Study On The Characteristics Of Reservoir-induced-seismicity In The Lower Reaches Of Jinsha River

In this paper,we use the seismic data recorded by the reservoir network in the lower reaches of the Jinsha River from 2016 to 2018,to study the relocation,activity parameters and source parameters of these earthquakes,in order to analyze and summarize the general characteristics of reservoir seismicity.First of all,we use the double-difference method to relocate the earthquakes in the region in order to get the accurate location information of the earthquakes.Double-difference location requires accurate arrival time difference of the same seismic phase,and there must be a large error in the earthquake report of manually picking up the seismic phase,so we first use the cross-correlation algorithm to correct the partial arrival time difference,and then bring it into the double-difference location.The location results show that the earthquakes are mainly concentrated in the following areas from north to south:(1)the Yanjin-Mabian fault,which is distributed in the northwestsoutheast direction;(2)the Xiluodu reservoir area,along two parallel northwest-southeast belts;(3)the Baihetan dam site,which is distributed in clusters,and the overall depth is shallow;(4)the Xiaojiang fault zone is distributed in the northwest-southeast along the fault zone.;(5)in the aftershock area of the Ludian M6 earthquake,the earthquakes are the most concentrated,and distributed along two vertical south-north and east-west belts;(6)in the Huize basin,the earthquakes are scattered and not banded,and the overall depth is shallow.Then we use the maximum likelihood method to calculate the spatial distribution of b-value in the region.We first find the most suitable method by comparing a variety of methods to calculate the b-value,and then apply it to the calculation of the b-value in the region.Before the calculation,we use the K-K method to delete the aftershocks in the catalogue to ensure the independence of the events.The results show that:(1)the b-value in the aftershock area of the Ludian M6 earthquake is obviously lower than that in the surrounding area,which is consistent with the variation law of-b value after the large earthquake;(2)there are two obvious high b-value areas near Huize basin and Zhaotong,indicating that the earthquakes in these two areas mostly occur in the form of microearthquakes.;(3)the b-value of Xiluodu reservoir area does not show the high b-value which is common in reservoir earthquakes,but is close to the-b value of Yanjin-Mabian fault and Xiaojiang fault.Finally,we use the spectral ratio method to calculate the source parameters of earthquakes with M≥2.0 in the region,we calculate 197 main events with level 2.0 above and about 500 reference events corresponding to them.The calculation results show that:(1)there is a very strong correlation between the earthquake moment and the magnitude,while the stress drop is mostly concentrated in 0.1-10 MPa,and the correlation with the magnitude is not obvious,but roughly shows a trend of increasing with the magnitude;(2)compared with the earthquakes near the fault zone,the stress drop of several earthquakes in Huize basin is obviously lower;(3)the earthquake in Xiluodu reservoir area does not show the common low stress drop of reservoir earthquake,but is similar to the earthquake on Yanjin-Mabian fault and higher than that on Xiaojiang fault.Based on the above results,we mainly draw the following conclusions:(1)the results of previous studies show that,compared with natural earthquakes,reservoir earthquakes or other earthquakes induced by water infiltration,except for a few banded distribution along the fault,most of them show the characteristics of clustered distribution in some specific parts,and shallow depth,in addition they higher b-value and smaller stress drop;(2)the earthquakes near Huize are basically consistent with the above characteristics,while the earthquakes in Ludian aftershock area show almost completely opposite characteristics.combined with the local actual situation,there are a large number of hot springs in Huize basin.we can think that they are the typical representative of reservoir earthquake and natural earthquake respectively;(3)there is a high correlation between seismicity and water storage in Xiluodu reservoir area,so they can be regarded as reservoir earthquakes,but their b-value and stress drop obviously do not accord with the above characteristics,that may be related to the strong tectonic stress in southwestern China;(4)there is an obvious stress concentration near Baihetan Dam,so the seismic risk after impoundment in the future should be paid more attention to.