Research On Accurate Location And Formation Mechanism Of Micro-earthquake In Dongqing Reservoir Of Beipanjiang,Guizhou Province

Dongqing Reservoir is located at the junction of Zhenning County and Zhenfeng County in Guizhou Province.It began to accumulate water in August 2009.Beipanjiang Seismic networks recorded a large number of micro-earthquakes after the impoundment of Dongqing reservoir.Attributed to the low accuracy of the fast earthquakes location algorithm used in the network,if we wanted to research the seismic activity of the reservoir area more deeply,a more accurate seismic activity location algorithm is needed.Therefore,this paper introduces a more accurate method,double differential location method,to relocate by the records.Accuracy results can provide more information for understanding the patterns of earthquakes happened in Dongqing reservoir and more geophysical evidences for understanding the generation mechanism of Dongqing reservoir in Beipanjiang.Firstly,the theory of double difference location method is applied to relocate the seismic events in the monitoring area from October 1st,2010 to December 31 th,2016(105.4°-106.0°E,25.2°-25.7°N)and accurate positioning results of 2144 events are obtained.After relocation,RMSCT decreases from 0.2126 s to 0.0723 s,and the mean positioning errors of EW,NS,vertical are 184.5m,208.5m and 303.5m,so the results were well improved.Secondly,based on the results,this study analyzes the seismic activity.It is shown that seismic activity were gradually enhancing after impoundment.The happening frequency of earthquakes in Dongqing reservoir area decreased from February 2010 to November 2015,and the seismic activity were weakened.During that time,Dongqing reservoir area entered into the strain accumulation period.In the end of 2015,the seismic activity suddenly increase,which may be due to Dongqing reservoir's strain having been accumulating to a higher level after the release of strain energy.Then,according to the rule of seismic activity,spatiotemporal evolution mode of the zone was analyzed.It is indicated that,when impounded initially(1 Feb,2010 to 30 Sep,2010),the eastern and northeastern side of the dam appeared a number of earthquakes with shallow depth of hypocenter,which is strongly related with the process of impoundment.With the time of impoundment goes by,the earthquakes seems to migrate to the southern Bacao fault of the dam.In addition,with the passage of water,the earthquake has moved away from the dam farther and deeper.Finally,according to the spatiotemporal evolution mode,combined with tectonics,lithology and the change of water level,the generation mechanism of earthquakes is discussed.When impound initially,the main factor of the induced earthquakes were water load of reservoir;after impoundment when it is stable,the main factor was porous pressure caused by permeation of water in reservoir.On the basis of the distribution of hypocenter depth of earthquakes clusters near the fractures,the inclination and dip angle can be calculated.