Tomographic inversion of Pn travel times in China

Abstract

Pn travel times are useful for studying crustal and uppermost mantle structure and regional tectonics because they are affected by crustal velocity and thickness as well as uppermost mantle velocity and anisotropy. We obtained 57,740 Pn travel time picks from 5433 earthquakes and 307 stations from Chinese national and provincial earthquake bulletins and the International Seismological Center bulletins to invert for Pn velocity variation and anisotropy and station delays in China. Our inversion reveals significant features that correlate with surface geology. The main results are as follows: (1) The Pn velocities show a mosaic of very fast and very slow anomalies, mirroring the heterogeneous geology of China at the surface. The Pn velocities are high beneath the major basins in the west (Sichuan, Qaidam, west Tarim, Tulufan, and Junggar) and low in areas of active volcanoes (Myanmar and western Yunnan) and Quaternary volcanism in northern Tibet, in seismically active areas in north China and Tien Shan, and in the southern part of south China (the Hainan plume). (2) The Pn anisotropy beneath the major basins in the west is generally weak. Strong anisotropy is found beneath high-deformation regions (the Tibetan Plateau, southeastern margin of the Tibetan Plateau, western Tien Shan, and part of north China), suggesting the anisotropy is likely related to recent large-scale tectonic activity. (3) A large area of north China shows prominent low Pn velocity beneath Archean basement with thin crust. Our observations are consistent with rifting, lithospheric thinning, and mantle upwelling in the region. The Pn anisotropy is consistent with a dextral simple shear in the NNE direction in the lithosphere mantle during the last (and ongoing) major deformation period. (4) The Pn velocity in northern Tibet is generally lower than that in the south. Southern Tibet has significant E-W structure. Low-velocity anomalies can be traced from northern Tibet across southwestern Tibet and south central Tibet to near the India plate. Anisotropy is absent beneath much of the Himalaya block, but consistent anisotropy with E-W fast direction is present beneath the Lhasa block and large anisotropy (up to 4%) is observed in low-velocity regions of the northern and western Tibet. Complex station delays in the eastern margin of the plateau suggest that the whole crust may be highly deformed. The anisotropy pattern in the southeastern margin of the Tibetan Plateau suggests a mantle lithospheric deformation similar to the clockwise rotation of material observed at the surface. (5) Crustal thicknesses inferred from our station delays are consistent with previous models, which correlate well with surface topography. Copyright 2004 by the American Geophysical Union.