Inversion Of Coseismic Fault Slip Distribution Of 2013 Lushan Ms7.0 Earthquake Based On Triangular Dislocation Model

The refined fault slip distribution model provides an important basis for exploring the causes of earthquakes and studying the rupture mechanism,and is an important guide for earthquake emergency rescue,regional earthquake trend prediction,and earthquake disaster prevention and mitigation work.April 20,2013 at 8:00,the Ms7.0 earthquake erupted in Lushan,Sichuan,China,causing significant loss of life and property.This earthquake is similar to the Wenchuan Ms8.0 earthquake in that it is a destructive earthquake formed by the extrusion and collision of two major blocks during the eastward movement of the Qinghai-Tibet Plateau due to the support and blockage of the Sichuan Basin.Unlike the Wenchuan earthquake,the Lushan earthquake is a blind reverse-fault type earthquake with a complex fault structure,and oversimplification of the geometric structure of the fault will have a greater impact on the reliability of the inversion results of the coseismic slip distribution and on the interpretation of the rupture process.In this paper,the geometric structure and coseismic slip distribution of the Lushan earthquake fault are studied using the results of 33 continuous GPS stations and 22 leveling points in the earthquake area with the observation of coseismic deformation and a total of 1993 aftershocks from April 2013 to May 2014.The main research work and conclusions of this paper are as follows:(1)The geometric parameters of the single plane fault of the Lushan earthquake were inversed using Bayesian method based on GPS data,and the results of the inversion showed that the seismogenic fault of the Lushan earthquake had a strike of 207.56° and an inclination of 43.22°.By combining the results of aftershock precise localization,a curved fault structure was constructed on the basis of the single plane fault,in which the dip angle of the fault gradually decreases with depth,and its dip angle changes continuously from 63.9° high dip angle at the surface to 15.8° low dip angle at the bottom,reflecting its characteristics of steep at the top and slow at the bottom.(2)Based on the geometric model of the curved fault,the inversion of the isoseismic sliding distribution based on the triangular dislocation model using the Helmut variance component estimation method combined with GPS and level data shows that the Ms7.0 earthquake in Lushan is dominated by an obvious reverse motion,with only one rupture peak area and no obvious rupture directionality.The fault rupture surface has no obvious slip near the surface,and the maximum slip is 0.98m,located near the depth of 13.5km.The released energy is 1.05×1019 Nm,and the corresponding moment magnitude is Mw6.63.The fitted residuals of GPS and leveling data are 2.8mm and 7.9mm,respectively,which are reduced by 1.3mm and 1.6mm compared with the planar rectangular dislocation.(3)The effects of different fault geometries and dislocation models on the coseismic slip distribution of this earthquake are investigated.Based on the rectangular dislocation model,the maximum slip in the inversion is 0.68m under the planar fault structure,and the residuals of GPS and level data fitting are 4.1mm and 9.5mm,respectively;under the curved fault structure,the maximum slip in the inversion is 0.91m,and the residuals of GPS and level data fitting are 3.2mm and 9.0mm,respectively.Based on the triangular dislocation model,the maximum slip of the inversion is 0.97m(regular triangle)and 0.91m(irregular triangle)under the condition of planar fault structure,and the residuals of GPS and leveling data fitting are 3.1mm and 8.8mm(regular triangle),3.2mm and 9.0mm(irregular triangle).The inversion results show that the maximum slip based on the triangular dislocation model is larger than that based on the rectangular dislocation model for both planar and curved faults,and the triangular dislocation model fits better,which indicates that the curved fault model constructed by using triangular dislocation elements is not only closer to the real fault production,but also can better explain the surface observed displacement.