Gansu and its adjacent areas are on the northeastern margin of the Qinghai-Tibet plateau. The study area locates at the junction of Alashan,Qilian,Qaidam and Ordos blocks. The tectonic structure is complicated in this area. The research of tectonic stress field in the study area has an important scientific significance for earthquake prediction of Gansu. Characteristics of present-day tectonic stress field is studied based on 245 M_L≥3.5 focal mechanism solutions since January 2001 to June 2012 in Gansu and its adjacent areas.
For solving the stress field,we follow Wan's(2000)method to solve the rake angle according to two nodal planes' strikes and dips of the focal mechanisms. We treat the stress tensor solution of each fault plane as the optimal solution of the area when the slip on the fault plane minus the angle between shear stress direction and strike is minimal. We use F test to get the confidence interval of the four parameters, as well as the confidence interval of the principal compressive and tensile stress axes. We divide the whole area into 1°×1°grids. In order to cover the whole study region and get the smoothed stress field,we select the data within the square areas of 2°×2° centered at the grid point. The direction and the relative size of the tectonic stress field in 2°×2° grid in Gansu and its adjacent areas are presented for the first time. These results show that the dip angles of maximum principal compressive stress axes of the whole region are generally small. And the dip angles of maximum principal tensile stress axes vary widely,which are small in the western part,and relatively larger in the east. It indicates that the tectonic stress field with horizontal action of compression acts as the major in the west. Many deep large strike-slip faults in Qilian seismic belt correspond with the results. The geological structure of southeastern area of Gansu is complex,and the characteristics of stress field indicate that strike-slip reverse faulting is likely to occur. Result of the inverted tectonic stresses also shows that maximum principal compressive stress axis strikes near NE on the plate margin of Tibet plateau. This is caused by Qinghai-Tibet plateau moving to the northeast. At the outer edge of the Tibet plateau plate,its strikes show a radiated pattern,i.e. NS in west part,NNE in the middle section,and NNW in the east. The reason is that Qinghai-Tibet plateau,when moving to the northeast,is encountered with stable Ordos and Alashan blocks,therefore the material moves to southeast. The stress directions show that the source of the stress field in Gansu and its adjacent areas is extrusion of the India plate to Eurasian plate. Our inversion results are similar with the previous results,indicating the correctness of the approach. The relative stress values are more than 0.5 in the western areas and less than 0.5 in the eastern areas. According to Wan's research(2011), the maximum principle compressive stress solutions are more reliable in the western area,while the maximum principle tensile stress solutions are more reliable in the eastern areas.
The results indicate a non-uniform stress field spatial distribution in Gansu area,and have certain reference significance for explaining the seismo-geological background and earthquake prediction research.

In this paper,we present a method which allows to calculate the mean stress field according to the total seismic moment released by earthquakes.The exact method is as follows: First,we calculate the scalar seismic moment released by each earthquake according to the statistical relationship between earthquake magnitude and its seismic moment; Second,we calculate the seismic moment tensor released by each earthquake according to the relationship between focal mechanism solution and seismic moment tensor; Then,we can get the total seismic moment tensor released in a specific time period of the study area; Finally,we calculate the eigenvector and eigenvalue of the total seismic moment tensor,the obtained eigenvector is corresponding to the mean stress field direction released by the study area. We tested the method by using the synthetic focal mechanism to which random error was added and with the focal mechanism data of Tangshan aftershock zone.The testing results show that,the released stress field of the study area obtained by our method is in consistency with the regional stress field. So our method can be applied to solve regional stress field.The more focal mechanism data used,the more stable the result would be,and closer to the real regional stress field. One of the advantages of this method is that it uses magnitude as the weight of each earthquake,so the contribution difference of the earthquake size in the stress field inversion can be better reflected. Another advantage is that it does not need to know which nodal plane of the focal mechanism is the real fault plane when we calculate stress field.

The precisely located earthquake catalogue is important to seismicity, seismic tomography and crustal stress inversion studies. It also has great application value in rapid report of an earthquake that just occurred. By considering the arrival time uncertainty, and the constraints on station elevation and seismic depth, we propose a relatively accurate method to estimate hypocentral location and its uncertainty based on inversion theory. Our method can combine the arrival times of Pg wave, Sg wave, Pn wave and Sn wave in hypocenter location, so it increases the location accuracy by involving more data; and it can be also used in local and regional earthquake location simultaneously. In order to test our location method, we located earthquakes by using the simulated data with different uncertainty of Pg,Sg,Pn,Sn arrivals. The result shows that the location determined by using our method is more accurate than that by using other method. We apply it to earthquakes occurring in the period from 2001 to 2008 in Sichuan area, and obtained a more clustered hypocentral distribution convergent to the fault zones. The result provides a solid foundation for studies of seismicity, geometry of the active faults and seismic tomography in Sichuan region. It is also helpful to study the seismicity precursors before the Wenchuan earthquake.