| Changning-Xingwen area in the southeast of Sichuan province is rich in natural resources,and the regional geological structure environment is very complex.In recent years,moderate-strong earthquakes have occurred frequently in this area,which has attracted extensive attention from the academic circle.It is of great significance to understand the focal features,causes and hazards of seismic activity in this region for the safety of industrial production and the safety of life and property of residents.In this paper,the temporal and spatial distribution,source parameter relationship,b value and stress drop of seismic activity in Changning-Xingwen area in southeastern Sichuan province from 2019 to 2020 are studied,and the relationship between seismic activity and hydraulic fracturing in the study area is explored.Firstly,according to the catalogue of China Earthquake Networks,earthquakes above 0.0 in the study area from 2019 to 2020 were relocated using the doubledifference positioning method.After relocation,the earthquakes were more concentrated in space distribution,and the earthquakes in the study area were obviously divided into two clusters,south and north,with the focal depth within 25 km.The northern part of the anticline is the focal area of the 2019 Changning MS6.0 earthquake sequence,The southern syncline is the Changning shale gas development zone,where intensive hydraulic fracturing Wells are distributed.Compared with the northern zone,the seismic activity has a more prominent spatial clustering in the same period,and the focal depth in 2020 has increased compared with the previous year.It is speculated that the temporal and spatial evolution of seismic activity in southern China may be related to hydraulic fracturing operations in industry.Secondly,we calculate the time variation of the minimum complete magnitude and b-value in the study area from 2019 to 2020 by using the relocated earthquakes catalog according to the maximum curvature method and maximum likelihood method.The results show that the region has good seismic observation capability and the b-value is generally low(less than 1.0).According to the time variation of b-value,the seismic activity is divided into four periods,and the seismic activity is more frequent in the period of low b-value.According to the four stages of b-value with time variation,the spatial distribution of b-value in each stage is calculated.Comparing with the epicenter location of earthquakes above ML4.0 occurring in each stage,it is found that these earthquakes are mainly distributed in the region with low b-value.This result indicates that the region with low b-value has higher stress and higher seismic risk.Thirdly,the source parameters of earthquakes above ML3.0 after the Changning MS6.0 earthquake in the study area are calculated by multi-platform joint inversion method.The results show that the stress drop and corner frequency have obvious variation trend with earthquake magnitude,but they are not linear correlation.There is an obvious linear relationship between the logarithm of seismic moment,the logarithm of source radius and magnitude.The calibration relationship of these parameters is fitted in this paper,which can be used as a reference for other researchers to study this area.The spatio-temporal relationship between stress drop and significant earthquakes above ML4.3 is studied in this paper.The results show that the high stress drop earthquakes after removing the influence of magnitude are in good agreement with these significant earthquakes in time and space,which has certain indication significance for the prediction of strong earthquakes.Last,we use the ETAS model fitting for the earthquake sequence of shale gas area in the south of the study area.The result show that the background seismicity in Changning shale gas mingling area is characterized by large fluctuation and high proportion.In this study period,the proportion of earthquakes induced by internal and external forces to the total seismic activity(f.s.)is about 79.1%,while the aftershocks triggered by earthquakes themselves are relatively small and the attenuation rate of aftershocks is fast.It conforms to the characteristics of induced earthquakes. |
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