关键词: 波密震群, 季节性降水, 降水规模和降水过程, 介质岩性, 断层构造, 地震触发, 孔隙压力变化

Abstract: A significant seismic swarm occurred in Yigong, Bomi, Tibet, in July and August of 2020. 25 earthquakes with ML?4.0 occurred during about 30 days and the maximum magnitude is ML4.9. The statistical results on historical earthquake catalogue show that earthquakes in Bomi region mostly occurred during July and August and concentrated in a very small area (about 15km?20km) in space. The maximum magnitude is basically stable in the range of ML4.5~5.0, and the annual average energy release is roughly equivalent to one earthquake with ML4.9. The seismicity in Bomi region is closely related to the precipitation scale and precipitation process in the first half of the year. The swarms occurred during the periods with the peak precipitation, and almost all the earthquakes started after the end of the first significant precipitation process in the year. The contrastive analysis shows that the strength of the seismicity is qualitatively proportional to the starting time of precipitation above scale, the days of precipitation above scale during first half year, as well as the increasing rate of precipitation during April to June. The seismicity in Bomi swarm region mainly concentrated in the middle section of the NE-trending Lequ Zangbo revier and its branches. In summer, it becomes a area for fast convergence of water from surrounding regions, which provides basic conditions for fluid-triggered earthquakes in July and August every year. The lithology in the earthquake densely distribute area is mainly quartz sandstone and siltstone with relatively higher permeability, which is convenient for fluid penetration to increase the pore pressure of underground media, and then triggering seismicity. This local area is confined by many faults. The thrust faults may act as a "water retaining wall", which has a certain confining effect on water infiltration and diffusion. The normal faults have better fluid conductivity, which is convenient for rapid fluid infiltration. Under the action both of the gravity and load pressure of the surface water, the fluid infiltrates rapidly along the fracture zone and the sandstone rock medium with good permeability, resulting in the rapid increase of the pore pressure in the underground cracks, faults and porous media, therefore lead to the decrease of the strength of the faults or cracks, consequently triggering the seismicity. Simulation results on the temporal variation of pore pressure at different depths support the mechanism explanation on seismicity in Bomi region mensioned above.

Key words: Bomi earthquake swarm, seasonal precipitation, precipitation scale and precipitation process, lithology of medium, fault structure, earthquake triggering, variation of pore pressure