plunging symmetry axes. The domain-like structure of the Precambrian mantle lithosphere, most probably retaining fossil fabric since the domains' origin, supports the idea of the existence of an early form of plate tectonics during the formation of continental cratons already in the Archean. Similarly to different geochemical and geological constraints, the 3D anisotropy modelling and mapping of fabrics of the lithosphere domains contribute to tracking plate tectonics regimes back in time.

Detecting the limit of slab break-off in central Turkey: new high-resolutionPntomography results

Inversion of Pn traveltime residuals from a 39-station broad-band array provides a high-resolution image of the velocity structure in the uppermost mantle beneath central Turkey. Individually picked Pn phase arrivals from events recorded by the North Anatolian Fault Passive Seismic Experiment and the Kandilli Observatory were combined with additional events associated with the Eastern Turkey Seismic Experiment. Tomography results show no change in Pn velocity across the North Anatolian Fault, although longitudinal variations are evident. A region of very low Pn velocities (<7.8 km s−1) is imaged east of the Central Anatolian Fault Zone (CAFZ), with a transition to faster velocities (>8.1 km s−1) west of the fault. The sharp transition along the CAFZ, which follows the palaeotectonic Inner-Tauride Suture, may represent the location of the edge of the slab window, created when the oceanic slab broke off along the Bitlis-Zagros Suture around 11 Ma, as the Arabian plate collided with the Eurasian plate.

Seismic refraction traveltime inversion for static corrections in a glaciated shield rock environment: a case study

ABSTRACTA traveltime inversion technique is applied to model the upper ∼40 m of the subsurface of a glaciated shield rock area in order to calculate static corrections for a multi‐azimuth multi‐depth walk‐away vertical seismic profile and a surface seismic reflection profile. First break information from a seismic refraction survey is used in conjunction with a ray‐tracing program and an iterative damped least‐squares inversion algorithm to create a two‐dimensional model of the subsurface. The layout of the seismic survey required crooked seismic lines and substantial gaps in the source and receiver coverage to be accounted for. Additionally, there is substantial topographical variation and a complex geology consisting of glaciofluvial sediment and glacial till overlying a crystalline bedrock. The resolution and reliability of the models is measured through a parameter perturbation technique, normalized χ2 values, root means square traveltime residuals and comparison to known geology.

Mantle anomalies beneath southern Africa: evidence from seismicSandPreceiver functions

To understand deep structure and processes beneath southern Africa, we apply the double-stacking version of the S receiver function (SRF) technique to the recordings of the South African Seismic Experiment. In this technique the wavefields of S and SKS are separated by space—time filtration, and the receiver functions are constructed separately for the S and SKS seismic phases. The results are consistent with those obtained from the SRFs of the permanent stations BOSA and LBTB. Evidence for a reduced S velocity atop the 410-km discontinuity is present in both SRFs and P receiver functions: the S velocity contrast at the 410-km discontinuity is ∼40 per cent larger than the norm, and there are observations of S350p and P350s seismic phases from the negative 350-km discontinuity beneath the Kaapvaal craton. The S350p and P350s phases display a dependence on the azimuth, which can be caused by anisotropy in the layer atop the 350-km discontinuity. This dependence is consistent with observations of shear wave splitting in SKS. There are observations of the S450p phase from the 450-km negative discontinuity in the transition zone. The most anomalous transition zone is found close to the region where the Kalahari craton was located in the Mesozoic. Lateral variations of the S velocity are found beneath the 660-km discontinuity. Teleseismic S and P traveltime residuals with respect to IASP91 model are evaluated from the traveltimes of the P410s and Pp410s seismic phases. In the uppermost mantle these residuals as well as S410p traveltimes require a reduced VP/VS ratio (around 1.75 ± 0.01 versus 1.8 in IASP91), an effect of the depletion of the mantle lithosphere in basaltic material. In the models obtained by a joint inversion of the receiver functions and the teleseismic traveltime residuals the low velocity zone (LVZ) with the onset of low velocity at a depth of ∼140 ± 20 km is present at most locations. The minimum S velocity in the LVZ is ∼4.5 km s−1. The LVZ in our models is consistent with the S velocity, VP/VS velocity ratio and depth range of high-temperature lerzholites in the mantle xenoliths from southern Africa. Our most intriguing finding is a very low quality factor QS (on the order of a few tens) in the upper mantle. We interpret high attenuation, the LVZ and the low S velocity atop the 410-km discontinuity as the effects of plume-like phenomena in the upper mantle.

