Study On Focal Mechanism And Seismogenic Environment Of Mid-strong Earthquakes In Hubei Province

China is located between the two major seismic belts of Eurasia and the circum Pacific Ocean,with well-developed seismic fault zones.Moderate and strong earthquakes above 4.5 occur frequently,causing huge casualties and economic losses.Most of Hubei Province is located in the Yangtze block with relatively stable geological structure and few faults.In history,there were very few moderate strong earthquakes.However,it is actually found that there have been six moderately strong earthquakes with a magnitude of more than 4.5 in Hubei Province since there were earthquake observation records in1958,of which four occurred in the last ten years.At present,the research on the deep seismogenic environment and seismogenic mechanism of these moderate strong earthquakes is relatively simple,lacking systematic and comprehensive research.Based on the background noise imaging method,this thesis inverts the high-resolution three-dimensional crustal S-wave velocity structure in Zigui area,Hubei Province,and analyzes the relationship between the reservoir impoundment and the frequency of moderately strong earthquakes.Then,the CAP(Cut and Paste)method is used to invert the source parameter information of moderately strong earthquakes in Hubei Province,in which the source parameters of the Suizhou ML4.7 earthquake in 2006 and the Zigui Ms4.5earthquake in 2018 are obtained for the first time.By using the joint inversion method of receiver function and surface wave,the crustal S-wave velocity under the epicenters of six moderately strong earthquakes in Hubei Province was obtained for the first time.Finally,the seismogenic environment and focal mechanism of five moderate strong earthquakes are systematically compared and analyzed,and the seismic risk characteristics of Hubei Province are discussed.The main conclusions of this thesis are as follows:(1)Through the continuous seismic waveforms recorded by 24 mobile stations in Zigui area,Hubei Province,the three-dimensional S-wave velocity structure within 6 km of the near surface is obtained by using the background noise tomography.The results show that the tectonic evolution and sedimentary characteristics of different tectonic blocks are consistent with previous studies.In the vertical S-wave high and low speed junction area,earthquakes occur frequently,and the S-wave velocity in the fault area decreases significantly near the Yangtze River area,indicating that the Yangtze River water penetrates into the fault,The frequent earthquakes in Zigui area may be related to the water load and seepage of the Three Gorges Reservoir.(2)Based on the data of mobile and fixed seismic networks,using the CAP inversion method,it is obtained that the strike,dip and slip angle of the seismogenic fault plane of the Suizhou(SZ)earthquake in 2006 are 126~o/78~o/-30~o,NW strike and focal depth of 8km.The results show that the earthquake may be related to the NW Xiangfan Guangji fault zone and its syndromic secondary structural fractures;The strike,dip and slip angle of the seismogenic fault plane of the Zigui earthquake in 2018 are 61~o/58~o/173~o,NNE and NE strike,and the focal depth is 5 km,which is related to the Xinhua Longwangchong and Gaoqiao fault zones.(3)The S-wave velocity structure of the crust beneath the epicenters of six moderately strong earthquakes was obtained by using the receiver function H-k stack and the surface wave receiver function joint inversion method.The S-wave velocity of Badong(BD)M5.1 and Zigui(ZG)M4.6 earthquakes varied from low to high,with the variation of 4%and 7%respectively;The S-wave velocities of ZG4.7,4.5 and Yingcheng(YC)M4.9 earthquakes all vary from high to low,with variations of–1%,-4%and–2%respectively.The five moderately strong earthquakes in the Yangtze craton occurred in the vertical high and low velocity junction area,indicating that the vertical high and low velocity interbedding is conducive to the occurrence of moderately strong earthquakes.(4)The focal depth of moderately strong earthquakes in Hubei Province is mainly concentrated in the shallow upper crust of 0-10 km.The earthquakes of SZ4.7 in 2006,ZG4.6 in 2014 and YC4.9 in 2019 are tectonic earthquakes,which may be related to the pre-existing fault activation;The BD5.1 and ZG4.5 earthquakes in 2013 and 2018 may be reservoir triggered earthquakes.It is speculated that the impounding of the Three Gorges Reservoir and the downward seepage of reservoir water have changed the seismogenic environment.