The Velocity And Anisotropy Structure Of The Upper Mantle And Crust In Some Active Tectonic Regions

The crust and upper mantle are the main areas where the plate movements, ocean spreading, earthquakes, volcanoes, and geothermal activity occur. In-depth study of the structure of the crust and upper mantle is necessary to investigate the dynamic processes of the earth and the occurrence of disasters such as earthquakes and volcanoes. Ssing seismic data through a variety of geophysics methods, the research of the velocity and anisotropy structure of the crust and upper mantle plays an important role of geosciences. In the past a few years, global and regional seismic data increased rapidly with the development of digital seismographs and computerized processing capability, which present excellent opportunity to understand the fine structure of the crust and upper mantle. For the uppermost mantle, the Pn and Sn phases which critically refract at the Moho surface and propagate along the uppermost mantle are the best choice to study the velocity and anisotropy structure of the uppermost mantle. The Pn and Sn phases can provide higher ray coverage density and high-resolution imaging of velocity and anisotropy. For the crust, the surface wave can provide a wealth of information of different depths. Especially with the ambient seismic noise correlation technique, short period surface waves from seismic noise became useful to study the shallow crustal structure.The Tibetan Plateau region is the most important continent-continent collision zone, the tectonic activities and complex structure here make this region a hot spot of earth science. The tectonics of the area is currently controlled by the collision and continuing convergence of the Indian Plate and the Eurasian plates. The velocity and anisotropy variations of the uppermost mantle can provide constraints on the processes of continental deformation and differentiation, thus, a tomography study of the Tibetan Plateau region will be useful to better understanding of the history of plate movement and other important questions. We improved the tomography method and used large seismic data from the Chinese Earthquake Network catalogs and data from the International Seismological Centre catalogs, and then obtained high-resolution results of velocity and anisotropy structure of this region. The average Pn velocity under the study area is approximately 8.15 km/s and Sn velocity is 4.60 km/s. Generally, high velocities are found to be under the Indian Plate and in the Tarim and Sichuan basins, while low velocities are found under Myanmar Yunnan region and the Afghanistan region. An especially low Pn velocity zone is found under the area north of the Indus-Yarlung Zangbo suture. The low-velocity anomaly is probably associated with volcanic activity, and is likely a result of the higher temperatures and/or partial melting associated with volcanisms. Our inversion results also support the theory that the lithosphere of the Indian Plate subducted into the mantle, causing the upwelling of hot material in the north of the IYS. High velocity anomalies in the Indian Plate and low velocity anomalies in the Tibetan region are discontinuous at the collision region in the east-west direction, which suggests that the Indian Plate may subduct in different blocks. The preferred alignment of olivine crystals, caused by the creeping of material in the uppermost mantle, is believed to contribute to Pn velocity anisotropy. At the plate collisions, the fast Pn anisotropy direction is parallel to the collision arc. The anisotropy at the plate boundary may be caused by shear strains along the boundary; and the shear traction on the base of the lithosphere could orient the fast axis in olivine crystals parallel to the shear direction. The fast Pn anisotropy in the eastern Tibetan Plateau suggests that the preferred alignment of olivine crystals is in parallel to the plate movement direction. In eastern Tibet, the fast direction of Pn anisotropy is generally the same as the SKS splitting fast direction, this suggests that the material flow at the uppermost mantle is coupled with the deeper parts.Iran and the surrounding area are extremely complex due to the stacking of pre-, syn-, and post-collision tectonics structures in the region, creating an ideal area for studying a geologically young continent–continent collision belt. The current tectonic state of the region is controlled by the collision and continuing convergence of the Arabian Plate towards the Eurasian Plate. In this study, we present Pn velocity and anisotropy tomographic models obtained using both the new Iranian data and data from the ISC catalogs. This large dataset provides high-resolution results of velocity and anisotropy, and, therefore, more insight into the processes involved in continental deformation and differentiation. The average Pn velocity of this area is 8.0 km/s, the Sn velocity is 4.55 km/s. High velocity values occurring beneath the Zagros Mountains and the Caspian Sea. Under the Alborz and Caucasus regions, clear low velocities are found. The Arabian plate moved northward and subducted beneath the Iranian plateau. The old oceanic lithosphere, which has a higher seismic velocity, subducted under this region. The low Pn and Sn velocity anomaly under the Alborz and Caucasus regions may be associated with volcanic activity. The fast Pn direction is consistent with the crustal movement at the Caucasus region, implying the stress conditions exist in the uppermost mantle. At the Caspian Sea, the fast directions were probably frozen in during the formation of the oceanic lithosphere. Under the plate collision region, the fast Pn anisotropy direction is parallel to the collision arc and the strike of large reverse faults due to cross-fault compression and along-fault extension. We also get the crust model of the Iran region by P wave travel time curves.The Yellowstone volcano is the largest active volcano of the world, monitoring and researching of this area is an important issue for earth scientists. Previous studies showed that low velocity layer exists in the crust suggesting that magma exists in the crust here. The study of the magma feature will help us better understand the law of volcanic activities. We use ambient seismic noise from regional stations and get the short period surface wave through cross correlation, then study the shallow structure of Yellowstone region with surface wave dispersion analysis. We get a crustal model of the outside area of the Yellowstone volcano first, then study the S wave velocity of a low-velocity layer which been discovered by receiver function method. The results show that the velocity of this layer is about 1.3km/s, the surface wave dispersion curve which calculated from this model is most consistent with the observed curve. This study also proves that large scale magma exists in the shallow crust of Yellowstone volcanic region.