| Summary: | 碩士 === 國立臺灣海洋大學 === 應用地球科學研究所 === 96 === Abstract
Ten Micro-Ocean-Bottom Seismometers (MicrOBS) and a Multi-Channel Seismic (MCS) profiling were implemented across the Gagua Ridge in September 2006. First, an initial model was built from reflection times of the MCS data and a density model inverted from gravity data. Subsequently, we picked refracted and reflected arrivals from four-component OBS data to invert the velocity-interface structure layer-by-layer. Sedimentary model based on OBS/MCS data shows that the largest thickness of the Huatung Basin and the Eastern Trough of Gagua Ridge are about 4 km and 3 km, respectively. Two thrust faults, one west dipping about 45 degrees beneath the Eastern Trough and another east dipping 60 degrees beneath the Western Trough, are imaged in the active basement near the Gagua Ridge. East of the Gagua Ridge, the small velocity variation but with large velocity (about 5.5~6.0 km/s) of the upper crustal structure is observed whereas the large velocity variation but with small velocity (about 3.5~5.0 km/s) is found west of the Gagua Ridge. We also observed from the OBS model that the crust is thicker about 14 km thick below the Gagua Ridge, but it remains about 9~10 km thick as the normal oceanic crust on the both sides of the Gagua Ridge. We conclude that the asymmetrical structures west and east of the Gagua Ridge and formation of the Gagua Ridge within the oceanic crust imaged from OBS data may have resulted from compression due to the northwest convergence of the Philippine Sea Plate east of Gagua Ridge.
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