We have obtained velocity images of the uppermost mantle beneath China by performing tomographic inversion of both Pn and Sn traveltimes. From the Annual Bulletin of Chinese Earthquakes, 99,139 Pn arrivals and 43,646 Sn arrivals were selected. Pn anisotropy was also obtained simultaneously with Pn velocity. Average Pn and Sn velocities are 8.05 and 4.55 km/s, respectively, and maximum velocity perturbations are about 3–4%. The Pn and Sn velocities are low in eastern China and high in western China. Particularly high velocities are associated with old basins (for example, Tarim, Junggar, Turpan‐Hami, Qaidam, and Sichuan) and stable craton (for example, Ordos). Low Sn velocities are found mainly throughout North China. In addition, velocities are relatively low beneath the central Tibetan Plateau and the North‐South Seismic Zone (along 103°E). In Tarim and central China where we observe strong anisotropy, the fast Pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from Global Positioning System. Beneath the India‐Eurasia collision zone, the Pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation. Both the velocity variations and anisotropy indicate that the Tibetan Plateau was extruded, and the mantle material beneath the plateau has flowed around the East Himalaya Syntax, while the remaining material has diverted northwestward beneath the Tarim Basin.

A three‐dimensional (3‐D) P wave velocity model is developed for the crust and uppermost mantle of China and the surrounding area by applying the tomography method of Zhao et al. using 500,000 high‐quality P wave first arrivals extracted from the Annual Bulletin of Chinese Earthquakes (ABCE). This tomographic method can accommodate velocity discontinuities such as the Moho in addition to smooth velocity variations. The spatial resolution is 1° × 1° in the horizontal direction and 10 km in depth. The velocity images of the upper crust correspond well with the surface geologic features such as basins and the Tibetan Plateau. High‐velocity anomalies are found in the lower crust beneath the Precambrian regions (Tarim Basin, Ordos Basin, Sichuan Basin, and western half of Songliao Basin). The highest‐velocity anomaly is beneath the Sichuan Basin. High‐ and low‐velocity anomalies imaged beneath the Bohai Gulf are associated with the presence of a major Cenozoic rift system. In the lower crust beneath the South China Block, P wave velocities are lower in the north than in the south. The Indochina Block shows low velocities both in the crust and in the uppermost mantle due to volcanism. The Pn velocities in the Tibet area are higher than those in other areas largely due to thicker crust. Tomographic model significantly reduces the traveltime residuals. Tests conducted by relocating large explosions and earthquakes validate the 3‐D velocity model.

A new tomography method, the adaptive moving window, is introduced and applied to construct the velocity structure of the crust and upper mantle of China and surrounding areas. More than 345,000 high-quality compressional body-wave phase data extracted from the Annual Bulletin of Chinese Earthquakes spanning from 1990 to 1998 are used. The area of interest is represented horizontally by 2338 points with 1Њ intervals. Each point is assigned a window (a cell or a region centered at each point) whose size is varied depending upon the ray path density. A five-layer 1D model from surface down to uppermost mantle is then determined at each point by performing a Monte Carlo random search where earthquake locations are held constant. Combining and smoothing the obtained 1D models, an equivalent 3D model is achieved. The predicted travel times through the 3D model match very well with the observed ones from local to regional distances. The model has a good correlation with tectonic features and is generally consistent with the existing models constructed by other researchers. Our model gives detailed information about structure and is feasible for application to high-quality earthquake location problems.

[1] We have obtained Vp and Vs velocity images of the uppermost mantle beneath the continental collision zone from the Alps to the Himalaya by performing tomographic inversion using both Pn and Sn travel times. 654,999 Pn arrivals and 121,838 Sn arrivals were selected from the joint database including ISC/EHB, Iran bulletin and the Annual Bulletin of Chinese Earthquakes. Average Pn and Sn velocities are 8.04 km/s and 4.60 km/s, respectively, and maximum velocity perturbations are about 6%. Pn velocity correlates well with topography. In general, mountains, with high elevations, show low velocity, while the seas, basins and plains with low elevations show high velocity because the mountains are collision zones with strong tectonic activity and the low elevation areas are stable plates. The large tectonic lines are boundaries between high and low Pn velocity, such as plate boundaries, sutures and faults between plates and orogens. Sn velocity shows a very similar pattern to Pn velocity. A geodynamic cartoon is proposed to show the relationship between velocity and tectonics, indicating that the sutures are boundaries with high dip angles and the plate boundaries are low angle reverse faults in the region.

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The Natural Science Foundation of China.

[1] We use 280,000 local and regional arrival times from 13,105 local earthquakes recorded by 220 seismic stations to determine a three-dimensional S wave velocity structure of the crust and uppermost mantle under China and surrounding regions. We use a traveltime tomography code that accommodates a heterogeneous crust and varying Moho depth. The grid for tomographic inversion is 1°Â 1°in the horizontal direction and 10 km in depth. Our results show that large velocity variations of more than 6% exist in the crust and upper mantle in the China region. The velocity image of the upper crust correlates with surface geological features. Crustal heterogeneity is clearly observed, and velocity changes are visible across some of the large fault zones. In Tibet, low velocities exist in the thick crust, and high Sn velocities exist in the southern and eastern parts. Low-velocity zones beneath volcanic sites and the rifts are clearly observed in our tomographic results. Under the Tengchong, Hainan, and Changbai volcanic areas, strong low velocity zones are visible down to the upper mantle, indicating the existence of magma chambers beneath the volcanoes.