Study On Seismic Activity Characteristics Of Baihetan Reservoir And Nearby Areas Before Impoundment

Seismic activity in high seismic intensity area has attracted wide attentions of seismologists both at home and abroad.With the development of engineering technology,China have built lots of large water reservoir dam in the high seismic intensity area of Western China.The big capacity in high intensity area,may bring potential earthquake risks.Once the earthquake occurs,it is likely to be"small earthquake big disaster".Few earthquake research has been done in the Baihetan Reservoir area before the impounding.Under such background,it is nessary to know the siesmic activity in the Baihetan Reservoir and its surrounding area before the impounding,so as to provid references for the reservoir earthquake research after the impounding in the Baihetan Reservoir and the surrounding area.We select the catalogues from China Earthquake the Networks Center from200901 to 201705,and analyze the characteristics of regional seismicity and the spatial distribution of seismic activity parameters.The seismic data of Qiaojia array from Institute of Geophysics CEA,are used to study seismic precision location and seismic source parameters.The main conclusions of this paper are as follows:（1）The major seismogenic structures of strong earthquakes in the study area are the southern segment of Zemuhe Fault,the northern segment of Xiaojiang Fault,the Zhaotong-Ludian Fault and Yiliang Fault.The present seismicity（20090801-20170531）is mainly distributed in the Zhaotong-Ludian Fault and the northern section of the Xiaojiang Fault,and there are many shallow earthquakes around the Zhaotong City,Qiaojia and Dongchuan County,which may be related to human engineering activities.With the Yiliang Ms5.7 and Ludian Ms6.5 earthquakes,the seismicity of the study area is increasing gradually.（2）After deleting aftershocks,the present earthquake catalogue has calculated the completeness of earthquake magnitude Mc is 1.6,indicating that the seismological records above the M_L1.6 are complete and the b value of the whole study area is 0.966.On this basis,the spatial distribution of seismicity parameters a,b,Mc and R values are analyzed.The seismicity in Qiaojia and Ningnan County near the Baihetan Reservoir is strong,and the small earthquakes around Yudong Reservoir may be the result of the joint effect of the water saturation of the surrounding heterogeneous rock mass and the Longshu Fract.（3）Using the seismic data of the Qiaojia array in three years,7132 earthquakes in the study area is relocated.The spatial distribution characteristics of the seismic source and the relationship between the seismic source and the reservoir or the active structure are analyzed.The results show that there are two aseismic regions on both sides of the reservoir area.According to previous experience,after the storage of the Baihetan Reservoir in the future,it is possible to observe the changes of the seismic activity.（4）According to the principle of good surrounding station and good waveform record,32 events were selected to calculate the Q value and site responses.Compared with other results,the results show thatQ（f）=45.82f^0.926presents a lower Q value and a high attenuation coefficient,reflecting the lower degree of uniformity of the media in the Baihetan reservoir area.In general,the magnification of sites vary steadily between1⁴,and site response of some stations is significantly enlarged in a certain frequency band,which may be related to the special environmental conditions,topography and geomorphology.（5）On the basis of known Q values and site responses,another 860 events（M_L1.0⁴.5）were calculated.The results show that the relationship between the local magnitude and the moment of the earthquake or the radiated energy or moment magnitude are strong,but the relationship between the local magnitude and the stress drop or rupture radius is weak.Overall,the five increase with the magnitude of the earthquakes.The results of stress drop indicate that the stress drop in Baihetan reservoir before impounding is higher than that in other areas.Through the sequence diagram of stress drop,it is found that before the M_L4.5 earthquake,the M_L3.0³.9 earthquake occurred higher stress drop.