Using Seismic Data to Explore the 4D Seismogenic Structures
| Autor: | Liao, Jun-Fu, 廖峻甫 |
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| Rok vydání: | 2015 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 103 Taiwan is situated in the collision boundary of the Eurasian Plate and the Philippine Sea plate. The Philippine Sea Plate moves in NW direction with convergent rate of 8.2 cm/yr. The ongoing orogenic process causes a series of imbricate fold-thrust belts in the Western Foothill of Taiwan. It leads to complex seismogeneric structures beneath which are not well understood. In central Taiwan, there were several disastrous earthquakes occurred, such as 1904 Douliu earthquake, 1906 Meishan earthquake, 1935 Hsinchu-Taichung earthquake, the great 1999 Chi-Chi (ML=7.3 ; depth=8km), 2013/03/27 (ML=6.2 ; depth=19.4km ) and 2013/06/02 (ML=6.4 ; depth=14.5km ) Nantou earthquake sequence. One of the major objectives of this study is to investigate the evolution of velocity structures before and after the occurrence of the abovementioned earthquakes. The other important goal is to exam the temporal variations of velocity structures including Vp, Vs and Vp/Vs beneath central Taiwan since the occurrence of the Chi-Chi earthquake. Our study area is from 120.0o E to 122.0o E in longitude, while the latitude is between 23.5o N and 24.5o N. We have used the travel time data compiled by the Central Weather Bureau (CWB) from 1991 to 2014 to build a 3D velocity map from the crustal area to the uppermost mantle. We also divided the whole study time span into 6 segments to investigate velocity structures in different time periods. The software package called LOTOS is applied to perform tomographic inversion. The main advantage of the method is to allow the travel times of deeper earthquakes occurred outside study area in the inversion process to improve the resolution of the velocity structure at deeper depths. The results show that the temporal variation of Vp is less significant than Vs, and Vp does not show prominent perturbation before and after the occurrence of a large earthquake. However, Vs and Vp / Vs in the source area demonstrate significant temporal and spatial difference before and after major shocks. In general, before the strike of an earthquake in central Taiwan, Vs begins to decreased (Vp/Vs is increased) in focal area, which may caused by the increasing of microcracks produced by accumulated stress. But in the vicinity of source area, Vs is increasing which may be due to closing of cracks. Our findings suggest that the monitoring Vs and Vp/Vs structures may used as an earthquake precursor to reduce seismic hazard in central Taiwan. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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