区域宽频地震波形反演所揭示的2022年青海门源M_(S)6.9地震震源破裂过程

Rupture process of the 2022 M_S6.9 Menyuan,Qinghai earthquake revealed by inversion of regional broadband seismograms

基于有限断层模型反演方法,利用区域宽频带数据反演了2022年1月青海门源M_(S)6.9地震的震源破裂过程,并结合地质构造与地震重定位结果判断发震断层走向.综合反演结果表明:此次地震的发震断层走向为WNW向,主要以走滑为主;破裂主要发生在震源两侧,可能存在着双侧破裂,在震后2 s和9 s出现破裂极大值,最大错动量约为1.5 m,位于深度约6km处,发生明显破裂的深度约为16 km,地表破裂长度约20 km;此次地震释放的标量地震矩为1.23×10^(19)N·m,相当于矩震级M_(W)6.7,地震能量主要在前15 s释放;发震断层面的倾角为84.6°,接近于垂直,由于破裂范围较大,所以发生明显错动分布的地表投影也长达34 km.

Finite fault inversion is an effective method commonly used to study the rupture process of earthquakes and has been widely applied to seismic source rupture process research.In this study,we utilized the method to investigate the rupture process of the M_S6.9 Menyuan,Qinghai earthquake in 2022 using regional broadband seismic waveform data.Considering the quality and distribution of the network data,we selected 20 regional seismic stations with high signal-to-noise ratio and epicentral distances less than 500 km.We preprocessed the observed waveforms by removing instrument response,demeaning,detrending,and filtering.We also calculated the near-field Green's functions using the frequency-wavenumber(f-k)method.First,synthesize the waveforms using the Green's function synthesis theory and compare them with corresponding observed waveforms.Then,perform fault model inversion where the displacement response of each sub-fault at any station can be represented as a function of the sub-fault's slip,direction,rise time,and rupture velocity.Additionally,in the process of reducing errors,we use wavelet transform to establish the objective function and use simulated annealing(SA)to search for the global optimal solution of each sub-fault parameter,so that the waveform residual can be minimized in the wavelet domain,ultimately obtaining the best spatio-temporal distribution model of the source fault slip.In this study,high frequency signal of regional network is used.Low-frequency information constrains the overall characteristics of the earthquake source,but is less sensitive to detailed rupture features such as rupture rise time or rupture velocity changes.However,sudden changes in slip amplitude or rupture velocity will radiate intense seismic signals with higherfrequency.Therefore,using higher-frequency signals for finite fault inversion can effectively improve spatial and temporal resolution,and fully utilize the wide-band information of seismic waves to better understand the earthquake source rupture process.The following results were obtained:(1)The seismic fault is nodal planeⅠ(strike 104.2°,dip 84.6°,rake-5.4°).The rupture is mainly concentrated in two areas.The first area is a circular region with a radius of approximately 3 km above the hypocenter,with a maximum displacement of about 1.5 m,located at a depth of about 6 km underground,where the depth of obvious rupture is about 16 km.The earthquake caused a rupture to the surface with a maximum displacement of approximately 0.5 m,and the surface rupture length is about 20 km.Second,along the strike direction,the circular area with a radius of about 3 km is at the lowerright of the hypocenter,with a maximum displacement of approximately 1.2 m,at a depth of about 14 km.(2)The scalar seismic moment released by the earthquake is 1.23×10^(19)N·m,corresponding to M_W6.7.The earthquake rupture lasts about 17 s,with the maximum release of energy occurring around 8 s.The energy was mostly released before 15 seconds.(3)From the perspective of rupture direction,the rupture mainly propagates along the ESE direction.The maximum rupture values appear on both sides of the epicenter at 2 s and 9 s,reflecting the characteristics of bilateral rupture.According to the surface dislocation distribution map,the dip of the seismic fault surface is 84.6°,which is close to vertical.Therefore,the surface projection with obvious dislocation distribution is as long as 34 km.(4)Two obvious features can be identified:first,the fault plane has a large dip and releases a high amount of energy,resulting in significant surface ruptures with a length of approximately 18 km;second,there is bilateral rupture,accompanied with large displacements observed on both sides of the epicenter at 2 s and9 s.The regional tectonic position also confirms that the initial rupture occurred along the Tuolaishan fault in an approximate E-W direction and triggered the NW-SE trending Lenglongling fault,which is closely related to the complex tectonic environment in the area.