Tomography and Location Problems in China Using Regional Travel-Time Data

摘要

Phase data from the Annual Bulletin of Chinese Earthquakes (ABCE) are being collected and used for tomographic inversion and event location problems within China. So far, we have seven years of data in computer form and six more in catalog form. Current efforts focus on regional tomography of China, comparing locations between the ABCE and other earthquake catalogs, and developing station corrections for International Monitoring System (IMS) stations in and around China. We used the Pn phase data from the ABCE catalog to image the uppermost mantle velocity and anisotropy structure beneath China. Raypaths cover most of central and eastern China; coverage in western China and Tibet is poor. The data quality is exceptional, with Pn phases routinely identified and picked for distances from 1.5 to 9 degrees. Over 25,000 arrivals have been used in the Pn tomography algorithm. The average uppermost mantle velocity beneath China is 8.0 km/s. The Tarim, Junggar, Tsidam, and Sichaun basins have the highest Pn velocities (over 8.2 km/s). These places are cratonic terrains that were accreted to southern Asia before the Indian-Asian collision. The high velocities imply higher density mantle that may have aided in the development of these basins. The eastern Tien Shan has normal Pn velocities of 8.0 to 8.1 km/s. Pn velocity beneath Tibet decreases from south to north as previous studies have also found. Late station delays in and around Tibet attest to its 70-km thick crust. Along the southeastern Tibet margin, low Pn velocities are found, suggesting that high temperatures and possible partial melt exist in the uppermost mantle there. A region of high anisotropy surrounds Tibet. Eastern China has lower Pn velocities and thinner crust as a result of Cenozoic extension of eastern China. A very low Pn velocity.