Joint Inversion For Crustal Structure And Azimuthal Anisotropy:Application To The Sichuan-Yunnan Region,SW China

The Sichuan-Yunnan region is located in a transition site between the active Tibetan Plateau and the stable Yangtze craton,which has complicated tectonic deformation and severe seismic hazards.A detailed investigation of the crustal structure in SW China may play a key role in understanding the mechanism of crustal deformation and the dynamics of earthquake occurrence.In this thesis,we invert multiple seismological datasets jointly to constrain the crustal isotropic and azimuthally anisotropic structure in the Sichuan-Yunnan region.With Rayleigh wave dispersion,ZH ratio,P-wave receiver function and the initial model from well logging measurements and surface wave tomography,we perform stepwise-linearized joint inversion for the crustal shear-velocity model.Compared to previous tomography results,we observe higher shear-velocity in the sedimentary rocks within the Sichuan Basin,corresponding to direct velocity measurements by sonic logs of old and compacted Mesozoic sedimentary rocks in the shallow Sichuan Basin.Our model reveals widespread low-velocity zones in the mid-lower crust with distinct spatial variations,and some of the boundaries correlate well with major fault systems.Between two main mid-crustal low-velocity channels,a prominent high-velocity region surrounded by earthquakes is observed in the inner zone of the Emeishan large igneous province(ELIP)and around the Anninghe-Zemuhe fault zone.Based on the results,we suggest that mid-lower crustal ductile flow and upper-crustal rigid fault movement play equally important roles in controlling the regional deformation styles and earthquake distribution in SW China.Our results also resolve thick crust-mantle transition zones beneath the eastern Tibetan Plateau and the inner zone of the ELIP due to "top-down"and "bottom-up" crust-mantle interactions,respectively.Study of azimuthal anisotropy also helps understand the tectonic evolution.We estimate the crustal azimuthal anisotropy of the Sichuan-Yunnan region with a joint analysis of Ps waves converted at the Moho from radial and transverse receiver function data.We also perform the harmonic analysis and the bootstrap approach to test the reliability and stability of the measured anisotropy.Stations in the Songpan-Ganzi fold belt show large splitting times and well-aligned NWW-SEE fast directions.In contrast,stations in the Sichuan basin show very little anisotropy.In addition,the mid-crustal low shear-velocity zones are accompanied with distinct crustal anisotropy with fast wave direction parallel to the strike of the large strike-slip faults nearby.To further determine the source of azimuthal anisotropy in depth,we develop a Fortran package by using the neighborhood algorithm to invert for layered azimuthal anisotropy using both surface wave dispersion and receiver functions.We demonstrate the feasibility and reliability of the proposed method and codes with synthetic tests.Based on the isotropic velocity model derived in this thesis and the crustal layering results from deep seismic sounding,we apply this method to the real seismic data in the Sichuan-Yunnan region.Some stations in southeastern Yunnan show weak azimuthal anisotropy in the upper crust while strong azimuthal anisotropy in the mid-to-lower crust.We suggest that the azimuthal anisotropy in this region is caused by the alignment of the mid-to-lower crustal melts or minerals.