| dc.description.abstract | Complex tectonic structure in Taiwan is strongly affected by the arc-continent collision between Philippine Sea Plate and Eurasia Plate. Geophysical studies involving wave propagation within complex media requires the fundamental travel-time information for structure imaging and related studies. The goal of this thesis have three folds, mainly on travel-time information contents, involving research efforts (1) to evaluate Taiwan 3-D reference models, (2) to propose a simple and effective methodology for absolute earthquake location and (3) to investigate a 2-D velocity structure profile across Western Central Taiwan through travel-time inversion.
For the past 15 years, various 3-D models were proposed including Roecker et al., (1987), Rau and Wu (1995), Ma et al. (1996), Cheng et al. (1999), Kim et al. (2005) and Wu et al. (2007). Previous study on Rau and Wu (1995) and Ma et al. (1996) has been intensively studies (Li, K. S., 2002). For most recently published 3-D velocity models, including both Kim3D and Wu3D, the corresponding 3-D travel-time trajectories that correlate with wave-fronts were compared to evaluate its feasibility on various possible applications shown in this thesis. The travel-time computation is base on the modified expanding wave-front tracking approach proposed by Vidale (1988, 1990) through finite-difference approximation. By incorporating neo-tectonic basin structure in the Taiwan Strait and western foreland basin (Chen, 2006), the newly established 3-D reference models (MKim3D and MWu3D) can be individually identified and evaluated through the comparison on the differences in travel-time values, earthquake location and initial model setup for DiLi test shot profile. The results from both synthetic and real data applications all indicate that both Kim3D and MKim3D have smaller travel-time differences than Wu3D and MWu3D and thus have more reliable applications in there related research topics. By comparing the efforts to incorporate basin structure into existing models also indicate that spatial averaging of velocity variations in Wu3D model has to be taken care of in order to produce reasonable model. Such efforts can be visualization through comparison of travel-time differences before and after the modification.
The purpose of TAIGER experiment during 2006 to 2009 is to investigate main seismogenic structure features of the crust and preferable even up to upper mantle through both active and passive source experiments. The primary goals for DiLi test shot experiment is to understand the wave propagation and attenuation characteristics across western foreland basin and the safety concerns while utilize 500 Kg explosive as effective energy source. One of the seismic dataset recorded by Texans recording array in the DiLi experiment is used for this study.
A simple and effective approach through 3-D travel-time calculations for absolute earthquake location problem is proposed. By combing efficient 3-D calculation of travel-times and the construction of necessary database, non-linear location problem can be solved through recursive forward simulation of travel-times and the minimization of root-mean-square travel-time errors. Both synthetic tests on errors involved in finite-difference computation, consideration of spatial coverage of stations and influence on the existing velocity model were performed in order to understand the fundamental issues involved in absolute earthquake location problem. Study results show that the proposed algorithm is effective and the efficiency is subject to how detail the travel-time databank is demanded, the capacity of the hard disk and the efficiency of data fetching mechanism. Both synthetic and real data tests all show that the proposed methodology is workable for in-land earthquakes but not for the offshore earthquake as demonstrate from PingTung Earthquake synthetic case study. Further modification of the proposed algorithm can be pursuit by using S-P time and databases produced from the currently developed methodology.
Wide angle refraction and reflection (WARR) study of DiLi test shot data involving data processing indicated that minimum phase and predictive de-convolution procedures are necessary in order to reduce strong oscillating source wavelet signature. The de-convolved data has the advantage for travel-time picking and phase identification. Band-passed filtering (10-40 Hz) of the recorded data enhanced the possible reflection (?) and/or diffraction (?) arrivals. Shallow basin structure and faulted block determined from first-arrival travel-times can be well defined both from near-source CMP and WARR sections. Four major faults with clear velocity change are clearly identified. However, the questionable arrivals at 11 and 15 seconds are not well constrained through 2-D travel-time forward modeling and inversion. The final model obtained from identified phases show two distinctive reflectors at 12 km and a clear reflector patch located at depth of 24 km. The interpretation of final velocity structure can be correlated well with results from seismic survey and gravity studies and the background geology. The apparent 3-D propagation effects and/or possible diffractions (?) for 11 and 15 sec phases that were caused by the complex wave propagation phenomena can be verified through explicit consideration of 3-D effects. The quality and quantity of the DaLi test shot dataset can be improved for future detailed velocity structure investigation in central Taiwan. | en_US |