Seismic Tomography In The Crust And Upper Mantle

Seismic tomography, one of the most powerful scientific research tool for the exploration of earth’s interior structure, has profound impact on the process of earth science development. In this study, we used a large number of high-quality arrival times recorded by the seismic stations located in Alaska subduction zone and Longmen-Shan Fault belt to image the deep structures under the two regions, respectively.On one hand, the Alaska subduction zone, is a typical region of the oceanic plate colliding with continental plate, is selected as an example for determining the seismic images in the crust and upper mantle. For the first time we used 562,891 P- and 156,321 S-wave arrival times data to have determined Vp and Vs structures simultaneously in the crust and upper mantle in Alaska. At the same time, the Poisson’s ratio(σ) images were calculated from the inverted velocity models. Our tomographic results provide crucial information for a better understanding of seismic structures and the geometry of the subducting slabs under south of Alaska. The P-wave velocity is similar to that of the S-wave velocity and coincides well with the Poisson’s ratio image. Strong high velocity(Vp, Vs) and low Poisson’s ratio(σ) anomalies with 200 km depth extension are clearly imaged along the Alaska subduction zone, indicating the subducting Pacific and Yakutat slabs. The high-velocity and low-(σ) zone show well consistent with the spatial distribution of the earthquakes along the slab subducting. Most of large earthquakes(M>6.5) are located in the boundaries between the high- and low-velocity zones, suggesting strong interplate coupling of the subducting slabs. Widely lowvelocity anomalies with high Poisson’s ratio are revealed in the mantle wedge of the subduction zone, which might reflect the magmatic flow associated with the slab subduction. Our study indicates the dipping angle of the slab changes from steep in the Wrangell Block to flat under the Bristol Bay in south of Alaska, which is associated with the Yakutat plate under the Wrangell Block and the Pacific plate subducting under the Bristol Bay. A strong low-velocity body in the upper mantle is observed under the conjunction of the Kenai Peninsula and the Kodiak Island, suggesting the collision of the Yakutat and Pacific plates as well as the strong upwelling of the mantle asthenosphere that might have thinned the subducting plate there.On the other hand, the Longmen-Shan fault belt, which is a typical continent-continental collision region, is selected as a study area for crustal and upper mantle seismic tomography.To investigate the generation mechanism of the Lushan earthquake and its relation to the 2008 Wenchuan earthquake(Ms 8.0), 50 temporary seismic stations were installed in the source area following the Lushan earthquake. Crustal stress data were also collected along the Longmen-Shan Fault zone(LMFZ) to reveal its influence on the generation of the Lushan earthquake. Seismic imaging and crustal stress analysis have revealed that the Lushan earthquake occurred in a distinct area with high velocity(Vp, Vs), low Poisson’s ratio(σ) and high crustal stress. The high-velocity zone at the Lushan source may reflect Precambrian metasedimentary or igneous rock in the seismogenic layer, which enables the accumulation of high crustal stress to generate large earthquakes. However, a sharply contrasting gap zone with low-velocity, high-(σ) anomalies is clearly imaged in the upper crust under the concatenated area between the Lushan and Wenchuan earthquakes. Seismic images indicate that the lowvelocity gap zone is associated with fluid-bearing ductile flow from the lower crustal materials of Tibet being pushed into a weakened segment of the LMFZ. This study suggests that the 2013 Lushan earthquake may have been triggered by high crustal stress accumulation together with high coseismic stress further increased by the Wenchuan earthquake in the metamafic seismogenic layer. Contrasting rheological variations in the crust and crustal stress changes along the LMFZ control the rupture processes that generated the Lushan and Wenchuan earthquakes, as well as the generation of new earthquakes in the future.In this study, two typical regions, including the oceanic-continental plate coliision and continent-continental plate collision, are selected as the study regions for seismic tomography in the crust and upper mantle. A large number of high-quality travel times have been used to determine deep structures in the crust and upper mantle. These typical seismic tomographic studies could provide the important information on a better understanding of structural heterogeneities, earthquake generation and plate coupling in the subduction zone and mountain building region. Simultaneously, our study indicates that the method of seismic travel-time tomography is an effective way to image deep structure and to investigate earthquake generation in the collision zones of the Earth.