| dc.description.abstract |
The Taiwan mountain belt belongs to a collision zone, located between the Eurasian plate (EP) and the Philippines Sea Plate (PSP). In such a complex tectonic regime, lots of seismic activities are expected. However, due to the limitation of the onland seismic stations, the seismotetonics setting of the related offshore area is usually not well understood. In order to better understand the tectonics of the offshore area, we have combined the Ocean bottom Seismometer (OBS) and the onland seismic stations. Different from the onland station, OBS data can record micro-earthquakes and tremors that are outside the range of the onland seismometers. The joint use of the OBS and the onland seismic stations can help understand detailed structures in the offshore area of Taiwan.
The first topic of this dissertation is to understand the mechanism of gas emission off SW Taiwan. The area off SW Taiwan belongs to an incipient stage of orogeny and consists in numerous active mud diapirs and mud volcanoes. Gas emissions out of the seabed off SW Taiwan are a common phenomenon. However, the mechanism of gas emissions is still poorly understood. For that, we collect the gas-related impulse tremor and long-duration tremor by using OBS around mud volcano MV1. We show that the gas emissions out of the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially with the diurnal and semidiurnal constituents. For each tidal day the strong gas emissions and long-duration tremors occur mainly at the rising periods to the higher high water and the falling periods to the lower low water. Both the gas emissions and tremor activities are generally quiet near the tidal datum. On average, the daily intensity of gas emissions and the magnitude of tremors are positively proportional to the daily tidal range; we suggest that the larger slope site has a greater gas concentration in the shallow sediments.
In the second topic, we address the seismotetonics of Nanao-Hoping forearc basin off NE Taiwan. In the Taiwan area, over 70 percent of the earthquakes occurred in the offshore area. Based only on the onland seismic network, the observation and monitoring of those earthquakes are apparently insufficient. In order to better understand the nature of the seismic behavior in that region, 6 OBS were deployed around the Nanao basin immediately after the Mw 6.3 earthquake on April 20, 2015. We have combined the recorded OBS data with 8 CWB onland stations for a detailed earthquake analysis. In total, 501 earthquakes have been relocated by using HypoDD. The distribution of relocated hypocenters shows a NW-SE (NE-SW??) trending and the focal depths range from 15 to 35 km. The earthquake locations display two major features: (1) A group of events at depths between 20 and 30 km beneath Nanao Basin is aligned with the subduction interface. We consider that this group belongs to subduction earthquakes, because the earthquakes cluster in the vicinity of the expected subduction interface. (2) Another seismicity is beneath the NW Nanao Basin and Hoping Rise and shows a high-angle south-dipping structure. We suggest the feature is associated with normal faults. Two parallel right-lateral transform faults on Hoping Basins and Ryukyu accretionary prism (Yaeyama fault) could cause a transpressional stress and induced a local deformation on subducting PSP slab. The subducting PSP is therefore locally arching, which has generates a series of normal faults on the NW Nanao basin and Hoping Rise. | en_US |