Model Update December 2008: Upper Mantle Heterogeneity beneath North America from P-wave Travel Time Tomography with Global and USArray Transportable Array Data

Online material: MIT P -wave tomography model for the United States created using travel-time residuals from the USArray Transportable Array from April 2004 to December 2008. As the Transportable Array of USArray (http://www.iris.edu/USArray/), the seismology component of EarthScope (http://www.earthscope.org/), progresses eastward across the United States, the seismological database available for studies of mantle heterogeneity beneath the North American continent is rapidly expanding. Burdick et al. (2008) published a tomographic model of mantle heterogeneity beneath North America based on USArrayTA P -wave travel time data through November 2007. The purpose of this article is to announce the availability (as an electronic supplement through the SSA Web site. www.seismosoc.org) of model MITP\_USA\_2008DEC, which is based on USArrayTA data through December 2008. By the end of 2008 USArrayTA had begun to illuminate mantle structure below the center of the continent, where systematic high-resolution tomography was previously unavailable. Specifically, data from stations east of the Rocky Mountains provides new constraints on the heterogeneity of the stable North American craton and allows for direct comparison with mantle heterogeneity beneath the tectonically active western margin. Compared to the previous version (Burdick et al. 2008), the current model update refines estimates of mantle heterogeneity around the Yellowstone hotspot, better defining its boundaries and revealing a strong slow anomaly beneath the transition zone. Large-scale structural features in the west, such as the Basin and Range and the Cascadia subduction zone, where data coverage was already good, remain similar to those in the original model, which suggests that the tomographic images of them are now robust. Our tomographic inversions of USArray data are based on the method developed by Karason (2002) and described by Li et al. (2008). We perform global inversions in order to account properly for mantle heterogeneity outside the study area and use an adaptable grid …

Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole, Structural Interpretation Supported by Magnetic, Gravity and Petrophysical Data

The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01 profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels, were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the “slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity image down to 15 km. Although the contact of Saxothuringian and the Tepla-Barrandian Unit (TBU) is collateral with the S01 profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular, the Moldanubian and TBU contact beneath the Western Krusne hory/Erzgebirge Pluton, the buried contact of the Lusatia unit and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs beneath the Doupovske hory and the Ceske středohoři volcanic complexes. The intermediate mid-crustal P-wave velocity lows are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic and petrophysical data.

Integrated seismic model of the crust and upper mantle of the Trans-European Suture zone between the Precambrian craton and Phanerozoic terranes in Central Europe

The structure and evolution of the Trans-European Suture zone (TESZ), contact between Precambrian Europe to the northeast and Phanerozoic terranes to the southwest is one of the main tectonic questions in Europe. The knowledge of the crustal structure, lithosphere–asthenosphere boundary and mantle transition zone between two seismic discontinuities at depths “410” and “660” km, is one of the most important issues to understand the Earth's dynamics. To create a mantle model of the TESZ and surroundings we used different seismic data collected along the 950 km long POLONAISE'97 profile P4. Previous results of 2-D ray-tracing and P-wave travel time modelling and new results of P-wave travel time residuals methods and receiver function sections provide facts about the seismic structure from the surface down to 900 km depth. In the TESZ a large basin, about 125 km wide, is filled with sedimentary strata (Vp < 6.0 km s − 1 ) to about 20 km depth. This basin is asymmetric with its northeast margin being most abrupt. The crystalline crust under this basin is only about 20 km thick today indicating that the lithosphere of Baltica was either thinned drastically or terminated along the northeast margin of the basin. The East European craton (EEC) has a ~ 45 km thick three-layered crust. The crust of the accreted terranes to the southwest is relatively thin (~ 30 km) and similar to that found in other non-cratonal areas of Western Europe. The lower crust is relatively fast (Vp > 7.0 km s − 1 ) along most of the P4 profile. However, lower values to the southwest may indicate the termination of Baltica. High velocity (~ 8.35 km s − 1 ) uppermost mantle lies beneath the Avalonia/Variscan terranes, and may be due to rifting and/or subduction. The seismic lithosphere thickness for the EEC is about 200 km, while it is only 90 km in the Palaeozoic platform (PP). The mantle transition zone is shallower and about 30 km thicker under the EEC, which could be due to thermal conditions (lower temperature) and/or the presence of water and FeO. The result of this paper is a new compiled and integrated seismic velocity model, available in digital form down to 900 km depth ( http://www.igf.fuw.edu.pl/p4-mantle), which can be used as a preliminary model of the crust and upper mantle in the TESZ area in Central Europe.

