THE VELOCITY STRUCTURE OF THE CRUST AND UPPER-MANTLE UNDER CHINA FROMBROAD-BAND P AND PP WAVE-FORM ANALYSIS

We model 45 P and PP waveforms at epicentral distances of 14 degrees t o 40 degrees, primarily sampling western and central China, to determi ne regionalized P-wave velocity variations for the crust and upper man tle in this region. Observed P and PP waveforms are matched by 1-D for ward modelling using the reflectivity technique. Our approach is to fi rst find a homogeneous average velocity model for China which can matc h the observed broad-band waveforms filtered with a long-period instru ment response. This model is then used as a starting model for the mat ching of the broad-band waveforms to investigate the lateral velocity variations. Our preferred average model WCH has a 50 km thick crust wi th a velocity of 6.4 km s(-1), a P-n velocity of 8.15 km s(-1) and a l ow-velocity zone between 100 km and 175 km depth. It has intermediate characteristics between an active tectonic region and a stable shield, compatible with previous S-wave modelling results. Modelling of the b road-band waveforms indicates significant lateral deviations from mode l WCH. For the Tibetan plateau we observe larger crustal thickness (55 -70 km) and P-n velocity (8.15-8.25 km s(-1)) in combination with a lo w-velocity zone that varies in magnitude throughout the Tibetan region . Lower velocities than our average model may be present below 200 km depth. Our modelling indicates a thin lid of about 30-45 km under Tibe t. We infer from our results and previous work that crustal shortening and thickening is the most likely process responsible for the central and northern Tibetan plateau's thick crust and high average elevation , and that the Indian plate has probably only underplated the southern most part of the plateau. Mantle convection associated with the crusta l shortening process has led to strong lateral heterogeneity in the up per mantle under Tibet. Higher P-n velocities in western Tibet might i ndicate that this region is presently undergoing crustal shortening. P aths traversing eastern China require a constant velocity lid of about 100 km thickness, a P-n velocity of 8.12-8.14 km s(-1) and a crustal thickness between 27 and 35 km. A preliminary model for northern China suggests a crustal thickness between 43 and 50 km, a P-n velocity of about 8.0 km s(-1) and a 100 km thick lid with a positive velocity gra dient for this region.