Study On The Characteristics And Causes Of Seismic Activity In Changning Area,Sichuan Province

Seismic activity induced by fluid injection is a research hotspot in the field of geosciences today.The Changning area,located on the southwestern edge of the Sichuan basin,has frequent seismic activity in recent years.Since the end of 2018,there have been many earthquakes with M_S>5 in this area.The relationship between seismicity and water injection for salt mines and shale gas mining in the area has attracted widespread attention.Determining the cause and mechanism of seismic activity in Changning area is of great significance for analyzing earthquake risk,mitigating earthquake disasters,and ensuring resource exploitation.Using the data recorded on 86 seismic stations around Changning area from 2015 to 2019,we study the three-dimensional velocity structure,seismic spatiotemporal migration characteristics,seismicity and source parameters in the Changning area.Combined with the geological background,industrial water injection and previous research results,we analyzed comprehensively the causes and mechanism of seismicity in Changning area.Firstly,the 3D V_P,V_S and V_P/V_S structures and seismic locations in Changning area are inversed by using the V_P/V_S model consistency-constrained double difference seismic tomography method.The crustal velocity structures in Changning area have significant heterogeneities.Earthquakes in Changning-Shuanghe anticline area are mainly distributed along the axis of the anticline,and the focal depths gradually deepen from southeast to northwest,concentrated in a significant anomalous body of high-velocity and low-V_P/V_S.The earthquakes in Jianwu syncline shale gas mining area are mainly concentrated on the edge of low-velocity anomalies,corresponding to the relatively low-V_P/V_S area.The Changning M_S6.0,Xingwen M_S5.7,and Gongxian M_S5.3 earthquakes are all located in the area where the crustal velocity structure changes dramatically,indicating that the crustal structure plays a controlling role in the occurrence of earthquakes in this area.Based on the seismic locations obtained by joint inversion,the double-difference earthquake location method combined with waveform cross-correlation technique is used to accurately locate the earthquakes from December 2018 to July 2019.The results show that the seismicity in the Changning area is composed of multiple seismic belts with different strikes,and the focal depths are generally shallow,reflecting that the seismicity in the Changning area is related to the reactivation of pre-existing small-scale faults.The earthquakes after relocation are selected as the templates,and the microearthquakes in the Changning area from December 2018 to July 2019 are detected and located by using the Match and Locate method.The number of events detected was 8 times of that recorded in the network earthquake catalog.Based on the detected earthquake catalogue,the frequency and intensity of seismic activity near the epicenter of Xingwen M_S5.7 earthquake are significantly increased when the Xingwen M_S5.7 earthquake was approaching.According to the spatiotemporal migration characteristics of foreshocks,it can be inferred that the occurrence of Xingwen M_S5.7 earthquake is related to hydraulic fracturing.Before the occurrence of Changning M_S6.0 earthquake,the earthquakes in the Changning-Shuanghe anticline are mainly concentrated in the southeast of the main earthquake.Near the occurrence of the Changning M_S6.0 earthquake,the foreshocks did not increase significantly.Based on the earthquake catalogue improved by microearthquake detection,the ETAS model was used to analyze the characteristics of seismicity in the Changning area,and the spatiotemporal distribution characteristics of the b-value in the Changning area are calculated and analyzed.From December 2018 to the time before the Changning M_S6.0 earthquake,the background seismic activity in the Changning-Shuanghe anticline area is relatively stable,while the background seismic activity in the Jianwu syncline area showed significant fluctuations,reflecting the influence of long-term water injection in salt mine and short-term water injection in hydraulic fracturing area on seismicity,respectively.The aftershock productivity in the Jianwu syncline area is low and aftershock attenuate is fast,while in the Changning-Shuanghe anticline area,the aftershock productivity is relatively higher and the aftershock attenuate is slower.There are obvious differences in the spatial distribution of b-values in Changning area,and earthquakes with larger magnitudes are mainly concentrated in the low b-value areas.The source parameters of 442 M_L1.3?4.7 earthquakes in Changning area are calculated by spectral ratio method,and the relationship between the source parameters and the spatiotemporal distribution characteristics of stress drop are analyzed.The stress drops of earthquakes in Changning area are in the range of0.02?7.26 MPa,and more than 90%of the earthquake stress drop is less than 2 MPa.The stress drop generally increases with the magnitude,but the relationship with the focal depth is not obvious.Before the occurrence of the Changning M_S6.0 earthquake,the stress drop of earthquakes in the focal area are generally at a low level.After the occurrence of the main earthquake,the stress drops of the aftershocks increased significantly,and then decreased rapidly.The earthquakes with high stress drop are mainly concentrated in the northwest of Changning aftershock area,where several M_S>5 aftershocks have occurred.The stress drop of earthquakes in Jianwu syncline are slightly lower than that of the Changning-Shuanghe anticline area,but the difference is not significant.Comprehensive analysis suggests that the continuous active earthquakes in the southeastern side of the Changning-Shuanghe anticline in the study area in recent years are closely related to water injection of salt mine,and long-term water injection has promoted the occurrence of the Changning M_S6.0 earthquake.The area on the northwest side of the Changning-Shuanghe anticline is less affected by the long-term water injection of salt mine and the seismic environment and seismic activity characteristics in this area are closer to those of natural earthquakes.Seismic activities in the Jianwu syncline area,including Xingwen M_S5.7 and Gongxian M_S5.3earthquakes,are related to shale gas hydraulic fracturing activities.In addition,the seismic activities in the study area are also controlled by the geological structure background and regional stress conditions.The frequent seismic activity in Changning area in recent years is the result of the combined effect of geological structure background,regional stress field and industrial water injection.