Tomography and thermal state of the Cocos plate subduction beneath Mexico City

The geometry and thermal state of the subducting Cocos plate beneath Mexico City has been enigmatic because of the absence of a deep Wadati‐Benioff zone. We present a tomographic image of the slab based on inversion of 8869 teleseismic P wave travel times measured on a portable broadband seismic network. The images combined with receiver function analysis show that the slab runs flat from the coast to near Mexico City, where it dives into the mantle just before the Trans‐Mexican Volcanic Belt with a dip of ∼75°. It continues down to a depth of ∼500 km at a distance of 400 km from the trench, where the tomography reveals that the dipping portion ends. As well as standard block tomography, we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the (15 Ma) age of the Cocos plate. The combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older, thicker, nontruncated plate.

Path-average kernels for long wavelength traveltime tomography

SUMMARY Although much effort goes into improving the resolution of tomographic models, investigating their quality has only just started. Probabilistic tomography provides a framework for the quantitative assessment of uncertainties of long-wavelength tomographic models. So far, this technique has been used to invert maps of surface wave phase velocities and normal-mode splitting functions. Including body waves would substantially increase the depth resolution in the lowermost mantle. In surface wave tomography, the construction of phase velocity maps and splitting functions is a well-defined inverse problem, and the depth inversion is less well constrained but characterized by a small number of dimensions suitable for a Monte Carlo search. Traveltime tomography is mostly based on ray theory that covers the 3-D Earth, thus the dimension of the inverse problem is too large for a Monte Carlo search. The ray-mode duality suggests to apply the path-average approximation to body wave traveltimes. In this way the measured traveltime residual as a function of ray parameter can be inverted using path-average kernels, which depend on depth only, similar to surface wave tomography. We investigate the validity of the path-average approximation for delay times in both the forward and the inverse problem using the velocity model S20RTS as well as random models. We numerically illustrate the precision of such kernels compared with ray-theoretic and finitefrequency ones. We further invert traveltime residuals, calculated from Fermat rays, using the path-average kernels. We find that the agreement between classical ray theory and path-average theory is good for long wavelength structures. We suggest that for mapping long wavelength structures, body waves can be inverted in two steps, similar to surface waves, where the ray parameter and the vertical traveltime play the role of frequency and phase velocity, respectively.

Estimation of the 2D crustal velocity structure in the Hidaka and Iburi Subprefectures, Hokkaido, Japan

Applying the iterative shooting/bisection technique for rapid forward modeling to the seismic explosion data, we could refine the crustal velocity structure model of the western part of the Hidaka collision zone, Hokkaido, Japan. We used only the precise P-wave first arrival data obtained by the Research Group for Explosion Seismology, which set up a 113.4-km-long profile in August 2000 along with 327 observation points and four shot points with TNT charges from 100 to 300 kg. We could estimate a two-dimensional inhomogeneous crustal velocity structure model with a velocity decrease in the eastern direction at a depth of 15.7 km, several portions of velocity reversals with depth and a low velocity anomaly proposed in previous studies. The root-mean-square of travel-time residuals was improved from 0.398 s for the previous structure model to 0.176 s for the present model with a reduction of 55.8%.

