| On April 20,2013,a strong earthquake of Ms 7.0-magnitude(hereinafter referred to as the Lushan earthquake)occurred in Lushan County,Sichuan Province,causing large casualties and property losses.The epicenter of the Lushan earthquake is located at the southernmost part of the Longmenshan fault zone,about 80 km away from the 2008 Wenchuan earthquake.Therefore it has drawn a lot of attention from seismologists and brought extensive researches,mainly including locating the mainshock and aftershocks,determining their focal mechanisms,inverting the fine structure around the aftershock zone,determining fault geometry and studying the rupture process.These studies have found that the mainshock of the Lushan Earthquake occurred at a depth of about 13 km,aftershocks basically distributed in the deeerp part(below 8 km),and there was no apparent rupture on the surface.The geometry of the Lushan earthquake fault is a"Y"type conjugate fault,and the mainshock is a thrust-type earthquake which occurred on the right blind reverse fault of the conjugate fault.The velocity model and slip distribution shows low velocity,high Vp/Vs and nearly no slip in the shallow parts of the Lushan aftershock zone,which may reveal the highly development of cracks,and the development of shallow fractures implied the existence of anisotropy,which has not been studied in detail.In order to better determine the anisotropic structure of this area,we have applied multiple methods to perform systematic and comprehensive research about anisotropy,mainly in the following three aspects:(1)A week after the 2013 Lushan earthquake,we deployed 29 temporary seis-mic arrays around the aftershock zone and have recorded about 4200 seismic events.About 1000 events with larger magnitude and clear S wave arrivals were picked to do shear wave splitting analysis.In order to obtain reliable shear wave splitting measure-ments,we applied two methods:cross-correlation method and eigenvalue method,to perform shear wave splitting analysis.And two different strict criteria were used to re-move "NULL" measurements obtained from those two methods.After removing those"NULL" measurements,the "High Quality" results of two different methods are con-sistent with each other,the fast wave polarization direction is generally close to the faults strike.However,by analyzing the fast wave polarization direction recorded at each station,we find that there are still some of them diverges from the fault strike,but nearly parallel to the regional maximum horizontal compressive stress direction.There-fore,we considered that upper crustal anisotropy around the Lushan aftershock zone is bi-modal,which are both structure controlled and stress induced,instead of a simple mechanism.The delay time between slow shear wave and fast shear wave is mainly between 0.05 and 0.15s,but does not increase with event depth.Since the aftershocks are basically distributed below 8-10 km,we speculate that the strong anisotropy in this area is basically located above depth 8-10 km.(2)The delay time between the fast and slow shear wave measured by the shear wave splitting analysis is the effect that accumulated along the ray path,thus can not give three-dimensional spatial distribution of anisotropy strength.To further character-ize the spatial distribution of anisotropic intensities,we used the measured slow wave time delays and perform three-dimensional shear wave splitting tomography.The three-dimensional anisotropic percentage model shows strong anisotropy above 8-10km depth but low anisotropy below it,validating the speculation in shear wave splitting analysis.From the fault-normal cross-section and horizontal section,it can be seen that two strong anisotropy stripes on the left and right are correspond to two branches of the conjugate fault,respectively,and the left strong anisotropy stripe has higher strength than the right one.This indicates that the anisotropy around the Lushan aftershock zone is likely sources of structure-controlled anisotropy.However,since this shear wave splitting to-mography method does not consider the fast wave direction,the spatial distribution of the fast shear wave direction remains to be studied.(3)Body wave traveltime anisotropy tomography is a method that can simulta-neously characterize the spatial distribution of anisotropic magnitude and fast direc-tion,but mostly uses P-wave traveltime only.Based on the newly developed body wave P and S traveltime anisotropy tomography method that uses both P wave and S wave,we achieved joint body wave traveltime anisotropy tomography based on existing P wave traveltime version.We constructed checkerboard model to perform synthetic test,the results show that considering the anisotropy effect can better resolve isotropic velocity model.The recovered anisotropic checkerboard model using both P and S wave travel time is better than only P wave travel time,with both anisotropy magnitude and fast direction.We apply this method to Lushan aftershock zone and obtain three-dimensional spatial anisotropic magnitude and fast direction distribution model.The anisotropic magnitude distribution is consistent with the anisotropic percentage model by shear wave splitting tomography,and has better resolution since the ray coverage is much denser.Strong anisotropy is mainly located in the shallow part(8-10km)of up-per crust,and the correspondence between two strong anisotropic stripes and conjugate faults can be seen more clearly.From the fault-parallel section,it can be seen that the anisotropy magnitude distribution along fault strike derived by two methods is different.The anisotropy magnitude obtained by the body wave traveltime tomography is more concentrated,while the result from shear wave splitting tomography is more divergent.The distribution of the fast direction shows that fast direction in strong anisotropic re-gion is relatively uniform and approximately parallel to the strike of the fault,while in weakly anisotropic parts the distribution of fast direction is more scattered,nearly paral-lel to the regional maximum horizontal stress direction.This agrees well with the results of fast shear wave polarization by shear wave splitting analysis,further illustrating the mechanism of anisotropy in this region is both structural-controlled and stress-induced.By synthesizing the results of velocity and anisotropy model in this paper,and comparing with the rupture distribution of the Lushan earthquake,it is found that large rupture anomalies are located in high velocity and weak anisotropic area,revealing that structure has a certain control on the distribution of larger earthquakes.For the first time,the three-dimensional anisotropic model around Lushan aftershock zone has been studied by using both shear wave anisotropy tomography and body wave travel time anisotropy tomography method.It has been found that two different anisotropy to-mography methods have obtained relatively consistent results,indicating that these two methods can reliably determine three-dimensional anisotropy distribution in the crust. |
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