| In recent years,the energy problem has become very serious with the rapid economic development.Breakthroughs in the techniques of horizontal drilling and hydraulic staged fracturing have made large-scaled shale gas exploitation possible;at the same time,the seismic events induced by fracturing,especially the occurrence of moderately strong earthquakes,have brought huge losses to the people,and have attracted much attention from all sectors of society.Located in the southwest of Sichuan Basin,Changning is rich in salt mines and shale gas resources,making it one of the largest shale gas exploration areas in China.A three-dimensional velocity structure model based on dense array data and high-precision earthquake positioning can deepen the understanding of underground medium structure,facilitate the judgment of spatial location of faults and the characterization of fault geometry.The seismicity analysis on this basis is helpful to the disaster risk assessment of Shale gas Remotely triggered earthquakes in Changning area.The Institute of Geophysics of the China Earthquake Administration has deployed a dense seismic array in Changning region supported by a national key research and development project.In this study,we use the continuous waveform data recorded by this array from April 28,2021 to October 31,2021,and utilize the PhaseNet method to pick up the body wave travel times.After REAL correlates the earthquakes,absolute and relative location methods are used to relocate the seismic events to build a high-precision earthquake catalog.Based on this,the three-dimensional velocity structure of Changning and its surrounding areas is inverted with a double difference tomography method,and the shallow crustal Vp and Vs and Vp/Vs models are obtained.Based on the high-precision earthquake catalog constructed from the observed data by the array from April 28,2021 to June 30,2022,the spatiotemporal variation characteristics of the b value in Changning area are analyzed,and the relevant causes of the b value variation are discussed.The main conclusions obtained are as follows:(1)The earthquakes in Changning area are distributed in clusters or belts.The earthquake distribution in Gongxian area is dominated by the Changning structural belt,which generally extends in a NW-SE direction.Earthqakes in the west are deep while and ones in the east are shallow.The seismic events in Junlian District show an obvious strip shape,with the dominant orientation in the northeast southwest direction.From the profile,the earthquake zones have steep slopes,which corresponds to the fact that faults in the area are steeply dipped.The focal layer is shallow,usually in 2-6 kilometers.(2)The three-dimensional body wave velocity models of Changning area by the double difference tomography shows that the crustal shallow structure has obvious heterogeneity,and the P and S wave velocity structures have highly consistent characteristics.At shallow depths,velocity in the axial part of the Baixiangyan Shizitan anticline and in the Shuanghe anticline is relatively high while velocity in the Jianwu syncline is relatively low,which is consistent with the shallow geological structure of the region(3)The magnitude of earthquakes has a trend of gradually increasing with hypocentral depth.Earthquakes with larger magnitudes are generally distributed at depths of 5 kilometers or more,while small earthquakes are relatively close to the ground.Seen from the profile,earthquakes are mainly located in areas with low wave velocity ratios.Earthquakes with magnitudes above 4.0 are located at the boundaries of abnormal bodies.(4)By using the maximum likelihood method,the temporal and spatial distribution characteristics of b value in Changning region are obtained.The results show that the average b value of Changning area is 1.06.The value of b in the Shuanghe salt mine area is 1.25,while the b value of the Changning Zhaotong shale gas exploration area is 1.01,indicating that the shale gas exploitation area may have a higher stress level.The spatial distribution of b values has significant heterogeneity,which to some extent reflects the heterogeneity of underground media.The spatiotemporal distribution of b value shows sensed earthquakes are more likely to occur in areas with low b values,and there are usually a decrease in b value before earthquakes occur,which is probably important for analyzing the risk of induced earthquakes. |
