Research On Characteristics Of Seismic Activity At The Northern Area Of Red River Fault

On May 21,2021,the Yangbi M_S6.4 Earthquake occurred in Yangbi,Yunnan Province,which caused a large number of casualties and property losses.The earthquake is located on the north end of the Red River fault,two earthquakes above M_S5 have also occurred in 2013 and 2017 in the same area.The seismic activity in this area has increased significantly in recent years.The seismic research in this area is mainly focused on the single earthquake sequence in 2021.The study of the characteristics of seismic activity and its spatiotemporal evolution in this area for a long time scale,as well as the inversion of the focal parameters of typical strong earthquakes,are of great significance for understanding the tectonic settings and seismogenic mechanism in this area and can provide an important reference for the seismic risk assessment in this area.This dissertation studied the focal mechanism and earthquake source parameters of the main seismic events in the northern area of the Red River fault zone since 2013 based on the waveform data of seismic stations.The main contribution and conclusions are listed as follows:(1)The focal mechanism of three earthquake sequences is inversed by the CAP method,the results show that the M_W5.37 and M_W4.96 earthquakes in 2013 are both normal types,and the subsequent aftershock sequences are dextral strike-slip types.The M_W4.91 mainshock and M_W4.88 foreshock in 2017 are dextral strike-slip types,and the Yangbi M_W6.29 mainshock in 2021,which occurred about 15km southeast of the previous events,is also dextral strike-slip type,with about 70%of strike-slip events and30%of normal events in its fore-and aftershock sequence,we speculated that the seismogenic fault is the secondary fault of Weixi-Qiaohou fault.(2)We calculated the corner frequencies and stress drops of 17 typical strong earthquakes by spectral ratio method,the corner frequencies of 2013,2017,and 2021mainshock are 0.68±0.03hz,1.59±0.05Hz,and 0.415±0.01Hz respectively,and the stress drops are 11.98-1.52/+1.66MPa,39.84-3.64/+3.88MPa,65.35-4.61/+4.84MPa.The stress drops of 17 relatively strong earthquakes reflect the self-similarity of strike-slip events,while the possible fluid diffusion on the coseismic fault plane caused the self-similarity breakdown of the normal events.It may be due to the diversity of structural deformation,structure,and lithology on the fault,which caused great differences in friction coefficient,fracture scale,fracture fluid,and other properties in transversal and longitudinal fault plane.In terms of potential earthquake risk,the seismogenic process of strike-slip may trigger greater rupture and have greater potential damage.(3)We fitted the spatiotemporal ETAS model based on the earthquake catalog of Yunnan from 1970 to 2021.The background earthquake probabilities of the earthquake sequences in 2013,2017,and 2021 are 0.76,0.77,and 0.96 respectively.From the statistical law,the three earthquake sequences are more likely to be background earthquakes.We calculated the Coulomb stress changes excited by three earthquake sequences in 2013,2017,and 2021.The results show that there is no trigger relationship between the three earthquake sequences.The results of the ETAS model based on statistical law and Coulomb stress changes based on the physical model both show that the three earthquake sequences are independent background earthquakes,indicating that the region may be in a stable stress loading state.The result of Coulomb stress changes excited by the mainshock of the earthquake sequences in 2021 shows that the central and southern sections of Weixi-Qiaohou fault and the middle section of Eryuan-Midu fault are in the stress loading zone,which has a high seismic risk in the future.