学位の種別: 課程博士審査委員会委員 : (主査)東京大学准教授 芦 寿一郎, 東京大学教授 須貝 俊彦, 東京大学教授 山崎 俊嗣, 東京大学准教授 池田 安隆University of Tokyo(東京大学

Misawa Ayanori

三澤 文慶

UTokyo Repository

論文の内容の要旨 
 
論文題目 2004 年スマトラ島沖地震震源域付近の地質構造と変動地形に関する研究 
(The study on geological structure and tectonic relief around the epicenter of the 2004 Sumatra Earthquake) 
 
氏 名 三澤文慶 
 
On 26th December 2004, the Sumatra–Andaman Earthquake (Mw 9.1-9.3) occurred offshore 
northwestern Sumatra and subsequently northward ruptured the megathrust for over 1,300 to 1,500 km 
wide along the Sunda Trench. The great earthquake generated huge tsunami that struck many countries 
around the Indian Ocean. In particular, a huge tsunami of more than 20 m in height struck the west coast 
of Aceh in northern Sumatra. Since the earthquake, several international marine geological and 
geophysical surveys have been investigated morphologies, geological structures, and seismic activities in 
the Sumatra forearc region, including the Sunda Trench, trench slope, forearc high and the Aceh Basin. 
The generation mechanism of the huge tsunami during the 2004 great earthquake remains unsolved, yet 
several different fault models have been proposed by previous studies. Hirata et al. (2008, 2010) proposed 
that the secondary tsunami derived around the Middle Thrust in the forearc high region of the Sumatra 
forearc. However, a seismic reflection survey had not yet been conducted in the forearc high region. If the 
2004 coseismic rupture reached the seafloor along the Middle Thrust, deformations of the seafloor 
contributing the great tsunamis would be recorded in the shallow parts of sedimentary sequences. In order 
to discuss the presence of deformations documented within sedimentary sequences, acquiring  
high-resolution reflection data in the forearc high region of the Sumatra is required.  
The objective of this study includes elucidation of detailed geological structures and fault distributions 
around the Sunda Trench, trench slope, and forearc high regions offshore northwestern Sumatra, using the 
high-resolution seismic reflection images and swath bathymetric data. The data used in this study include 
the high-resolution multi-channel seismic (MCS) reflection data obtained during the KH-10-5 research 
cruise using R/V Hakuho-Maru in November 2010 and the swath bathymetric data compiled from 
NT05-02, KY09-09, and KH-10-5 research cruises. This study also demonstrates previously 
unrecognized deformational features such as the uppermost deformation within seafloor and sedimentary 
sequences in the forearc high region.  
The high-resolution MCS profiles images clearly down to 1.6 sec. (TWT; Two-Way Travel Times) in 
the trench and the trench slope regions. Moreover, the MCS profiles also successfully obtained clear 
images down to 2.7 sec. (TWT) in the lower trench slope region and down to 2.0 sec. (TWT) in the 
forearc high region. These seismic profiles and bathymetric map clearly documented the system of faults 
and folds trending NW-SE or NNW-SSE direction in the Sumatra forearc region including the five major 
thrust faults systems defined by Sibuet et al. (2007) are confirmed: Deformation front, Main Thrust, 
Lower Thrust, Middle Thrust and Upper Thrust. Strikes of these thrusts are NNW-SSE direction, 
corresponding to the morphological trends. Based on interpretation of the seismic profiles and 
swath-bathymetric map, detailed distributions of fault and fold are elucidated in the Sumatra forearc 
region offshore northwestern Sumatra.  
  Distinct vergence is identified in the Sumatra forearc; the lower trench slope is characterized by 
landward vergence, while seaward vergence is dominantly developed in the forearc high region. In the 
lower trench slope region, the MCS profiles clearly showed seaward-dipping thrust faults and 
landward-vergent ramp anticlines. These faults and anticlines play major roles in the growth of the 
accretionary prism along the Sunda Trench. In the forearc high region, landward-dipping thrust faults are 
dominant and tend to strike NNW-SSE near the seaward rim of each ridge. Major thrusts defined by 
Sibuet et al. (2007) are also recognized on the high-resolution MCS profiles and bathymetric map. In 
addition, anticlinal ridges have been identfied in the forearc high region. Anticlinal ridges in the forearc 
high region are considered to have been formed due to the compressional stress generated by subducting 
Indo-Australia plate in the Sunda Trench.  
  In the forearc high region, a number of piggyback basins are recognized between anticlinal ridges. 
Thickness of the piggyback basins sediments can be imaged down to a maximum of 0.7-0.8 sec. (TWT) 
below the seafloor. Older and lower parts of this basin-fill consists of relatively continuous and landward 
tilted reflectors that onlap the flanks of the basin. On the other hand, the uppermost part of the basin-fill 
sediments is parallel and flat-lying in most of the piggyback basins of the forearc high region. In several 
of the piggyback basins, this study recognizes deformations in the uppermost part of basin-fill sediments. 
Many of them are distributed along the Middle Thrust. Therefore, this study suggests that the Middle 
Thrust had been recently active mostly among major thrust faults within the forearc high region. 


The study on geological structure and tectonic relief around the epicenter of the 2004 Sumatra Earthquake

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The study on geological structure and tectonic relief around the epicenter of the 2004 Sumatra Earthquake

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