Seismic Observation And Analysis In Southwestern Yunnan

Controlled by geodynamic background of India-Eurasia collision, Sichuan and Yunnan area in southwestern China develops intensive tectonic activities, and develops double arc rotational tectonic system which comprised by Sichuan Yunnan outer arc zone and western Yunnan inner arc zone. Western Yunnan inner arc zone is one of the area which has the most complex tectonic framework and strongest seismic activity.For the surface geology investigation is hard to decide the attitude of active faults in depth, a portable seismic network with dense short-period seismeters deployed by Inst. Geomechanics, CAGS in Ruili during2010.3-2011.3, and the purpose is research the spatial distribution, correlations and seismic activity of major active faults in the area by microearthquake data. We got527microearthquakes and their focal parameters in Ruili area through generic processing such as earthquake location and error analysis. By analysis of the spatial distribution of these microearthquakes, we suggest that the seismic activity in Ruili area is not strong recently, and the activity of Wanding fault is higher than Longling-Ruili fault. Besides, there shows a high seismic activity N-S tectonic belt, southeast of Ruili.Fortunately, our seismic network recorded the Ms5.9earthquake occurred at the Yingjiang town on March10th,2011.-60km far away from Ruili, but also recorded the foreshocks and aftershocks. All the earthquakes were located firstly with the HYPO71and then relocated by HYPODD. The results show that seismic activity in the Yingjiang area became higher since December,2010; then three earthquake swarms and several foreshocks stronger than Ml4.0occurred before the mainshock; Both the mainshock and all the foreshocks stronger than Ml4.0occurred at/around the intersection of the NNW-SSE Sudian Fault and the NE-SW Dayingjiang Fault. Vertically, foreshocks and aftershocks aligned in a SE-dipped belt around the intersection of the two faults, indicates that the Dayingjiang fault dips SE at/around the mainshock. Although Focal mechanism of the main shock indicated that the Dayingjiang Fault is the seismogenic fault of the mainshock, however, most foreshocks and aftershocks distribute as a belt along the NNW-SSE Sudian Fault, indicating that the seismicity of Yingjiang during2010-2011is mainly controlled by the Sudian Fault. In order to understand why mean that the seismogenic fault of the mainshock is different with the regional active seismogenic fault, we analyzed the relationship between regional strong shocks happened in2008-2010and the2011Yingjiang earthquake by Coulomb Stress analysis. At first, the strong shocks increased coulomb stress in a plane parallel to the Sudian fault in a large area along/close to the Sudian fault at the depth of-5km, which can interpret why the foreshocks and aftershocks of the2011Yingjiang earthquake distributed along the Sudian Fault. The three strong shocks, especially the Ms6.1earthquake occurred at Sudian Fault, increased coulomb stress at the intersection of the Sudian Fault and the Dayingjiang Fault, at both the possible fault planes of the2011-Yingjiang-earthquake focal mechanism down to10km deep, however, the increase of coulomb stress along the plane parallel to the Dayingjiang fault is obviously higher than the other plane which is close to the strike of the Sudian Fault, which can interpret that although the seismic activity of the Sudian Fault is stronger than the Dayingjiang Fault, the Dayingjiang Fault is the seismogenic fault of the mainshock...