Fine Velocity Structure Of The Longmenshan Fault Zone By Double-difference Tomography

On12May2008, the Mw7.9earthquake occurred on the Longmenshan fault inSichuan province of China which induced huge economic losses, huge casualties andgeological disasters. Researchers have done lots of studies on geological background,seismogenic structure, and mechanism of this earthquake. Geological surveys,inversion of surface rupture process, inversion of GPS and InSAR data, andaftershock relocation all indicate that the Wenchuan earthquake resulted from therupturing on a high-angle listric thrust fault. Although many results support this view,the morphology of this fault zone and the rupture mechanism remain controversial.The focused issue is how the fault extends in the deep crust, and what is thestructural context below the Longmenshan fault zone and adjacent areas.Geophysical exploration to the earth interior is the main technique to reveal deepstructure and seismogenic setting. Recently, with the rapid seismic networkdevelopment, double-difference seismic tomography has become a method for jointinversion of three-dimensional velocity structure of the fault and seismic eventrelocation. This technique is based on the double-difference location method andstandard tomography, making use of both absolute and more accurate relative arrivaltimes by neighboring earthquakes, to jointly inverse aftershock relocation and seismicvelocity structure. The double-difference seismic tomography has been widely used,such as inversion of fine velocity structure, high resolution imaging of subductingslabs and volcano velocity structure. In addition, high resolution velocity structure offaults obtained by this method is of great significance for understanding tectonicprocesses, earthquake generation, and earthquake interaction.A transportable dense seismic array with297broadband seismic stations wasdeployed in western Sichuan in October of2006by the State Key Laboratory ofEarthquake Dynamics, Institute of Geology, China Earthquake Administration, namedWestern Sichuan Seismic Array under the support of973-project. The transportabledense seismic array recorded a large number of earthquakes which allow an intensivestudy on the Wenchuan earthquake.In order to know more about the upper crust velocity structure and the faultstructure in crust, this thesis makes full use of P-wave travel time data recorded by theWestern Sichuan movable seismic array and the earthquake emergency responsestations during May2008to October2008and conducts joint inversion of upper crustvelocity structure below the Longmenshan fault zone and seismic event relocationwith double-difference seismic tomography. The purpose is to attain accuraterelocation of theWenchuan earthquake sequence and three-dimensional P-waveseismic velocity structure for a further research of the mechanism of the Wenchuanearthquake and to provide evidence for establishment of earthquake dynamic models.The results can be summarized as follows:(1) The joint inversion obtained the seismic event relocation similar to that based on the one-dimensional velocity model. The aftershocks relocation on the southsegment of the surface rupture zone is closely correlated to the surface faults. Whilethe aftershocks relocation on the north segment of the surface rupture zone iscontrolled by the velocity structure. The largest depths of aftershocks on the southsegment are slightly shallower than that on the north segment. Major aftershocks arelargely confined to above22km-24km depth with average depth about15~18km onthe entire aftershock zone.(2) The joint inversion yields the seismic P-velocity structure under theLongmenshan fault with depth range about0~30km. Velocity structure above15kmhas a close relation with surface faults. Velocity structure under20km indicates anortheast and northwest staggered pattern. The south segment of the Wenchuanearthquake rupture zone shows a high-velocity characteristic, but the bottomboundary is not found. Velocities at different depths indicate obvious northwestwardinclination of the Longmenshan fault. Comparing the seismic velocity structure withseismic event relocation, it is seen that the velocity heterogeneity is the main factorcontrolling aftershock distribution. High velocity in the southern Longmenshan faultzone corresponds to the thrust dominant area. And it is also a major factor influencingrupture propagation.(3) The joint inversion also shows that there exists an aftershock distribution beltfrom Xiaoyudong to Lixian, which is perpendicular to the strike of the Longmenshan.The velocity result also proves the existence of obvious lateral velocity variation. So itcan be predicted that there exists a buried fault in this direction which producedrupture at the surface of northeast segment. Seismic event relocation shows that thenortheast segment of surface rupture does not extend along the Qinchuan fault,velocity structure also proves that. Based on the results above, this thesis thinks thatthe northeast segment of the surface rupture zone extends along a new line. Lateraftershocks of the Wenchuan event happened primarily on this section.(4) The joint inversion results show fine seismic velocity structure and eventrelocation across the main Wenchuan earthquake area and is perpendicular to theLongmenshan fault zone. It indicates an obvious high velocity characteristic betweenthe Wenchuan-Maoxian fault and Yingxiu-Beichuan fault extending from surface to30km depth. The Wenchuan earthquake caused rock brittle fracture above20km. Theresults confirm that the Wenchuan earthquake was generated by a high-angle listricthrust, and the Guanxian-Jiangyou fault merges the Yingxiu-Beichuan fault at thedepth of~20km. The results also confirm the fault zone model proposed by previouswork. It is a pity that this work does not find the existing low velocity structure in thedepth range20km~30km. This work only uses catalog time data of the Wenchuanaftershocks to inverse the velocity structure. Most events took place above24km, so itis quite difficult to give a reliable constraint for lower crust. The boundary of the highvelocity zone under the Longmenshan fault and fine velocity structure need furtherresearch.