摘要
The Yangbi M_(S)6.4 earthquake occurred on May 21,2021 in western Yunnan,China,where moderate earthquakes strike frequently.It exhibited a typical“foreshock-mainshock-aftershock”sequence and did not occur on a pre-existing active fault.The seismogenic environment and mechanism of this earthquake have aroused considerable research attention.In this study,we obtain the three-dimensional v_(P),v_(S)and v_(P)/v_(S)images using the v_(P)/v_(S)consistency-constrained double-difference tomography method,which improves the accuracy of v_(P)/v_(S)models.We focus on characteristics of v_(P)/v_(S)images in areas with a lateral resolution of 0.1°,and reveal the seismogenic environment of the Yangbi M_(S)6.4 earthquake.The conclusions are as follows:(1)Low velocity and high-v_(P)/v_(S)anomalies are revealed at different depths around the northern segment of the Red River fault.v_(S)and v_(P)/v_(S)images along the Weixi-Qiaohou-Weishan fault and the buried faults on its west show obviously segmented feature.(2)The source region of the Yangbi M_(S)6.4 earthquake is located in a low-v_(P)/v_(S)zone implying high medium strength.High-v_(P)/v_(S)anomalies in its NW direction indicate cracks development and the existence of fluids or partial melts,which are unfavorable for stress accumulation and triggering large earthquakes.Such conditions have also prevented the earthquake sequence from extending northwestward.(3)With the southeastward extrusion of materials from the Tibetan Plateau,fluid migration was blocked by the low-v_(P)/v_(S)body in the source region.The high-v_(P)/v_(S)anomaly beneath the source region may implies that the fluids or partial melts in the middle and lower crust gradually weakened medium strength at the bottom of the seismogenic layer,and preparing the largest foreshock in the transition zone of high to low v_(P)/v_(S).Meanwhile,tectonic stress incessantly accumulated in the brittle upper crust,eventually led to the M_(S)6.4 earthquake occurrence.
基金
This work was jointly supported by the National Key R&D Program of China(No.2021YFC3000700)
the National Natural Science Foundation of China(No.42174066).
