| 摘要: | 蘇門答臘隱沒帶地震活動研究：大地震序列，地震危害和隱沒板塊特性
摘要
蘇門達臘隱沒帶是世界上非常活耀的聚合型板塊邊界之一，本研究將沿著隱沒帶前緣地震活動做研究，並提出新的觀點說明在目前未被定義的活動斷層和隱沒帶特性的相互作用下所產生危險性是非常重要。隨著地球物理觀測的進步和資料累積，蘇門達臘板塊構造的機制提供了良好的研究問題的機會。
蘇門答臘隱沒帶主要是由澳大利亞海洋板塊隱沒至Sunda大陸板塊之下所形成，隱沒同時也產生巨大地震並伴隨著巨大的災害和損失，不僅影響當地區域同時也影響全世界。而在現今的蘇門答臘隱沒帶的板塊運動，主要是由三個的斷層系統所組成: 1.隱沒帶上的逆衝斷層, 2. Mentawai斷層(MF)(右走向滑移斷層), 3.蘇門答臘斷層帶(SFZ)。在本論文研究則涵蓋了這三個主要的斷層系統，因此沿著這個隱沒帶做研究將會有研究尺度大小的問題，由於地球物理資料上的蒐集需要沿著這些系統的邊緣觀察，雖然會有一些限制但是是無可避免的。因此本論文將可提供新的資訊和和更高水準地觀測解析度。本論文將在第一章節詳細討論本研究區域的板塊結構和本研究的關係。
本研究區域資料由印度尼西亞氣象、氣候和地球物理局(BMKG)所提供的地震時間序列目錄，將在本研究第二章說明。在研究區範圍由BMKG所記錄到的地區尺度地震兩年(2009年至2011年)資料比國際機構，例如國際地震中心(ISC)所提供的免費數據資料具有較高解析度的資料品質。因此我們利用高密度和高解析度的地震資料透 hypocentre double difference的技術再次將地震重新定位，而得到高階析度的速度模型。從結果顯示，資料的品質得到大幅的改善，明顯的觀察到大的地震序列，例如: Padang地震、Banyak地震和Mentawai海嘯地震序列。在巴尼亞克群島(Banyak Islands)的地震序被確認是向東南傾且伴隨著上部的分歧斷層(splay fault)活動。而在此也發現Mentawai空白帶只發生少量地震，我們將視為此區域地層材質較為粗糙而導致該地區的能量持續累積，而這些斷層將可能成為海嘯災害的主要威脅。
雖然在第二章節中所提到地震目錄上所使用到的時間尺度上是受到限制的，但地震沿著蘇門答臘斷裂帶(Sumatra Fault Zone,SFZ)的發生是可以被觀察到的。在SFZ上的地震活動規模介於6至7.7之間，在這區域是能夠產生大規模的地震的。我們將在第三章中探討SFZ的北端構造和可能的危害。在本章重新處理了兩條震測反射剖面，從結果反射訊號結果顯示一些混沌相(chaotic facies)是和SFZ有者強烈的相關性，這些證據表示了在這區域裡曾經發生大型的海底地滑.海嘯發生的情境是通過模擬地震引致海底斷層錯動和引致海底滑坡兩種形式產生的水體變動為海嘯源。在極端的案例下，結合此兩種機制並透過數值模擬的計算，如果地震規模達（Mw）達到7 或更大，將會引發寬約300公尺，長約600公尺範圍的海床滑動,並可能在沿海地區產生浪高4公尺的海嘯。因此在本章節中非常強烈的顯示該地區的走向滑移斷層系統所觸發的海底地滑和海嘯產生具有強烈的相關。這些機制或可解釋部分歷史上曾發生的海嘯並未伴隨明顯的陸地受到強烈震害的記載，這也暗示了在本地區建立監測海底地滑引致海嘯的早期預警系統的必要性。
本文所討論的兩個章節(第二章和第三章)，包含了蘇門答臘隱沒帶的主要斷層系統。然而其它的地震活動也是非常重要的，我們時常忽視了深度範圍大於100公里的中等深度地震，然而在我們所認知的蘇門答臘構造環境具有一個靠近海溝的弧前島嶼，存在於海洋板塊隱沒至20到30公里深處，這些島嶼為我們研究隱沒帶地震提供了很好的研究機會。在第四章中，我們利用測站蒐集到的地震波資訊估計地震波通過板塊構造中低速波導層所產生的頻散(disperrsed)現象。利用分析頻散速度曲線，瞭解來自海洋地殼中間深度的地方(大於等於100公里深)板塊速度結構。從頻散的結果顯示速度約減少了2% 到4% ，和世界其他年齡相近的隱沒帶所表現的結果也是一致的(例如:阿拉斯加隱沒帶Alaska subduction zone)。從觀測到的不同頻率地殼波速延遲現象可以顯示地殼的低速帶的厚度，而此低速帶的發現是非常重要的，有助於了解中深層的地震發生以及板塊隱沒時所發生過的經歷和過程。
;Study on Seismic Activity at The Sumatra Subduction Zone: Large Earthquake Sequences, Seismic Hazard and Subduction Slab Properties
ABSTRACT
The Sumatra subduction zone exhibit as one of the most actives convergence plate boundary. The study on the seismic activity along this margin will give new perspectives on how the interaction between the earthquake occurrences on the governing active unidentified-identified faults, the hazard that might result by these faults activity and the properties of the subducting slab are considered important. The tectonic setting of the Sumatra, as well as the increasing amount of observational geophysical data, gives the opportunity to study these problems.
