碩士 / 國立臺灣大學 / 地質科學研究所 / 105 / In recent years, noise cross-correlation functions (NCFs) from ambient noise between paired stations provide the unprecedented interstation path coverage within highly instrumented regions for high resolution shear wave speed tomographic imaging. Since 2006, a number of OBS experiments from the Institute of Earth Sciences (IES), Academia Sinica of Taiwan and TAIGER, a US-Taiwan cooperative research project, have been conducted in deep sea east of Taiwan to expand the path coverage of seismic network and improve the resolution of the subseafloor velocity structure.
In this study, we use continuous recordings of vertical records and differential pressure gauges in 67 OBSs deployed offshore eastern Taiwan during the past 10 years to investigate the subseafloor structures from extracted Rayleigh waves and the source origin contributing to the short period secondary microseism and generation of very long-period infragravity waves. We apply a wavelet-based method to characterize the time-varying spectral properties and measure the frequency-dependent group and phase velocities of these waves. The results show fundamental mode Rayleigh waves in both the vertical records and DPG derived NCFs with a dominant period of 3-8 s and relatively slow speed of 0.3-1.5 km/s. The infragravity waves at periods ranging from 50 to 180 s found in DPGs records and some of the vertical records show the speed of 0.05-0.1 km/s.
With these robust dispersion measurements, we employ an accurate normal mode summation method to calculate the sensitivity kernels of phase velocity anomalies with respect to fractional perturbations in P- and S-wave velocity Vp and Vs as a function of period and depth. A damped least-squares linear inversion is then conducted to constrain the radial velocity structures beneath the seafloor offshore eastern Taiwan. Our results show that the subseafloor S-wave velocity structure in Okinawa Trough (OT) is slowest among all the investigation areas offshore eastern Taiwan, with a very high Vp/Vs ratio in the topmost 2 km of the crust. The S wave velocity in the Ryukyu forearc is relatively higher but also with a high Vp/Vs ratio in the shallow depths, while that in the Huatung Basin is high with a low Vp/Vs ratio. Because of the lack of constraints on the shear velocity structure offshore eastern Taiwan, the linear inversion results are highly dependent on the assumed initial models. A nonlinear Bayesian inversion is needed to obtain the probability distribution of the possible 1-D structures in the future study.
| Identifer | oai:union.ndltd.org:TW/105NTU05138025 |
| Date | January 2017 |
| Creators | Weiwei Wang, 王葳葳 |
| Contributors | Shu-Huei Hung, 洪淑蕙 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 98 |
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