宁夏地区小震重定位
Relocation of Small Earthquakes in the Ningxia Area

应用双差地震定位法与遗传算法进行定位计算,并对结果进行比较,发现其具有一致性和差异性。根据宁夏地区数据资料的实际情况,采用遗传算法收集整理1991年以来该区地震震相记录资料,获得至少具有3个S-P台站以上记录的地震事件共4 688个,重新定位结果基本勾画出地震在块体边界的空间展布特征;地震深度主要位于5~25 km的上、中和下地壳中上部,基本反映陆陆板块的活动边界形态。
Ningxia and its neighboring area are located in two blocks of the northeastern margin of the Qingzang-Tibet plateau:the Erdos block and the Alashan block.The geological structure in this area of the plateau is known to be unstable.Over recorded time,many large or great earthquakes have occurred in this area.Based on previous research results,we employed two different earthquake location methods,i.e.,double-difference and genetic algorithm,to recompute the location of small earthquakes that were recorded by the Ningxia Regional Earthquake Monitoring Network from 1991 to May 2014.This process consisted of first employing the theoretical travel time method to delete or modify the seismic events whose arrival time error exceeded 10 s.Next,we selected seismic events that contained S and P waves recorded by at least three stations.In addition,Pg,Sg,Pn,and Sn seismic phases are included in each earthquake event.The velocity model was obtained from four seismic sounding profiles that crossed the research area.In addition,a horizontally layered velocity model was used.The double-difference method results showed that the seismic source depth was not so different from that obtained using the genetic algorithm method.But only a few earthquake events (approximately 25% of the total 4 688 seismic events) can meet the relocation requirement in the double-difference method.Therefore,the genetic algorithm method can be used to recalculate and reanalyze the depth characteristics of small earthquakes and their relation to the deep fault in the research area.Relocation results showed that the seismic zone distribution by depth is more apparent.Statistical analysis,taking the relocation calculation into account,showed that more earthquakes happened from 5 to 30 km.The average depth was 18 km.The space distribution of small earthquakes in this area was obtained.To facilitate the analysis of the relationship between the occurrence of earthquakes and faults in this region,we divided the geographic research region into four districts on the basis of their tectonic characteristics and concentration region of small earthquakes.These districts were named A,B,C,and D. In each district,the seismic profile perpendicular to fault was obtained.In district A,the results showed that the seismic depth was mainly distributed from 5 to 25 km.The earthquake distribution width was 50 to 60 km.In