Surface-wave group velocity tomography of East Asia

Abstract

Abstract Group velocities of both Rayleigh and Love waves are used in a tomographic inversion to obtain group velocity maps of East Asia (60–140°E and 20–60°N). The period range studied is 30–70 s. The Tibetan plateau, a region undergoing intense north-south compression, appears as a prominent low-velocity (about −15% from the average) structure in this area; central Tibet appears as the area with the lowest velocities, and southern Tibet, south of the Zangbo suture zone and the Himalayas, is an area of high velocities. The Tibet low-velocity area extends northward to the southern Tarim basin. The North China-Korean platform, an area traditionally recognized as the oldest platform in China, does not have a consistent crust and upper-mantle velocity signature. The North China plain, a part of the platform where extensional tectonics dominates, is an area of high velocities, probably as a result of thin crust. The other parts of the platform have relatively low velocities. The south China block, the least tectonically active region of China, is generally an area of high velocity. For periods longer than 40 s, a high group velocity gradient clearly exists along longitude 105°E; the velocities are noticeably higher east of this longitude than west of it. This transition corresponds closely to that in the Bouguer gravity map. In general, processes in the crust and upper mantle owing to recent tectonic activities, rather than the age of the basement rocks, seem to have dominant control of crust and upper-mantle velocity structures.