Study On The Method Of Determining Asperity In Haiyuan Fault Under Multi-Source Observation Constraint

As an important model for explaining the seismic rupture mode,the asperity model plays an important role in studying the stress accumulation of faults and the location of earthquake initiation.The Haiyuan fault zone,as a large-scale left-lateral strike-slip fault zone on the northeastern margin of the Qinghai-Tibet Plateau,reflects the movement and geological structure of the Indian plate and Eurasian plate,and is an ideal test area for studying seismic rupture patterns.Therefore,it is of great scientific significance to accurately detect the distribution of asperity bodies in the Haiyuan fault zone and to study the geological tectonic movement and seismic activity of the Haiyuan fault zone.In order to study the distribution of asperity bodies in the Haiyuan fault zone,this paper first uses the b-value method to detect the asperity bodies in the Haiyuan fault zone,and uses the national seismic catalog data to calculate the b-value distribution in the Haiyuan fault zone.Considering the influence of the Haiyuan earthquake in 1920 and the Gulang earthquake in 1927,this paper divides the Haiyuan fault zone into the Tuolaishan-Lenglongling fault,Jinqianghe-Laohushan fault and Haiyuan fault(narrow sense).Seismic time(Haiyuan Earthquake,Gulang Earthquake)is divided to analyze the change of b value on the fault and the distribution of asperity bodies.The results show that the b value in the western part of the Tuolaishan-Lenglongling fault drops,and there are potential asperity;the b value in the middle and west parts of the Jinqianghe-Laohushan fault decreases significantly,and there is a high probability of the existence of asperity;the west of the Haiyuan fault(narrow sense)originally had asperity,but the Haiyuan earthquake released the stress accumulation at this location,then it became a none-asperity body.In order to further determine the boundary of the asperity body and study the distribution and tectonic movement of the asperity body in the Haiyuan fault zone,this paper uses the geodetic method to detect the asperity body in the Haiyuan fault zone,using the GNSS velocity field data from 1998 to 2018,based on the block model and triangular dislocation element,inverting the inter-seismic slip loss rate of the Haiyuan fault zone.The results show that the slip rate of the Haiyuan fault zone gradually decreases from west to east,from 7.31 mm / yr in the Tuolaishan fault to 4.58 mm / yr in the eastern segment of the Haiyuan fault.According to the distribution of dip-slip rate and strike-slip rate,it can be seen that from the Tolaishan fault to the eastern section of the Haiyuan fault,the tectonic activity of the Haiyuan fault zone gradually shifted from the thrust structural movement to the strike-slip movement.The inversion results of the slip deficit rate show that there is an asperity body in the western part of Tuolaishan-Lenglongling;there is an asperity body in the middle and west parts of the Jinqianghe-Laohushan fault;this distribution result is consistent with the result of the b-value method.In order to improve the accuracy of asperity detection,this paper combines b-value method and geodetic method to propose a multi-source observation data fusion detection method that accurately locates the asperity boundary named double threshold search method(b value + slip deficit rate).The method combines the seismic catalog data and the GNSS velocity field data,based on the criterion that the b-value asperity body boundary should be most consistent with the slip deficit rate asperity body boundary.Then the optimal threshold combination of b value and slip deficit rate is obtained through threshold search,and then accurately determines the asperity boundary.The results show that there are two asperities in the Haiyuan faults zone:(1)the east-west boundary of the Tuolaishan-Lenglongling fault asperity body is 100.22 °E-101.22 °E and the lower boundary depth is 5.60 km;(2)the east-west boundary of the Jinqianghe-Laoshan fault asperity body is 102.46 °E-103.44 °E and the lower boundary depth is 12.23 km.