Accurate Location Of Small Earthquakes And Analysis Of Stress Field Characteristics In Chongqing And Its Adjacent Areas

Chongqing and its adjacent areas are located in the stable plate of Sichuan basin.Strong earthquakes are rare in history,but small earthquakes are active.This paper studies the seismic characteristics of weak earthquake areas,explores the seismogenesis,and provides basic data for seismic risk assessment from the perspectives of Double Difference Algorithm and the application of Composite focal mechanism solution to solve the stress field.The main contents include two aspects: one is to obtain the information of source depth and fault characteristics in the research area by Double Difference location;The other,the comprehensive seismic mechanism solution and crustal stress field information of the region are obtained by using P-wave initial motion data,and the seismogenic environment and dynamic background of the study area are analyzed by combining with the seismic mechanism of historically strong earthquakes,which are described as follows:Firstly,the seismic data from 2009 to 2019 years within the range of 28.0o~32.5 o north latitude and 105.0o~110.5o east longitude were relocated using the Double Difference Algorithm.The data were downloaded from the earthquake catalog system of China earthquake networks center.Among the 4,144 seismic events involved in the relocation,the number of P wave travel time is 1517038,and the number of S wave travel time is 1752458.The relocation obtained 3197 location events,covering Chongqing and its adjacent areas.After the relocation,the seismic distribution is more concentrated,forming strips near the fault zone or clusters at the junction of the fault zone.Based on the seismic distribution and regional tectonic characteristics,the research area is divided into six seismic cluster areas,namely,Hechuan-Changshou area,Rongchang area,Qijiang area,Shizhu-Wulong area,Wushan area and Wuxi area.The results of cluster localization show that the seismic distribution is more concentrated and linear along the fault zone,the seismic depth is more reasonable and the seismic structure is more closely related.The depth profile of the Hechuan-Changshou area reveals four NE trending bands and one near-vertical band,up to 15 km deep,indicating the development of faults in the lower abdomen of the area,which cutting of the cover basement.The NNW fault feature is obvious in the Shizhu-Wulong area and basement rift is cut into the middle and lower crust for focal depth is 28km;The small earthquakes in Rongchang area are densely distributed within a depth of 10 kilometers,which is consistent with the characteristics of waterflood induced earthquakes in gas field exploitation.There are a lot of bottom-belly faults in this area;Three seismic bands with high dip angle in the Qijiang region cut through the basement of the cover layer,indicating that there are bottom-belly faults in this area.The epicenter of the Wuxi earthquake is relatively shallow,which is presumed to be induced by karst collapse.There are three seismic bands with steep tendency in the depth range of 12 km,which are supposed to be hidden faults;The Wushan-Badong area is located at the intersection of three faults in the Three Gorges Reservoir Religion,where fault fracture zone and the limestone rock strata deposit.The source was shallow after the relocation,which according to the characteristics of reservoir induced earthquake.Secondly,the stress field of Chongqing and its adjacent areas is calculated by using the method of Composite focal mechanism.According to the results,the characteristics of tectonic stress field in this region are as follows: The composite focal mechanism solution of Chongqing and its adjacent areas is dominated by thrust and strike-slip,which is consistent with the background of NNW-SSE compression and NNE-SSW tensile stress field of Sichuan basin.In the southwest of Rongchang and Wuxi area,there are positive faults with local tension;The P-axis and T-axis are consistent with the background stress field as a whole,but in the north of Rongchang,both the P-axis and T-axis have a large deflection with the regional stress field,which may be related to the high-pressure injection of waste water in the gas field;The P-axis deflection occurred in the Wulong area located in the Qiyaoshan-Jinfoshan fault and the Fangdou mountain fault depression may be influenced by the Yangtze river and the Wujiang river system.The P-axis and T-axis at the junction of Wushan and Badong are deflected also,which may be related to the infiltration of carbonate sediments by reservoir water.Most earthquakes are located in the southwest and northeast of Chongqing.Gas field exploitation are concentrated in the southwest of Rongchang area and Wushan-Badong area is located in the Three Gorges Reservoir Region.The change of pore pressure in the karst fissure caused by the water storage of the reservoir is easy to destabilize the fault and change the stress,which leads to the earthquake.