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The Tectonic Stress Field in Southeastern Area of Gansu Province Deduced from P Wave Polarity Data

BU Yu-fei1,ZHANG Yuan-sheng1,WAN Yong-ge2,LIU Xu-zhou1,GAO Jian1(1.Lanzhou Institute of Seismology,CEA,Gansu,Lanzhou 73000,China; 2.Institute of Disaster-Prevention Science and Technology,Hebei,Sanhe 065201,China)  
To determine the crustal stress field based on the basis of earthquake waves,we analyze data from fixed seismic stations in Gansu and surrounding provinces and the broadband portable seismic array in southeast Gansu province,where three blocks converge to create complex tectonic conditions.Two methods used to directly obtain such stress fields include those based on focal mechanism data and p-wave first motion data.In order effectively use small and microscale earthquake data,the latter method is used in our study.On the basis of near earthquake observation data recorded from November 1,2010,to November 30,2011,we use Zhang's Method to relocate earthquakes occurring in this area.As a result,2 379 earthquakes and 12 129 p-wave polarity data are obtained through which the directions of principal stress axes are inferred.The inversion method and program are provided by Wan.Our study assumes a relative stress value of 0.5.We calculate only the 2-D stress field because most earthquakes have occurred in the research area at depths of 5~20 km;the average depth of 12 km was chosen as the calculating depth.The direction of tectonic stress field in a 0.25° × 0.25° grid in this area is presented for the first time.The orientation of P,B,and T axis azimuths are obtained by 1° × 1° × 1° grid searching.We treat these azimuths as stress flied directions when the contradictory ratio is the smallest.Results show that the maximum compressive stress axis strikes are presented in a radiated pattern;that is,they strike east-west from NEE in northern part to SEE in the east.These findings are similar to that reported in previous research,through which we can infer that the Qinghai-Tibet plateau moving to the northeast encountered the stable Ordos block and the South China block moving to the southeast;thus,the material moved southeast.The earthquakes occurring in this region were fewer and smaller than those west of 104° E;therefore,the stress field inversion result is primarily obtained from smoothing the results of surrounding areas.However,further analysis is necessary to explain why the principal compressive stress axes are steeper here;such research will be the focus of our next investigation.Maximum principal compressive stress axes are steep on the southwestern margin of the Ordos block likely because the material moved east along the West Qinling fault;therefore,the stress environment is tensional.Because the broadband portable seismic array in the southeast area of Gansu province covers density,the constraint mechanism and inversion results are reliable.Detailed characteristics and a relatively reliable direction of the tectonic stress field in southeast Gansu province are obtained in this research.We expect that our results are significant to geological background explanation and earthquake prediction research.
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