- 學位論文
利用地震資料探求孕震區四維地下構造
Using Seismic Data to Explore the 4D Seismogenic Structures
摘要
台灣位處於歐亞大陸板塊與菲律賓海板塊交界處，兩板塊間有劇烈之碰撞運動，為典型板塊碰撞下產生之大陸邊緣島嶼。菲律賓海板塊每年以約8.2公分的速度向西北方擠壓歐亞大陸板塊，兩板塊之聚合產生台灣島的造山帶，造就台灣中部中央山脈與海岸沖積平原區間一系列之覆瓦狀褶皺¬¬－逆衝斷層（fold-thrust belts）系統，導致地下構造非常複雜。歷史上台灣中部之災害地震頻傳，如1904年斗六地震、1906年梅山地震、1935年新竹-台中烈震，而近年來有1999年921集集地震序列及2013年南投地震序列，且地震序列前後皆有良好的波形紀錄。所以於921集集地震序列後，研究此區在速度構造於強震前後之變化情形亦為本研究欲探討的課題之一。 本研究利用中央氣象局即時觀測網（ＣＷＢＳＮ）所記錄的地震資料，選取的地震事件範圍為東經120.0度～122.0度，北緯23.5度～24.5度。以層析成像法來反演台灣中部的地下三維速度模型，此方法亦結合研究區外深源走時資料去改善深部構造之解析度。藉由反演所得之Vp、Vs及Vp/Vs構造觀察台灣中部地區地殼、上部地函區域及分析地震活動特性與其速度構造隨時間變化的關聯性，並推測其在地體構造上所代表的意義。本研究以Vs及Vp/Vs探求速度構造是否因壓力變化產生裂隙，使得地殼深處氣體或液體逸出或遷移，造成較大的孔隙壓，從而得以觀測斷層之活動性與Vs及Vp/Vs分布的關聯性。另外，利用速度構造隨時間上的變化亦可以做為探討大地震是否有前兆現象。根據研究結果，對大地震前後之於速度構造的影響有更進一步的了解。
並列摘要
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.
參考文獻
延伸閱讀
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