A Seismological Study Of Structure And Dynamics In The Earth

East Asia is located in the southeastern part of the Eurasian plate where four ma-jor plates are interacting with each other and form subduction zones, active orogens and broad continental diffusion that spreads thousands of kilometers. The present thesis fo-cuses on revealing the seismic structure beneath Mainland China and Northeast Japan by seismological studies, including seismic tomography, shear-wave splitting and stress tensor inversion. The results enable us to better understand the seismotectonics and geo-dynamics in these regions.Northeast Japan is one of the most active regions where many large earthquakes oc-cur frequently. Most of these large earthquakes occurred in the eastern margin of the Japan Sea and in the interplate thrust zone under the Pacific Ocean. To study the seis-motectonics in Northeast Japan, we determined high-resolution three-dimensional (3-D) velocity structure in the source areas of these earthquakes by jointly using the arrival-time data from many suboceanic earthquakes that are well relocated with sP depth phases. Many large intraplate crustal earthquakes occurred in or around low-velocity zones which may represent weak sections of the seismogenic crust. Strong heterogeneities are imaged above the subducting Pacific slab under the Pacific Ocean and most large thrust-type in-terplate earthquakes occurred in the high-velocity areas where the Pacific slab and the overriding continental plate may be strongly coupled. Compared with the adjacent low-velocity areas where fluids may exist, stress tends to accumulate near the high-velocity areas and leads to large earthquakes, such as the Tohoku-oki earthquake (M 9.0) occurring on March 11,2011.To better understand the geodynamics in the subduction zone, we determined high-resolution 3-D velocity structure as well as the anisotropic structure beneath Northeast iv Ph.D. Thesis, Nanjing University-by Z.Huang (DG0829022)Japan. A notable low-velocity zone is imaged in the mantle wedge with significant along-arc variations under the volcanic front, and it extends westward under the Japan Sea in the back-arc. P-wave anisotropy tomography reveals predominant E-W (subduction-parallel) anisotropy in the mantle wedge, which is consistent with the shear-wave splitting mea-surements for intermediate-depth earthquakes in the subducting slab. These results in-dicate that the fluids brought downward by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to surface by the up-welling flow in the mantle wedge. The subduction-parallel anisotropy is the result of the lattice-preferred orientation (LPO) of olivine caused by the back-arc spreading due to the subduction of the Pacific plate and the induced mantle-wedge convection. In contrast, high-velocity anomalies and nearly N-S (trench-parallel) anisotropy are imaged in the subducting Pacific slab. The anisotropy may either represent the original fossil anisotropy when the Pacific plate formed in the mid-ocean ridge or reflect the trench-parallel crys-tallographic and shaped preferred orientation in the subducting slab due to the slab bend-ing. Stress inversion result indicates that the maximal horizontal stress (WNW-ESE) is subparallel to the motion direction of the Pacific plate relative to Northeast Japan. The seismogenic layer in Northeast Japan is very weak because of the abundant fluids as well as the high-temperature magma beneath the volcanic front. Taiwan orogen resulting from the interaction between the Eurasian and Philippine Sea (PHS) plates is a subduction and collision zone. Teleseismic tomography using 5671 relative travel-time residuals in Southeast China indicates that the Eurasian plate is subducting eastward beneath the PHS plate under South Taiwan, while in North Taiwan, the subducted Eurasian plate is broken-off and the subduction is flipped, i.e., the PHS plate is subducting beneath the Eurasian plate. The subduction in East Asia may have driven extensive upwelling mantle flow from the lower mantle during the Cenozoic which results in extensive low-velocity anomalies in the upper mantle.Shear-wave splitting at 138 permanent seismograph stations provides important con-straints on seismic anisotropy and mantle dynamics under Mainland China. The results show that the fast orientations (p of the anisotropy (WNW-ESE) in eastern China are gen-erally consistent with the absolute plate motion (APM) direction of the Eurasian plate and the crustal movement revealed by GPS, suggesting that the anisotropy is mainly lo-cated in the asthenosphere resulting from the LPO of olivine due to the shear deformation there. The fast axes in western China generally agree with the strikes of the orogens and active faults, while they are perpendicular to the direction of the maximum horizontal stress, suggesting that the anisotropy in the lithosphere contributes significantly to the observed shear-wave splitting. The fast axes in western China are also consistent with the APM direction, suggesting that the APM-driven anisotropy in the asthenosphere is another source of the shear-wave splitting there. These results suggest that APM-driven anisotropy commonly exists under continents, similar to that under oceanic regions, even though the continental lithosphere has suffered extensive deformation.