Regional waveform inversion of 2004 February 11 and 2007 February 09 Dead Sea earthquakes

SUMMARY Two felt moderate size earthquakes with local magnitudes 5.2 on 2004 February 11 and 4.4 on 2007 February 09 occurred to the east of the Dead Sea and along the northern part of the Arava/Araba fault (ARF), respectively. Being well recorded by the national seismic networks operating on the Dead Sea region, they offer a good opportunity to update the crustal model, revealing the tectonic process and present-day stress field on the Dead Sea region. The observed group velocity dispersion of Rayleigh and Love waves picked from the broad-band records for the 2004 and 2007 main shocks are used to update the Jordan Seismological Observatory model, which shows a good match between the observed and theoretical dispersion. Our updated model shows small traveltime residuals during our location process and yields a high variance reduction for our regional waveform inversion. The earthquake hypocentre, determined from the observed P- and S-wave traveltime data, and regional waveform inversion indicate a source depth of ∼14 and ∼12 km for the 2004 and 2007 main shocks, respectively. Focal mechanism obtained from both first motion polarities of local-regional observations and moment tensor inversion of regional observations of 2004 earthquake reveal a mainly normal faulting, with minor strike-slip component, along WNW–ESE fault, whose orientation is nearly consistent with the transverse Zarqa Ma'in fault. However, the focal mechanism of 2007 main shock shows left-lateral, strike-slip faulting along a near-vertical, near-NNE–SSW striking fault plane whose orientation is in good agreement with the surface expression of the observed northern ARF. The focal mechanism for a felt foreshock of 2007 earthquake indicates a similar left-lateral strike-slip fault with a slight normal component. Moment tensors estimated from regional waveforms indicate predominantly double-couple mechanisms for the three studied events. These results can be used to calibrate ground motions, evaluate 3-D velocity models for simulation of large scenario earthquakes and seismic risk assessment along the Dead Sea Fault.

A Search for Temporal Variations in Station Terms in Southern California from 1984 to 2002

We use relative arrival times and locations for similar earthquake pairs that are found using a cross-correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002. We examine 494 similar event clusters recorded by Southern California Seismic Network (SCSN) stations and compute absolute arrival-time variations from the differential arrival-time resi- duals obtained following event relocation. We compute station terms from the robust means of the absolute arrival-time residuals from all events recorded by each station at 3-month intervals. We observe nine stations with abrupt offsets in timing of 20- 70 msec, which are likely caused by equipment changes during our study period. Taking these changes into account could improve the relative location accuracy for some of the event clusters. For other stations, we generally do not see systematic temporal variations greater than about 10 msec. Analysis of residuals along individual ray paths does not reveal any clear localized regions of apparent velocity changes at depth. These results limit large-scale, long-lasting temporal variations in P and S ve- locities across southern California during this time period to less than about � 0:2%. However, there is an increased fraction of individual travel-time residuals exceeding 20 msec immediately following major earthquakes from source regions near the main- shock rupture.

Latitudinal distribution of travel times in the upper solar convection zone

We applied the time-distance technique to GONG+ data in 2001 (at solar maximum) and 2006 (at solar minimum) to study the influence of surface activity on the latitudinal distribution of travel times of acoustic waves in the upper solar convection zone. We find that surface activity is the dominant source of travel time differences over the solar cycle. Removal of the surface activity with a masking method reveals a residual travel-time shift of 0.5 sec, corresponding to a surface temperature change of 0.25° K over the solar cycle.

