Seismic Swarms In Bomi,tibet,induced By Seasonal Rainfall And Discussionon Its Mechanism

Significant earthquake swarm of ML4.9 occurred in Yigong Township of Bomi,Tibet during July-August 2020,and earthquakes of ML?4.0 occurred 25 times in a short period of one month,which has attracted wide attention.In this paper,swarm activity in Tibet bomi area as the research object,the characteristics of previous seismic activity in the Bomi earthquake swarm region are comprehensively summarized,the relation between the local swarm activity and seasonal rainfall process is emphatically analyzed,the tidal modulation characteristics of the occurrence time of large earthquakes in the Bomi earthquake swarm are systematically studied,based on a simple one-dimensional fluid permeation and pore pressure diffusion model,the possible mechanism of seismic swarm activity is discussed in combination with the factors of precipitation process,geographical geomorphology,drainage distribution,medium and tectonic environment.The following conclusions are drawn:1.The proportion of large and small earthquakes in Bomi earthquake swarm is unbalanced,and the number of earthquakes with higher magnitude is more,and the GR relation b?0.3.The focal mechanism is about July 27 and 28 as the boundary,the early stage is mainly thrust and strike-slip,the late stage is basically EW trending tensile fracture,the consistency is good.2.The data since 1970 show that the seismic activity in the region of Bomi earthquake swarm basically concentrates in July and August every year in time,and it mainly concentrates in a very small regional range of 15km?20km in space.The maximum magnitude of the local earthquake in the year with earthquakes is basically stable in the range of ML4.5~5.0,and the annual average energy release is roughly equivalent to one ML4.9 earthquake.3.There is a close relationship between regional seismic activity of Bomi earthquake swarm and precipitation scale and precipitation process in the first half year.Seismic swarms occurred during the peak precipitation period,and all obvious swarms started after the end of the first significant precipitation process in the year.Quantitative comparative analysis shows that the earlier the start time of precipitation above scale,the more days of precipitation above scale in the first half of the year,the longer the time interval between the start of precipitation above scale and the earthquake,the faster the increase rate of the average monthly precipitation from April to June,and the greater the expected precipitation in June,the higher the seismicity level of the year.4.The time interval of some adjacent earthquakes of the Bomi Swarm is about 24 h or 12 h,which means that the seismic activity of the Bomi Swarm may be affected by the tidal variation of different periods.For 20 earthquakes of ML ?4.0 with focal mechanism results,the tidal normal stress,shear stress and Coulomb Failure stress of the focal mechanism solution on the two nodes before and after the earthquake are calculated respectively,the results show that tidal stress has a certain triggering effect on the seismic activity of the Bomi Swarm,65% of the earthquake cases show that the sliding characteristics of tidal stress,Most of these earthquakes with triggering effects occur in the period immediately after the maximum tidal normal stress.The enhancement of fault tension caused by tidal stress(weakening of compression)may be one of the main reasons for the tidal triggering of the Bomi earthquake swarm.5.The regional seismic activity of the Bomi earthquake swarm is mainly concentrated in the middle section and two sides of the branch basin of the NE-trending Lequ Zangbo.In summer,the surrounding water bodies rapidly converged,providing basic conditions for fluid-triggered earthquakes in July and August every year.The medium in this area is mainly sandstone rock,which has better fluid permeability compared with the surrounding area.This local area is bounded by several faults.On the one hand,the faults act as a "water retaining wall",which has a certain confining effect on the infiltration and diffusion area of surface aggregated water.On the other hand,the faults,especially the tensile faults,have better fluid conductivity,which is convenient for rapid fluid infiltration.Under the action of the dead weight of the fluid and the load pressure of the surface water,the fluid rapidly infiltrates along the fracture zone and the sandstone rock medium with good permeability,resulting in the rapid increase of the pore pressure in the underground fractures,faults and porous media,and the decrease of the strength of the faults and fractures,thus triggering the seismic activity.6.Combined with the contribution of accumulated precipitation,groundwater level change and warming and snowmelt to the rise of surface water level in the first half year,and based on the one-dimensional pore pressure diffusion model,the simulation study on the variation of underground pore pressure with time at different depths is carried out.The simulation results support the mechanism interpretation of regional seismic activity of Bomi earthquake swarm mentioned above.