The velocity structure of the crust and upper mantle under China from broad-bandPandPPwaveform analysis

Abstract

SUMMARY We model 45 P and PP waveforms at epicentral distances of 14 to 40, primarily sampling western and central China, to determine regionalized P-wave velocity variations for the crust and upper mantle in this region. Observed P and PP waveforms are matched by 1-D forward modelling using the reflectivity technique. Our approach is to first find a homogeneous average velocity model for China which can match the observed broad-band waveforms filtered with a long-period instrument response. This model is then used as a starting model for the matching of the broad-band waveforms to investigate the lateral velocity variations. Our preferred average model WCH has a 50 km thick crust with a velocity of 6.4 km s-', a P,, velocity of 8.15 km s-' and a low-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 broad-band waveforms indicates significant lateral deviations from model WCH. For the Tibetan plateau we observe larger crustal thickness (55-70 km) and P,, velocity (8.15-8.25 km sC') in combination with a low-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 Tibet. 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 southernmost part of the plateau. Mantle convection associated with the crustal shortening process has led to strong lateral heterogeneity in the upper mantle under Tibet. Higher P,, velocities in western Tibet might indicate that this region is presently undergoing crustal shortening. Paths traversing eastern China require a constant velocity lid of about 100 km thickness, a P,, velocity of 8.12-8.14kms-' and a crustal thickness between 27 and 35 km. A preliminary model for northern China suggests a crustal thickness between 43 and 50km, a P,, velocity of about 8.0kms-' and a 100km thick lid with a positive velocity gradient for this region.