An upper-mantle upwelling underneath Ireland revealed from non-linear tomography

SUMMARY We present results of a non-linear teleseismic P-wave traveltime tomography in Ireland. Relative traveltime residuals are calculated from the data set of the Irish Seismological Lithospheric Experiment (ISLE 2002/3) and permanent stations onshore Ireland. At each of the 28 station sites, a local crustal 1-D P-wave velocity (vP) model is determined because the 3-D crustal vP structure of Ireland is well known. These 1-D models are used for a traveltime correction for crustal effects. The corrected residuals indicate two large-scale traveltime anomalies of low amplitude: a negative traveltime anomaly in the western part of Ireland and a positive anomaly in the eastern part of the country. The subsequent traveltime tomography is calculated using an iterative non-linear inversion using a variable parametrization and 3-D ray tracing. Our inversion result contains a low P-wave velocity (vP) zone underneath central Ireland, at 31–120 km depth. The amplitudes of the low velocity zone reach up to −2.5 per cent vP at 31–60 km depth, but decrease to −0.5 to −1 per cent vP at 120–150 km depth. Reconstruction tests confirm that large anomalies in our model are well resolved, although the resolution decreases towards the edges of the model. We infer that the low velocity zone underneath central Ireland is related to lithospheric thinning towards the north, which was determined by a previous S-receiver function study. This anomaly is interpreted as presumably ancient or even recent mantle upwelling, related to the spreading of the Iceland plume head.

Automated velocity model building with wavepath tomography

Wavefield-continuation migration generally is recognized as superior to Kirchhoff methods in complex velocity models, such as below rugose salt bodies. It accounts for multipathing, sharp velocity contrasts, and the limited bandwidth of seismic wave propagation. Wavepath tomography builds the velocity model in a way that is consistent with the wavefield-migration operator. Traveltime residuals are back-projected along a wavepath instead of rays. The actual wavefield-continuation operator is used to represent the wave propagation between surface source/receiver pairs and subsurface reflection points. A wavepath is obtained by multiplying impulse responses from a surface location and a reflection point. The inversion matrix is kept to a manageable size by restricting the wavepath to the first Fresnel zone. The considerable expense of computing a single wavepath kernel in comparison to ray tomography is partially offset by the smaller number of back projections necessary to sample the velocity model adequately. We have used wavepath tomography to build subsalt velocity models using 2D synthetic data. Wavepath tomography was implemented in 3D.

Deep dynamical processes in the central-southern Qinghai-Tibet Plateau—Receiver functions and travel-time residuals analysis of north Hi-Climb

Teleseismic receiver functions and travel-time residuals along the north Hi-Climb broadband seismic array in the central-southern Qinghai-Tibet Plateau show that the lithosphere structures in the central and western Qinghai-Tibet Plateau are different. In the central Qinghai-Tibet Plateau, the Indian Plate is northward subducted beneath the Qiangtang block and arrives at the greatest depth beneath the central-southern Qiangtang block. The delaminated Indian lithospheric slab remains beneath the central Lhasa block to a depth possibly greater than that of the upper interface of the mantle transform zone. In the western Qinghai-Tibet Plateau, the Indian lithospheric plate is gently northward subducted and may have arrived to the south of Tarim plate. Due to the resistance from the gently northward subduction of the Indian mantle lithosphere in the western Qinghai-Tibet Plateau, the upwelling mantle material beneath the Qiangtang block moves mostly toward the east to bring about the lateral eastward flow of the deep mantle hot material in the central Qinghai-Tibet Plateau.

Distribution of local earthquakes in the crust beneath central Lake Baikal

We have located earthquake hypocenters in the area of central Lake Baikal using the Hypoellipse inversion of direct P traveltimes collected by a local seismological network of seven stations in 2001 through 2005. The location accuracy depends on the agreement between the reference velocity model and the real subsurface. Traveltime residuals have no bearing on the accuracy of depth estimates due to origin time errors. Hypocenter location with reference to a DSS-derived layered velocity model predicts that the crust is seismogenic between 10 and 22 km. The hypocenters cluster densely in belts trending in NE and SE directions along and across Lake Baikal. Earthquakes become deeper from 10 to 22 km toward the Selenga basin.

