Study On Focal Mechanism And Stress Field Distribution In The Anninghe Fault Zone And Its Surrounding Areas

The Anning River fault and the surrounding area are the middle sections of the eastern boundary of the Sichuan-Yunnan diamond-shaped block,The seismic activity has obvious segmentation features,and the distribution of faults is complex.There have been many earthquakes of magnitude 7.0 or above in history,and have been designated as key earthquake areas in recent years.Therefore,this paper will focus on the fault structural characteristics and stress field distribution in this area.Using CAP(Cut-And-Paste)and HASH methods and ML?1.0 high-precision seismic waveform data from January 2013 to December 2018(data from Xichang Mobile Seismic Observation Array,Yunnan and Sichuan seismic network),we have obtained 701 focal mechanism solutions.Then the source mechanism solutions with high quality were inverted to obtain the local stress field utilizing the spatial and temporal stress inversion(SATSI)method.By using Zmap software from the Matlab platform,we made the b value map of our research area by the maximum likelihood method.We will discuss the tectonic dynamics background and the seismogenic environment of the research area based on the results of this paper and the regional fault-locking features obtained from the GPS velocity field.The main conclusions of this article are as follow:(1)The seismic distribution in the region has obvious segmentation,and the earthquake is mainly concentrated in the area north of Mianning.The focal mechanism solutions of Lijiang-Xiaojinghe fault,Anninghe fault and Xianshuihe faule are mainly strike-slip and thrust type,while the strike-slip and normal earthquakes in the northern section of the Daliangshan fault dominate.(2)The maximum horizontal stress in the research area along the south end of the Xianshuihe fault southward through the Lijiang-Xiaojinhe fault,Anninghe fault and Limingjiu fault showed obvious clockwise rotation,from NW-SE in the north to NS in south,The type of stress field is from the south to the north from the thrust type to the strike type.In the northern part of our research area,the direction of the maximum horizontal stress shows a trend of counterclockwise rotation,which changes from NW-SE to near EW.However,in the northern section of the Daliangshan fault,the minimum horizontal stress dip angle is smaller than the maximum horizontal stress,and the seismic activity on the fault is mainly controlled by the NS tensile stress field.The Anninghe fault and surrounding areas are a clear stress field direction transition zone in the eastern source of the Tibet Plateau.(3)Combined with the source mechanism solution,the distribution of stress field and the previous research results of GPS velocity field,it is believed that the tectonic activity of the Anninhe fault and the surrounding area is mainly controlled by the interaction between the Tibet Plateautectonic block and the South China block,and the stress field rotates clockwise and is consistent with the distribution of the stress field over a large-scale background.This clockwise rotation appears as a thrust compression and strike-slip effect on the fault zone in the western part of the study area.However,in the east,it is the south-north tension to the Daliangshan fault,which reflects the clockwise rotation of the Sichuan-Yunnan block and the newness of the Daliangshan fault.(4)According to the b value map of our research area,the b value is lower in the area north of Mianning and higher in the south area.Due to the negative correlation between b value and stress,the analysis indicates that the level of underground stress accumulation in the northern part of the study area is higher than that in the south.The degree of fault locking in the northern part of the area is higher than that in the south,which is evidenced by the b value results,which also explains the segmentation characteristics of seismic activity.