The oblique subduction of Australian oceanic plate beneath the continental Sunda plate has produced large – mega earthquake with most of them has induced significant lost not only at regional scale, but also worldwide. Three major faults system occupied the tectonic processes at present day Sumatra subduction zone, i.e. 1. Thrust fault at the subduction zone, 2. The Mentawai Fault (MF) (right-lateral strike-slip) and 3. The Sumatra Fault Zone (SFZ) (right-lateral strike-slip). Thus, having a broad scale problem along this subduction zone, the study brought into this dissertation is covering those major faults system though some limitations are unavoidable due to the availability of the geophysical dataset and or observation along this margin. Though, the data are somewhat limited, however, most of the dataset is obtained from local observation with limited access. Therefore, this dissertation is expected to provide new information and at a higher level of observational resolution. The detailed structure of the dissertation and the relationship of the studies within each chapter is discussed in Chapter 1.
An opportunity to analyze the earthquake event catalogs with their travel time provided by the Badan Meteorologi, Klimatologi dan Geofisika (BMKG) Indonesia is established in Chapter 2. This work showed that the data records by the local institution BMKG on a regional scale within two years (2009 to 2011) cover a higher resolution of datasets compare to the freely available dataset managed by the international institution, e.g. International Seismological Centre (ISC). Accordingly, by having a dense and high resolution dataset, we relocate the earthquake events based on hypocentre double difference techniques with a refined velocity model. The result shows quality improvement of the dataset with clear large earthquake sequences being observed, i.e. the Padang earthquake, the Banyak earthquake, and the Mentawai tsunami earthquake sequences. A southeast dipping of earthquake lineation was identified within the Banyak Islands earthquake sequence and considered as the activity of the upper splay fault. Limited earthquake activity was identified at the Mentawai gap region and considered as a locked asperity. Those faults could be as a major threat of a great source of tsunamigenic fault.
Although only limited time range of the earthquake event catalog used in Chapter 2, limited earthquakes along the SFZ were observed. The seismicity along SFZ has its magnitude between 6 and 7.7. On that account, the SFZ is capable to produce a large magnitude of earthquakes. We investigate the northern end of the SFZ and its hazard in Chapter 3. In this chapter, based on two reprocessed seismic reflection profiles, the image shows that the extended Sumatra Fault Zone strongly related to some chaotic facies indicating an evidence of large triggered submarine landslide ever occurred in this region. Tsunami scenarios were simulated with a combined source of fault activity and submarine landslide. In extreme case, by these combined mechanisms, if the earthquake as large as 7 Mw or larger and triggered a submarine landslide with a length of 600 meters and width of 300 meters, it could produce a tsunami as high as 4 meters along the coast. Hence, in this chapter it is strongly showed the potential tsunami hazard related to the submarine landslide triggering of a strike slip fault system, this mechanism may explain also some history tsunami in this area without a clear earthquake signature inland, and imply the necessity of landslide tsunami early warning system in this area.
The two chapters (Chapter 2 and 3) discussed in this dissertation are covered the major faults system along the Sumatra subduction zone. The other earthquake activities that are considered important but being paid less attention and or sometimes neglected are the intermediate depth earthquake which has a depth range greater and equal to 100 km depth. Acknowledged the advantages of the Sumatra tectonic setting, i.e. which has the forearc islands close to the trench and just 20-30 km above the subduction plate interface, it gives the opportunity to investigate the detailed properties of the subducting plate. In Chapter 4, by utilizing the observation of body wave dispersion at the forearc stations, we could estimate the dispersion curve that caused by a waveguide structure which acts as a low velocity layer (LVL) from the former oceanic crust existed at the intermediate depth (greater and equal to 100 km). A velocity reduction of 2 – 4 % resulted from this dispersion showed an agreement to the observation at other subduction zones with similar age of the subducting plate around the world (e.g. Alaska subduction zone). The significant delay for a certain frequency range that observed from the dispersion curve may indicate the thickness of the LVL. The finding of the LVL is considered important in understanding the occurrence of the intermediate – deep earthquake as well as the fate of the subducting plate that undergo the subduction processes. |
