Study On Spatio-temporal Pattern Evolution Of Small And Micro Earthquakes Before And After Large Earthquakes Based On Spatio-Temporal Statistics

Earthquake is one of the most harmful and influential geological disasters in the world.In a short time,the occurrence of earthquakes may not follow the law,but by using a large number of seismic monitoring data for statistical analysis,we can explore the regularity of seismicity in a certain region.Some studies have showed that the spatial distribution of seismicity can be used as a guide to potential source area.Therefore,the study of space-time distribution pattern of small and micro earthquakes before large earthquakes has certain practical significance to the monitoring of large earthquakes.In a period of time after the big earthquake,the earthquake epicenter and the surrounding area often accompany a large number of aftershocks,some even cause a large number of casualties,which bring difficulties to the earthquake relief work.Understanding the temporal and spatial variation of aftershocks is helpful to scientifically guide post-disaster reconstruction and disaster prevention and mitigation.Therefore,the study on the evolution of the time and space pattern of aftershocks is also crucial.The data of small and micro seismic monitoring has the characteristics of time,space and attribute,which can be used to study the distribution pattern of the seismic point from the spatial statistic angle.Based on the seismic monitoring data,the spatial statistical analysis method is used to study the spatial and temporal patterns of the 3-year small and micro earthquakes before and after the Lushan 7 ML earthquake.Firstly,the variation of geographical distribution pattern of 3-year small and micro earthquakes before and after the Lushan 7ML earthquake is measured by mean center index and standard deviation elliptic index.The experimental results show that(a)the average center of small and micro earthquakes in the first 9 months before the earthquake of the Lushan 7-magnitude gradually approached the epicenter of the earthquake.In the 1th year after the earthquake,the average center of the small and micro earthquake is close to the epicenter of the earthquake and then gradually away from it;in the 2th year after the earthquake,the average center of the small and micro earthquake is gradually away from the epicenter of the earthquake.From the spatial distribution,as time goes on,the average center of small and micro earthquakes in the 3 years before the earthquake is constantly moving to Lushan County.(b)In the 1th year before the earthquake of Lushan 7ML,the concentration of small and micro seismic points is increasing,the spatial distribution of the direction of auxiliary axis is shrinking.The concentration degree of small and micro seismic point is decreasing in the 2 years after the earthquake,the spatial distribution of spindle direction and auxiliary axis is expanding.In the past 3 years before and after the earthquake,the 3-years small and micro earthquakes are generally distributed in the North East-south West pattern,coincide with the direction of Longmenshan fault zone.Then,the kernel density estimation method is used to analyze the temporal and spatial pattern evolution of 3-year small and micro earthquakes before and after Lushan 7ML earthquake.The experimental results show that although there are differences in number of small micro earthquakes in different time fragments before earthquakes,the nuclei of small micro earthquakes in the first 3 years before the earthquake of Lushan 7 earthquake are mainly distributed at the junction of Wenchuan County,Dayi County and Congzhou,and present the situation of Wenchuan agglomeration and other neighboring counties dispersed.In the 1th year before the earthquake,the nucleus gradually spread to the Lushan county,Lushan county from the original no concentration center slowly formed to Lushan county.In the 3 years after the earthquake,the concentration of small and micro seismic points in Lushan county has gradually weakened.Finally,spatial autocorrelation analysis is used to study the spatial distribution patterns and spatial autocorrelation of small and micro earthquakes in the past 3 years before and after the Lushan 7 magnitude earthquake.The experimental results show that the spatial autocorrelation of the small and micro earthquakes in the 1th-year study area before the earthquake is increasing,and the spatial distribution pattern is gradually becoming the aggregation mode from the stochastic mode.And the spatial autocorrelation of the small and micro earthquakes is gradually weakened in the 1th-year after the earthquake,and the spatial distribution pattern is the aggregation mode.The spatial distribution pattern of the small and micro earthquakes in the 3 years before the earthquake with 0.10 significant level is mainly the aggregation pattern,and the spatial distribution patterns of the small micro earthquakes in the 3 years after the earthquake with 0.05 significant levels are all aggregation patterns.To sum up,the standard deviation ellipse method and the spatial autocorrelation method can reveal the gradual increase of the small micro-seismic agglomeration in the 1th year before the earthquake,the trend of decreasing the aggregation of small micro-earthquakes within 2 years after the earthquake.By using the nuclear density estimation method,it is concluded that Lushan county formed a concentrated area centered on Lushan county from the original non gathering center in the first years before the earthquake,In the 3 years after the earthquake,the agglomeration of small and micro earthquakes in Lushan County is gradually weakened.The results of this paper can not only provide a statistical reference for the study of the temporal and spatial patterns of earthquakes by using seismic monitoring data,but also provide supporting decision support for earthquake prediction,disaster prevention and mitigation and post-disaster reconstruction.