| Summary: | 碩士 === 國立臺灣海洋大學 === 應用地球科學研究所 === 104 === Extension in Taiwan Strait was caused by flexural bending of the Eurasian Plate in response to the load of Taiwan orogenic belt. Mega earthquakes, such as Quanzhou Earthquake (Mw~8.0) in 1604, Nanao Earthquake (Mw~7 and Mw~7.5) in 1600 and 1918, Taiwan Strait Earthquake (Mw~6.8) in 1994 and Pingtung Earthquake (Mw~7.0) in 2009, were occurred by extension to form several fault zones (such as Littoral Fault Zone) in Taiwan Strait. In this study, ocean-bottom seismometer (OBS) data were analyzed along a seismic profile of HX07 (27 OBSs) across the Taiwan Shoal collected in 2015 and a seismic profile of MGL0906-04 (5 OBSs) across the Tainan Basin through TAIGER Project recorded in 2009 at the southern Taiwan Strait. Firstly, initial P-wave velocity-interface models were built from P-wave velocity models inverted by first arrivals of OBS data and other geophysical results in Taiwan Strait. Secondly, the refracted and reflected arrivals, propagating through the sedimentary and crustal structures, were picked from OBS data. Finally, by using the refracted and reflected arrivals, we inverted the P-wave velocities and interfaces layer by layer to obtain models along OBS profiles. Based on the model along MGL0906-04 across the Tainan Basin, the depth of basement became deeper from west (about 2.5 km) to east (about 10 km) since the Eurasian Plate was bending downward to the east by the load of the Taiwan orogenic belt. The Moho depth (about 29 km) across the Tainan Basin also became deeper near Taiwan (about 31 km). Moreover, large horizontal variations of the P-wave velocity (5.6-6.1 km/s) and a rough interface in the upper crust, found at the distance of 35-65 km in the P-wave velocity model along MGL0906-04, may have resulted from a loading of the Taiwan orogenic belt. From the P-wave velocity-interface model along HX07, the Moho depth is 28-31 km, and the P-wave velocity of the crust is about 5.0-7.1 km/s. At the middle part of the model along HX07, a depressed basement with normal faults for forming a graben is imaged. The basement at both sides of the horst has been raised relatively to the basement of the graben. And the normal faults between the graben and the horst caused large variation of P-wave velocity (5.0-5.8 km/s) in the crust. Furthermore, we also found faults, the Littoral Fault Zone, at the 25-65 km of the model along HX07. Therefore, we suggest that these fault zones and the large variations of P-wave velocity are where the earthquakes may occur in the future.
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