Abstract:The western segment of the Qilian Mountains,which is controlled by some tectonic blocks,has complicated geography and geology.This study looked at 14 strong-moderate earthquakes and 66 small-moderate earthquakes from 2001 to 2012,used the stress tensor averaging method to calculate the direction of the mean stress axis in the area,and then analyzed the characteristics of the stress field.Moreover,we compared the results with another inversion result by using the FMSI inversion process.Finally,we conducted a detailed study of the tectonic stress field characteristics of the western segment of the Qilian Mountains.With the stress tensor calculation method,we obtained the average direction of T,B,and P axes by calculating the average stress tensor of focal mechanism solutions from multiple earthquakes.If the earthquake has a relatively independent discrete distribution in the block,we can deduce the direction of the principal stress axes in the regional tectonic stress field on the basis of the average direction of T,B,and P axes.In our research,we selected two groups of data,strong-moderate earthquakes and smallmoderate earthquakes from 2001to 2012.We obtained 14focal mechanism solutions for strongmoderate earthquakes occurring since 1900,with 4above MS6.0and 10between MS5.0and MS 5.9.Furthermore,another was the earthquake that occurred on December 25,1932,the largeChangma MS7.6in the western Changma fault.In addition,we used the CAP method and the vertical P-wave and S-wave amplitude ratio method to calculate the focal mechanisms of smallmoderate earthquakes.The focal mechanisms show that the characteristics of focus are mainly strike-slip and thrust earthquakes.The quantification results for stress tensor in three regions show that the azimuth of maximum principal compressive stress,σ1,is NE,and the horizontally compressed acts show evidence of definite subarea characteristics.
