Detailed Temporal And Spatial Distribution Of Induced Earthquakes By Water Injection In Rongchang, Chongqing

Rongchang area,Chongqing,had exhibited low levels of naturally occurring seismicity in the history,and there was no record of damaging earthquake.However,following the injection of unwanted water from gas production,seismicity has increased dramatically and showed progressive increasing of magnitude since July 1988,even with earthquakes of magnitude above 5.Rongchang area is thus an idea site for studying seismicity induced by water injection.Studying seismic characteristics in Rongchang area is not only of theoretical significance for research on the distribution and controlling factors of induced seismicity but also of practical value for analyzing earthquake potential in Rongchang area.It also has reference function for research on the mechanism and forecasting of tectonic earthquakes.Based on the seismic data collected from reginal permanent stations and 6 temporal stations,we analyzed the seismic activity from October 2008 to July 2011 in Rongchang area.On the basis of HypoDD relocated results,we used Match&Locate method to detect and located the micro-earthquakes.We obtained the focal mechanism solutions of some earthquakes with 3.5 by using gCAP method.Then we analyzed the temporal-spatial distribution of earthquakes and discussed the characteristics of micro-seismicity before 5.1 occurred on September 10,2010.The main results obtained are as following.(1)We totally detected 3354 micro-earthquake events,which are nearly 5 times more than earthquakes in the seismic catalog issued by China Earthquake Networks Center.The magnitude of detected events is mostly from-1 to 1,and the focal depth is from 2 to 4km.The magnitude-frequency analysis shows that the catalog completeness is obviously improved after adding the detected earthquakes,with decreasing the lowest magnitude from 1.0 to 0.3?(2)Seismicity in Rongchang area showedthe characteristics of low magnitude and high frequency.The earthquakes hypocenters were mainly clustered along faults or buried faults and in a dominant depth range consistent with the depth of injection wells.It also shows a tendency to extend toward lateral from injection wells.The statistics of seismic parameters showed two periods with different characteristics.This indicated that seismicity was closely related to the diffusion of disposal water,that is,the fluid diffusion caused increasing of pore pressure and triggered the reactivation of the preexisting faults.The evolution of fault reactivation affected the seismic characteristics.(3)The focal mechanism solutions of 9 earthquakesof 3.5presented reverse faulting,as the same as the preexisting faults,indicating that earthquakes were surly related to reactivation of the faults.That the causative faults' striking,dip and rate separated in wide ranges indicated not only obvious changes in structure and striking of preexisting faults but also the effect of increasing pore pressure on the local stress field.(4)Before 5.1occurred in September 10 of 2010,seismicity firstly showed clustering in time and covered the most part of the seismogenic fault in space.Then an obvious seismic quiescence occurred and lasted about 3 months.The phenomenon is consistent with the mechanism of creep sliding and resistance-uniformization along fault zone,suggested on thebasis of laboratory experiments,and it may be one of patterns of sub-instability along fault zone.