Focal depth research of earthquakes in Mainland China: Implication for tectonics

Focal depth data of earthquakes in Mainland China are processed and analyzed in this paper, as well as the relationship between the focal depths and large-scale tectonic structures. As a basic parameter for earthquakes, focal depth is used to investigate deep environment of seismogenic regions, tectonic backgrounds for concentration and release of seismic energy, the inner crustal deformation and its mechanic features. Depth data of 31282 M_l⩾2.0 events with 1st class and 2nd class precision in Mainland China from Jan. 1, 1970 to May 31, 2000 are used to get spatial features of earthquakes distributed with depth and to provide average depth for each grid area throughout China. Researches show that the average depth (D) for all the earthquakes used in this paper is (16 ± 7) km, and (13 ± 6) km and (18 ± 8) km for the events in eastern China and western China, respectively. The area with the deepest focal depth is located in southwest Xinjiang region, near the western and southwestern ends of the Tarim Basin. The focal depth related to large-scale tectonic structures, for instance, D= (33 ± 12), (21 ± 10), (14 ± 7), (11 ± 5) and (10±4) km in Tibet plateau block, Xinjiang block, North China, Northeastern China and South China, respectively. The earth-quakes are deeper at the bounders of the integrated tectonic blocks, including the southwestern and northern brims of the Tarim Basin, southern brim of the Zhunge’r Basin and that of the Alashan block, as well as the eastern and western sides of the Edos block and the western brim of the Sichuan Basin. The earthquakes at the newly ruptured belts are relatively shallower, for instance, at the southwestern Yunnan seismic belt and the Zhangjiakou-Bohai seismic belt. The mechanic behavior, deformation and features for the crust and mantle structures are also discussed.