The Analysis On Focal Mechanism Solutions Of Minor Earthquakes Near Zipingpu Reservoir

The Zipingpu Reservoir is located at the turn of the Minjiang River on the central fault of the Longmenshan fault zone and is a special part of structural deformation.Zipingpu Reservoir has frequently experienced small intensive earthquakes after water storage started on September 30,2005.Most of these small earthquakes are concentrated in the reservoir area(103.3°N~103.7°N;30.85°E~31.15°E).According to the location of the small earthquakes and the characteristics of the focal mechanism solution,it is clear that the reservoir seismic group changes with the impoundment process in the regional tectonic setting,and the evolution process of the seismogenic structure and the incipient stress field with time and space is clarified.At the same time,the analysis of the origin mechanism of reservoir-induced earthquakes is of great theoretical and practical significance for whether the reservoir earthquake induced Wenchuan earthquake of magnitude 8.According to the seismic activity recorded in the dedicated seismic station network of Zipingpu Reservoir,people used the double-difference localization method to reposition the small microseismic(Lu Xian et al.,2010;Ma Wentao et al.,2011;Yan Yong et al.,2014)to determine most of the microseismic events.The focal depth is within 10 kilometers.The majority of focal mechanisms of the micro-seismicity of the Zipingpu reservoir identified by the P/S amplitude ratio method are thrust-type mechanisms(Hu Xianming et al.,2009).However,the traditional P/S amplitude ratio or P wave initial motion method to calculate the focal mechanism is limited to the limited use of observation data,leading to problems such as low accuracy and poor applicability.The CAP(Cut and Paste)full waveform fitting method can make full use of observation waveform data and perform progressive fitting of theoretical waveforms and observed waveforms.In the process,weights of body waves,surface waves,and other epicenter distances are given different weights.The utilization rate of observation data of the dedicated network of reservoirs has been increased several ten times,which has effectively improved the accuracy of the focal mechanism solution.Therefore,it is more reliable and universal.Using the CAP method can also be used as a test for the results of the original double-difference location and P/S amplitude ratio method.It is very common that the CAP method is used to calculate the focal mechanism solution for earthquakes with moderate to strong earthquakes and above.However,the application of the CAP method to the calculation of microseisms is still lacking relevant research cases.The main reason is that the microseismic observation data in the general regional network The signal-to-noise ratio is low,and it is believed that the reliability of the results is not strong,and the large amount of calculations has been ignored.However,the seismic stations for reservoirs have a small space,which is more than ten kilometers.The maximum amplitude recorded by the near station of the second-order small earthquake is comparable to that recorded by the regional station at the level 4-5.Therefore,this study believes that the CAP software can be used to calculate the 2-level focal mechanism solution using the reservoir-specific network,and it has high reliability.This paper provides a good example of the CAP method for calculating the focal mechanism solution of small earthquakes.In this study,CAP fitting calculations were performed on 61 short-cycle digital seismographs from 2005 to April 2008.The results were compared with those obtained by previous methods,and they were better in earthquakes with larger magnitudes.Consistency.Based on the analysis of the results of the seismic source solution for the Zipingpu reservoir seismic calculation,we found the following characteristics of the reservoir earthquake: First,the seismic events were mainly concentrated in four areas of Dujiangyan,Shuimo,Shenxigou and Taoziping,showing the characteristics of the earthquake swarm;Second,the focal mechanism solution is dominated by the thrust-type mechanism,which is the same as the thrust faults of the central fault and the Qianshan fault in the Longmenshan fault zone.Third,the seismic distribution of the earthquake swarm is mainly concentrated in Longmen Mountain from the reservoir profile.On the central fault zone and the Qianshan fault zone;Fourth,from the focal plane distribution map of the source stress of the focal mechanism solution,it is found that the direction of principal compressive stress is mainly perpendicular to the direction of the central fault,and is partially parallel to the fault,and the direction of the seismogenic fault is mainly In the northeast direction,it is in the same direction as the central fault and the frontal fault of the Longmenshan fault zone.Fifth,the change in the direction of the principal compressive stress from the principal compressive stress azimuth angle and the water level change over time is narrowed after the impoundment.Tend to the range of 180 to 270 degrees from the vertical frontal fault or central fault strike.Sixth,the focal source solution parameter dominant distribution rose diagram shows the maximum principal compressive stress.South East,consistent with the regional tectonic stress field in the direction of the main-to the north west.The small earthquakes from February to April 2008 were mainly concentrated in the vicinity of Dujiangyan.Their role in regional tectonics and their relationship with the Wenchuan earthquake are worth studying.Then we expanded the range of magnitude screening to obtain 28 focal mechanism solutions for reservoirs in the reservoir region(103.3°N~103.7°N;30.85°E~31.15°E)with a magnitude greater than ML 1.0 from February to April of 2008.The inversion results show that: First,from the source spherical distribution map,19 seismic events can be seen in the vicinity of Dujiangyan in the southeast direction of the Zipingpu Reservoir.The largest earthquake is the thrust type,and other earthquake-generating mechanisms carry the slip component in the thrust direction.,A small amount of components with normal faults;Second,from the depth map of the source ball,we found that the focal depths of the earthquakes are concentrated in the region of 13 km or more below the ground.Among them,the Dujiangyan earthquake clusters are clustered on the Qianshan fault zone,and other earthquakes are scattered on the central fault zone.3.From the change of the direction of the maximum principal compressive stress over time,it can be seen that the direction of the maximum seismogenic stress field changed from 150° to 350° during the period from February to March 2008 and decreased to 210° when the Wenchuan M8 earthquake struck.Between 250°,there is a phenomenon that the principal compressive stress axis tends to coincide with the regional structural stress field.The direction of the principal compressive stress direction from disturbance to regional structural tectonic stress field indicates that there will soon be a large earthquake(Gui Guiwei et al.,2011),that is,there is often a synergy of the seismogenic tectonic stress field before the major earthquake.Process(Ma et al.,2014).The direction of this main shock stress from random distribution to the trend of consistent changes before the Wenchuan earthquake shows that there is a convex-concave body near the Dujiangyan in the Longmenshan fault zone,which forms a stress concentration point.The activities of the Dujiangyan earthquake swarm led to the thrusting activity of the Qianshan fault in the Longmenshan fault zone,which intensified the activities of the central fault of the Longmenshan fault zone,accelerating the occurrence of the Wenchuan earthquake to some extent.Therefore,the Zipingpu water impoundment caused the reservoir earthquake,further modified the seismogenic structure,and finally intensified the activities of the central fault of the Longmenshan fault zone,accelerating the occurrence of the Wenchuan earthquake to some extent.