关键词: 漾濞M_S6.4地震, 震源机制, 构造应力场反演, R值, 相对应力值

Abstract:

According to the determination of China Seismological Network, at 21:48:34 Beijing time on May 21, 2021, an M_S6.4 earthquake occurred in Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province. The epicenter(25.67°N, 99.87°E)is located on the southwest boundary of the Sichuan-Yunnan rhombic block, with a focal depth of 8km. On the basis of the survey results of the Yunnan Eartquake Agency, the highest intensity in the earthquake area is VIII degree, and the long axis of the isoseismic line is NW-striking. We calculated the focal mechanism solutions of 11 M_S≥4.0 events of May 21, 2021, Yangbi earthquake by the CAP method. Combined with the collected focal mechanism solutions of the surrounding historical earthquakes, we inverted the tectonic stress field around the epicenter and its adjacent areas and simulated the relative stress value and the distribution of the focal mechanism solution in the Yangbi area. We analysis and studies the characteristics of tectonic stress field variation in the study area and its relationship with earthquakes, dynamic significance and seismogenic mechanism. The results show that: 1)The Yangbi earthquake sequence and the focal mechanism solutions of historical earthquakes near the epicenter are mostly of strike-slip type, followed by the normal-fault type. 2)The Yangbi earthquake sequence is mainly distributed in the depth range of 4~8km. The depth of earthquakes in the study area is mostly above 15km underground, and they mainly occur in the brittle upper crust. The tectonic stress field is strike-slip type in the Yangbi source area, and the maximum principal stress is the NNW-SSE direction, which is basically consistent with the known regional tectonic stress field. The regional inversion results show that the maximum principal stress axis(σ₁ axis)of the Sichuan-Yunnan rhombic block in the northeastern part of the study area is the NNW-SSE direction, and the minimum principal stress axis(σ₃ axis)is the NEE-SWW direction. In the Lanping-Simao block, the σ₁ axis becomes nearly NS, and the σ₃ axis is close to EW. For the Tengchong and Baoshan blocks in the southwest part, the σ₁ axis rotates in the NNE-SSW direction, and the σ₃ axis is in the NWW-SEE direction. Judging from the uncertainty range under the 95%confidence level of the maximum principal stress azimuth, the variation range of the inversion results of most grid points in the study area is within 20°, indicating that the inversion results are relatively stable. In general, the orientation of the maximum principal stress axis and the minimum principal stress axis show a clockwise rotation trend from northeast to southwest. And the maximum principal stress axis direction distribution is similar to the GPS horizontal velocity field and other stress data results. The stress orientation inside the Sichuan-Yunnan block, Tengchong and Baoshan blocks is relatively consistent. But the stress changes in the block boundary zone, showing a certain difference, which may be caused by the difference in the dynamic action, movement mode and speed of the block. The regional tectonic stress field is affected by the interaction between different blocks. 3)The R-value increases gradually from northwest to southeast in the study area. And the increase in the R-value shows that the intermediate principal stress is mostly characterized by tensile stress, indicating that the compressive stress required for material migration decreases relatively. Combined with the geological tectonic background of the study area, it is considered that the movement speed of the block(material)gradually slows down, which is consistent with the surface deformation observations. According to the simulation results of the relative shear stress and relative normal stress of the Yangbi earthquake, the NW-oriented nodal plane of the Yangbi earthquake is the seismogenic fault plane of the earthquake. Combined with the seismic relocation and tectonic stress field inversion results, We analyzed the seismic mechanism of the Yangbi earthquake as follows: Under the NNW-nearly NS-trending tectonic stress, the NW-trending subvertical faults intersect with the regional principal compressive stress at a small angle, resulting in right-slip shearing along the optimal release nodal plane, and finally rupture leads to the occurrence of earthquakes. The Yangbi earthquake is located in the southwestern boundary zone of the Sichuan-Yunnan rhombic block. In recent years, the 2013 Eryuan M_S5.5 and M_S5.0 earthquakes and the 2017 Yangbi M_S5.1 earthquakes were occurred along this boundary zone. The seismic activities here are very significant. The occurrence of moderate to strong earthquakes near the Sichuan-Yunnan rhombic block may indicate that the main active faults at the block boundary have accumulated a high level of elastic strain energy. In addition, the Yangbi earthquake manifested as a right-lateral strike-slip seismic activity on the NW-trending fault plane, and it is located southwest of the Sichuan-Yunnan rhombic block. It just shows that the Sichuan-Yunnan rhombic blockhas a SE-direction translational motion, which is consistent with the kinematic model of the Qinghai-Tibetan plateau material escaping eastward in the form of a rigid block, accompanied by a certain clockwise rotation, which is consistent with the long-term tectonic movement direction of the block. The Yangbi earthquake occurred under the dynamic background that the Sichuan-Yunnan rhombic block is continuously squeezed by the Qinghai-Tibetan plateau material. These research results have reference significance for understanding the seismic background and dynamic mechanism of the Yangbi earthquake.

Key words: the Yangbi M_S6.4 earthquake, focal mechanism, tectonic stress field inversion, R^{}value, relative stress value