| dc.description.abstract | Over the past 5 years, it has been demonstrated that the Time Domain Empirical Green’s Function (TDEGF) from ambient seismic noise cross-correlation can be used to investigate crustal and uppermost mantle velocity structure from many studies around the world. To decipher subsurface structures in various scales, researchers can utilize some existing continuous-recording seismic stations and/or deploy a new dense receiver array in the study region. In this thesis, we perform tomographic applications of ambient seismic noise analysis in the Taipei Basin, Taiwan Strait, and Taiwan Region for three arrays with very different scales.
The Taipei Basin is a high-level artificial noise metropolis and requires detailed shallow crustal structure. The high levels of ambient seismic noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic exploration or earthquake tomography methods. Analysis of the TDEGF amplitudes suggests that the dominant sources of ambient seismic noise are generated from the coastlines and shallow continental shelf of the Taiwan Strait, northwest of the study region. The ambient seismic noise tomography is feasible at periods 0.5-3 s, which is much shorter than most other studies. The lateral variation in Rayleigh wave phase velocities correlates well with surface geology and suggest that faults play an important role in the regional tectonic setting. High phase velocities mark the Tatun volcanic area, the Kuanyin Mountain dominated by Quaternary igneous rock, and the Miocene Western Foothills south of the Taipei Fault. Low phase velocities are along western and southeastern edges of the Taipei basin and the Pleistocene Linkou Tableland. Main faults in the region are either marked by low phase velocities or define transitions between regions of high and low velocity anomalies.
The Taiwan Region is located at a complex convergent plate boundary zone where the Philippine Sea plate interacts with the Eurasian plate. As a result, the lateral velocity variations show dramatic patterns among different geologic provinces. Rather than only focusing on the Taiwan Island as most previous studies, this thesis includes broader regions also with the Taiwan Strait and the eastern sea area. The 5-120 s phase velocity maps are constructed from analysis of ambient seismic noise and teleseismic Rayleigh waves. At 5-12 s, phase velocity distribution can compare well with surface geology. At 16-19 s, there is a saxophone shape low velocity zone beneath the Taiwan Island. At 20-24 s, phase velocity patterns beneath the Taiwan Island are almost contrary to 5-12 s. At 30-50 s, the Taiwan Island is a high velocity zone surrounded by low velocity zone. At 60-120 s, the Taiwan Strait shows low velocity and east of it shows high velocity.
The Taiwan Strait is located on the passive margin of collision zone and is also adjacent to South China Sea opening to the southwest. Despite that in the Taiwan Strait the tectonic structure is assumed relatively simple and seismicity is much lower than the Taiwan Island, intraplate earthquakes sometimes occurred but the mechanisms are not well constrained. Limited studies have focused on this region before and most previous ambient seismic noise studies focused on continental regions rather than across a strait. The data are from 50 broadband stations on both sides of the Taiwan Strait, in mainland China and the Taiwan region. The preliminary 5-30 s Rayleigh wave phase velocity dispersion curves show high velocities along mainland China coastlines and southern part of the Taiwan Strait while low velocities appear in the central and northern parts of the Taiwan Strait.
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