Rayleigh‐Wave Phase Velocity Distribution in China Continent and its Adjacent Regions

Abstract

AbstractUsing a two‐station analysis and vertical component records of Rayleigh waves from 102 seismograph stations in China and adjacent areas, we measured inter‐station fundamental mode Rayleigh‐wave phase velocity dispersion in the period band 20~120 s for 538 independent paths. The dispersion data were then used to invert for 21 phase velocity maps from 20 s to 120 s with 5 s interval in the continental China and its adjacent regions (70° ~ 140°E, 18° ~ 55°N). Checkerboard tests show that the lateral resolution is about 3° in central‐eastern China and about 5° in western China and adjacent regions. The spatial distribution of phase velocities in the study area is significantly different for the parts of China to the west and east of approximately 104°E. The phase velocity maps at shorter periods (20~35 s) are influenced by topography and crustal thickness. On these maps, the western part is characterized by low phase velocities and the eastern part shows relatively high phase velocities. The Tarim basin, the Qinghai‐Tibet plateau, and its eastern margin (the Songpan‐Garze block) form the most prominent low velocity structures in the study region. A large‐scale low velocity anomaly also appears in western Mongolia. The Sichuan basin, the Yangtze block, the south‐China block, the Songliao basin, the Sea of Japan, and eastern Mongolia are marked by relatively high phase velocities. At larger periods, the lateral size of the low velocity anomaly beneath central Qinghai‐Tibet becomes smaller whereas the phase velocities increase beneath the Himalayan thrust and Tarim Basin. This implies that the Qinghai‐Tibet low velocity area is surrounded by seismically fast structures to the south, northwest, and east, which may control a southeastern migration of mechanically weak, seismically slow material away from the central plateau. Meanwhile, a large area in the eastern part which appears fast at shorter periods shows relatively low phase velocity, suggesting a relatively thin lithosphere and a pronounced asthenosphere. As the low velocity anomaly beneath Qinghai‐Tibet plateau becomes weaker, the northern Indo‐China block and its adjacent off‐shore areas, East China Sea, Northeast China's deep‐earthquake region in Jilin Province, Sea of Japan, and the Sino‐Korean block to eastern Mongolia are marked by prominent low phase velocity anomalies at the period 120 s. However, high phase velocities appear in the upper Yangtze block (including the Sichuan Basin), which means that the ancient block is relatively stable. The North‐South Seismic Belt in China has relatively low phase velocity from periods 20 s to 120 s, and becomes a natural boundary separating the continental China into the eastern and western parts with different lithospheric phase velocity features.