Source Characteristics Of Small And Moderate Earthquakes And Tectonic Stress Field In Sichuan-Yunnan Region

The study area is located in the eastern margin of the Tibetan Plateau,which is a strongly deformed area of the collision and extrusion between the Eurasian and Indian plates.The region is characterized by multiple tectonic activities and the complex geological structure,which leads to the high frequency of earthquakes in the region,and is the natural laboratory to study seismogenic mechanisms as well as the key area for seismic hazard mitigation.In addition,the Sichuan Basin in the study area is one of the most important production areas for shale gas and salt mining in China,where large-scale salt mining and hydraulic fracturing of shale gas have led to a dramatic increase in seismicity of the industry and the surrounding area,and may have induced moderate earthquakes.Therefore,this study area is also an ideal area to investigate the mechanism of induced seismogenesis.The source characteristics of small and moderate earthquakes can provide important informations for the research of regional crustal stress state and rheological properties,the seismogenesis of strong earthquakes,fault distribution and geometry.In this thesis,we have obtained the results of focal mechanism and source depths of 1015M_L≥3.5earthquakes in Sichuan and Yunnan regions from 2009 to 2017 by using the generalized Cut-and-Paste（g CAP）method.The results show that there is obvious heterogeneity in seismicity on different active faults in the Sichuan-Yunnan region,and the seismicity is weaker in the Xianshuihe fault,the Anninghe-Zemuhe fault,the Xiaojiang fault,and the south-central segment of the Honghe fault,which ate the boundaries of the Sichuan-Yunnan block.The Sichuan-Yunnan region is mainly dominated by strike-slip earthquakes（55%）,and the majority of the earthquakes are distributed at source depths of 4～14 km.The normal fault earthquakes on the strike-slip faults may be related to the tensile or extrusion activity between the subblocks,or within the background of near-NS rightward shear,the tensile action generated by the overlapping of the clockwise rotational extrusion of the subblock with the tensile deformation at the end of the left-lateral strike-slip fault.In addition,the normal-fault earthquakes on the Longmenshan fault have a significant non-double-couple components component,and we suggest that these normal-fault earthquakes may be related to fluid effects,or they may be related to the tensional deformation between the Lixian hidden strike-slip fault and the Xiaoyudong strike-slip fault.The stress field of the Sichuan-Yunnan region are inverted by the stress tensor method with the focal mechanisms from our solutions and collected from published data.The results show that the majority of the Sichuan-Yunnan region is dominated by strike-slip stresses,and the maximum principal stressσ₁ is nearly horizontal.The normal fault stress is distributed near the Deqin-Zhongdian-Daju fault at the western boundary of the Sichuan-Yunnan block,and the NNE-SSW directional tensile effect is obvious.The direction ofσ₁ in the Sichuan-Yunnan region is generally bounded by the eastern boundary of the Sichuan-Yunnan block,and theσ₁ direction of the Sichuan-Yunnan block and the western region show clockwise rotation from the NE-SW direction of the Tibetan Plateau to the nearly NS direction of the Sichuan-Yunnan block,and then to the SSW-NNE direction of the Baoshan subblock in western Yunnan.At the eastern boundary of the Sichuan-Yunnan block,theσ₁ direction is nearly parallel in the NW-SE direction.In addition,the Longmenshan fault,the Xianshuihe fault,and the Northwest Sichuan subblock show complex stress distribution.Along the Longmenshan fault and the Xianshuihe fault,there are clear changes inσ₁ direction and obvious segmentation characteristics.The variation in theσ₁ direction of the Xianshuihe fault may reflect the evolution mechanism of the stress field.The change of stress field in the Longmenshan fault may be influenced by the accumulation of viscous materials from the middle and lower crust of the Songpan-Ganzi block at the margin of the Sichuan basin.The 2008Wenchuan earthquake also had a transient effect on the stress field.Within the Northwest Sichuan subblock,theσ₁ direction changes from NEE-SWW to NNW-SSE,indicating that the main force in this region changes from the lateral compression of the NE-oriented Indian block to the eastward migration of material from the Tibetan Plateau.Theσ₁direction in the Sichuan-Yunnan region is generally consistent with the direction of the GPS velocity field,which indicates that the tectonic stress field in the middle and upper crust is mainly related to the eastward migration of material from the Tibetan Plateau.In addition,the difference between theσ₁ direction and the GPS velocity field suggests that the stress field in the Sichuan-Yunnan region may also be influenced by the lateral compression of the Indian plate and the interaction between active blocks.In this thesis,the full moment tensor solutions,source depths,and fluid overpressure results of the 2019 Changning Ms6.0 earthquake and its Mw≥4.0 aftershocks are obtained by using the g CAP method,the depth seismic phase method,and the focal mechanism tomography method.A shallow source depth（1～4 km）is found for both the Changning mainshock and its strong aftershock.In addition,both the Changning mainshock and most of the strong aftershocks are characterized with significant non-double-couple.However,the 2019 Changning mainshock is dominated by the compensated linear vector dipole components,while most of the strong aftershocks are dominated by isotropic components.Meanwhile,the fluid overpressure of the 2019Changning mainshock is found to be much larger than those of most aftershocks.We infer that the mainshock is probably triggered by weakened fault strength with long-term fluid injection,and that its large non-double-couple components are associated with complex fault systems.For the strong aftershock,combining with the Coulomb stress analysis in the poroelastic medium,the aftershock may be associated with the change of poroelastic stress caused by the mainshock.