Imaging mantle transition zone thickness withSdS-SSfinite-frequency sensitivity kernels

SUMMARY We invert differential SdS-SS traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410- and 660-km discontinuities. This approach yields higher resolution images of transition zone thickness than previous stacking methods, which simply average/smooth over topographic features. Apparent structure measured using simple stacking is highly dependent upon the bin size of each stack. By inverting for discontinuity topography with a variety of bin sizes, we can more accurately calculate the true structure. The inverted transition zone model is similar to simple stack models with an average thickness of 242 km, but the lateral variations in thickness are larger in amplitude and smaller in scale. Fast seismic velocities in 3-D mantle models such as SB4L18 correlate with areas of thicker transition zone. The elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude, which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone. The anomalously thin transition zone regions are laterally narrow, and not broadly continuous. If these variations in transition zone thickness are interpreted as thermal in nature, then this model suggests significant temperature variations on small lateral scales.

Lithosphere of the Dharwar craton by joint inversion ofPandSreceiver functions

SUMMARY The Archean Dharwar craton in south India is known for long time to be different from most other cratons. Specifically, at station Hyderabad (HYB) the Ps converted phases from the 410and 660-km mantle discontinuities arrive up to 2 s later than in other cratons of comparable age, which implies lower upper mantle velocities. To resolve the unique lithosphere‐asthenosphere system of the Dharwar craton, we inverted jointly P and S receiver functions and teleseismic P and S traveltime residuals at 10 seismograph stations. This method operates in the same depth range as long-period surface waves but differs by much higher lateral and radial resolution. We observe striking differences in crustal structures between the eastern and western Dharwar craton (EDC and WDC, respectively): crustal thickness is of around 31 km, with predominantly felsic velocities, in the EDC and of around 55 km, with predominantly mafic velocities, in the WDC. In the mantle we observe significant variations in the P velocity with depth, practically without accompanying variations in the S velocity. In the mantle S velocity there are azimuthdependent indications of the Hales discontinuity at a depth of ∼100 km. The most conspicuous feature of our models is the lack of the high velocity mantle keel with the S velocity of ∼4.7 km s −1 , typical of other Archean cratons. The S velocity in our models is close to 4.5 km s −1 from the Moho to a depth of ∼250 km. There are indications of a similar upper mantle structure in the northeast of the Indian craton and of a partial recovery of the normal shield structure in the northwest. A division between the high S-velocity western Tibet and low S-velocity eastern Tibet may be related to a similar division between the northeastern and northwestern Indian craton.

P-wave velocity and gradient images beneath the Okinawa Trough

We determined the high-resolution 3-D P-wave velocity structure under the Okinawa Trough in the region between South Taiwan and North Kyushu (Japan) by inverting 102,511 absolute travel-time data from 5012 local earthquakes and 29,153 relative travel-time residuals calculated from 1873 teleseismic events. For the first time, a combined data set recorded simultaneously by three seismographic networks located in Kyushu, Okinawa Trough, Taiwan and Fujian province of China has been used, which enables us to determine the high-resolution P-wave velocity structure in the crust and upper mantle under the entire arc system. The resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the Okinawa Trough and dehydration of the Philippine Sea slab (PHS), and confirm some of the features proposed by previous studies. A high-velocity zone is imaged clearly at depths of 50–300 km along the Ryukyu Arc system from South Kyushu to North Taiwan, showing good agreement with the spatial distribution of seismicity under the subduction zone. The relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high-Vp regions, and we therefore interpret the high-Vp zone as the subducting PHS slab. Along the volcanic front from South Kyushu to North Taiwan, low-Vp areas are visible, which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting PHS slab. This fluid-related low-velocity zone is located at depths of 20–120 km along the arc with a total length of 1200 km from North Taiwan to South Kyushu. Using the inverted P-wave velocity model we also calculated the gradient images, which are independent of the selection of reference model or color scale, to provide further information and help us to understand and explain the tomoagraphic structures. Three sub-blocks of the Ryukyu Arc system are revealed from the inverted velocity images and the calculated gradient maps. These features of the velocity anomalies together with other parameters determined by the present study suggest that the Okinawa Trough extension, arc magmatism and seismicity are mainly attributed to the dehydration process associated with the PHS slab subduction and the oblique subduction direction of the plate.