| Summary: | 碩士 === 國立臺灣海洋大學 === 應用地球科學研究所 === 103 === The South China Sea (SCS), located at the junction of Eurasian plate, Indo-Australian plate and Pacific plate, is one of the marginal seas along the western Pacific. In the past, some studies investigated the plate features of the SCS by geophysical explorations using magnetic field, gravity, heat flow and seismic waves. For providing additional evidence in the tectonic evolution, the purpose of this study was to investigate the 3-D S-wave velocity structure in the SCS on the basis of Rayleigh-wave group and phase velocities. Earthquakes with magnitude from 5.5 to 7.0 and the focal depths of less than 100-km occurred from 1995 to 2012 at the region of 88-132E and 4S-32N were used to analyze Rayleigh-wave dispersion curves at periods of 12-150 seconds. At last, we adopted about 7000 Rayleigh-wave paths travelling across the SCS and its adjacent areas. Building a 3-D S-wave velocity structure by using surface waves requested a two-step inversion. First, the study area was divided into 375 sub-regions with each size of 22 in latitude and longitude. A block inversion with smoothing constraints, i.e., a tomographic method, was used to image 2-D maps of Rayleigh-wave group- and phase-velocity. Subsequently, the secondary inversion was to invert the S-wave velocity structure of each sub-region using its group- and phase-velocities. Finally, we combined the S-wave velocity structure of all sub-regions to construct the 3-D velocity model in the SCS. The model showed the lateral heterogeneity up to depths of 200 km, that is, the tectonic structure was complex in the crust and upper mantle under the SCS. The vertical velocity profiles along the EW-direction showed a high-velocity zone at a depth of about 50 km, which can be also found from north to south. This depth indicated the position where it is the lid of upper mantle. In additional, the crustal thickness decreased gradually toward the center of the SCS, where the crust is about 10-km-thick. The lithospheric thickness beneath the SCS is about 45-50 km. Low velocities in the Reed Bank and Dangerous Ground were related to the thick sediments; whereas the low velocity under the Tibet Plateau was in connection with its thicker crust. There are also low velocity in Sulu Sea and Central Philippine Islands, to be associated with high heat flow. The Nan-Uttaradit Suture and Sagaing Fault were also importantly geological boundaries, where the velocity differences can be obviously identified across these geological units. Along the NS-direction vertical profiles, the velocity variation in the Celebes Sea was relatively smooth than that in the Sulu Sea, in which the topography is complicated. This implied that the Sulu Sea has relatively higher action in plate tectonics than the Celebes Sea. The velocity discrepancy between the two sides of the Red-River Fault zone only was down to the crust, not to the lithosphere. For this reason, we inferred the Red-River Fault zone as a crustal fault.
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