Study On The Physical Process And Seismogenic Mechanism Of The Yangbi Ms 6.4 Earthquake In Dali,Yunnan Province

The Ms 6.4 earthquake occurred on May 21,2021 in Yangbi County,Dali Prefecture,Yunnan Province,which was the largest earthquake after the 2014 Jinggu Ms 6.6 earthquake,in western Yunnan.After the earthquake,the rapid field investigation and earthquake relocation reveal that there was no obvious surface rupture and the earthquake did not occur on pre-existing active fault,but on a buried fault on the west side of Weixi-Qiaohou-Weishan fault zone in the eastern boundary of Baoshan sub-block.Significant foreshocks appeared three days before the earthquake.These phenomena aroused scholars’ intensive attention.What the physical process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake are revealed by the foreshocks and aftershocks?These scientific questions need to be solved urgently.In this study,we use various seismological methods to comprehensively dissect this typical "foreshock-mainshock-aftershock" sequence,analyse the seismogenic environment,seismogenic structure and focal parameter characteristics,and build the seismogenic model of this earthquake.It is of great scientific significance for us to effectively discriminate the foreshocks to an upcoming large earthquake,to better understand the mechanism of foreshocks,and to establish the indices for discrimination medium-strong earthquake risk in mainland China.We use seismic tomography method to obtain three-dimensional(3D)velocity structure of the Yangbi Ms 6.4 earthquake source area and analyse the seismogenic environment.Based on the refined 3D velocity model,spatio-temporal migration image of the earthquake sequence is obtained by using the microseismic detection and relocation method,and characteristics of the b-value with time are analyzed.We also determine focal mechanisms of medium-strong earthquakes and reveal the seismogenic structure and spatial distribution of stress field in the source area.Finally,we determine source spectrums and calculate stress drops and source radius of earthquakes,and understand source characteristics and analyse the trigger mechanism of foreshocks.The source process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake are comprehensively revealed by combining results of the 3D velocity structure,earthquake relocation,b-value,stress drops and rupture radius.The main conclusions are as follows:(1)The source area of the 2021 Yangbi Ms 6.4 earthquake is located in the high-Vs,low VP/VS region.High VP/VS anomalies are in its middle and lower crust.It indicates that the source area is located in the strong asperity where stress is likely to be accumulated.Fluids or partial melts may exist in the deep focal region,which weakened the seismogenic fault strength.The high VP/VS material in northwestern source region led to the earthquake sequence not extending to NW-trending.(2)We obtain-3 times earthquakes than the manual catalog in the source area based on microseismic detection and location method,and the completeness magnitude decreases from 1.2 to 0.5.The b-value of the earthquake sequence decreased gradually from 8 o’clock on May 21.After the Ms 5.3 largest foreshock occurred at 21:21,the b-value continued to decrease to about 80%of the background b-value,and the mainshock occurred 27 minutes later.Both the significant decreased b-value and the high consistency of focal mechanism solutions in the foreshock sequence reflect the enhancement of regional stress and seismic risk before the mainshock.At present b-value is slightly higher than the background level,indicating that the risk of large earthquakes is gradually decreasing.(3)The relocation result reveals that the rupture scale of the Ms 6.4 earthquake was～20 km,and the rupture was unilateral in the SE direction,and focal depths gradually increased from NW to SE.The focal mechanism solutions show that the earthquake sequence was dominated by the high-dip strike slip earthquakes,with a few normal faulting earthquakes.The seismogenic fault of the Ms 6.4 earthquake is a NW-trending dextral strike-slip with normal component,dipping SW.Aftershocks reveal that there were two NW-trending faults in the SE segment of the source area.The source area was controlled by NNW horizontally compressive and NEE horizontally extensional stress.(4)High resolution spatio-temporal migration image were obtained by using the absolute location method and waveform cross-correlation relative location method,which reveals that foreshocks can be divided into three stages since May 18,2021.The first two sequences extended horizontally toward the Ms 6.4 mainshock,and gradually migrated to the upper crust within 10 km in depth.The third sequence was distributed discretely.Combined with results of stress drops and rupture radius of 334 earthquakes with M 1～5 magnitude obtained from source spectra,it reveals that the Yangbi Ms 6.4 earthquake may have both pre-slip and cascade model during nucleation process:The migration of the first two sequences and the largest Ms 5.3 foreshock may be related to aseismic slip,while the mainshock may be triggered by cascade model.