Gravity pattern in southeast margin of Tibetan Plateau and its implications to tectonics and large earthquakes

摘要

There are many active faults in the southeast margin of Tibetan Plateau, where three large active faults zones, the Longmenshan, Xianshuihe and Anninghe, merge to form a "Y" shape. Strong crustal deformation and a complicated fault distribution accompany strong earthquake activity in this zone. In this paper, we investigate a multi-scale gravity anomaly in the southeastern margin of the Tibetan Plateau using the wavelet transform; we find that the pattern of the gravity field is closely related to the fault system in the study area. Analyzing the characteristics of this Bouguer gravity anomaly at different orders indicates that the eastern Himalayan syntaxis has produced a strong eastward push during its northward movement, resulting in a shortening of the crust from west to east and a rapid uplift of the Tibetan Plateau. The Songpan-Garzê and Sichuan-Yunnan blocks have been forced to slip and extrude southward and eastward laterally. The distributions of seven large earthquakes from 1970 to 2018 reflects the relationship between large earthquakes and characteristics of the gravity anomaly. Comparing the tectonic backgrounds of several earthquakes reveals that the large earthquakes occur usually in the high gravity anomaly gradient zone, which corresponds in general to the boundary zones of the blocks. We infer that large earthquakes occur primarily in high Bouguer gravity anomaly zones in the upper crust, while low Bouguer gravity anomalies encompass the lower crust and the uppermost mantle.