Accretion, mass wasting, and partitioned strain over the 26 Dec 2004 Mw9.2 rupture offshore Aceh, northern Sumatra

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Abstract

The Aceh segment of the Sunda Arc absorbed the largest slip along the 26 December 2004 Mw9.2 subduction earthquake rupture and caused the highest and most disastrous tsunami run-ups. Sea floor disruption marks the frontal active part of the accretionary prism along that segment. An ROV, bathymetric, and seismic survey (R/V Natsushima, JAMSTEC 2-3/2005) revealed pervasive and deeply rooted fracturing along the upper trench slope. Grabens and ejecta clasts (from fluid escape?) along the crest of an anticline ridge coupled with mass wasting and erosional scouring on the slope below it. Specific features of the disruption and its patchy distribution suggest coseismic faulting and folding with gravitational collapse of the steepened anticline forelimb. The same ridge 40 km along strike was pristine. Disruption was also generally undetected by ROV on the Ache Basin side of the forearc ridge, suggesting a minor role for shaking. Only some of the faults in the imbricate and segmented accretionary structure ruptured, but together they may account for 2004 sea-floor rupture along the Aceh segment. This hypothesis accounts for unusual thrust aftershocks and suspended sediment along the frontal active part of the prism. It can also account for the huge tsunami heights in northern Sumatra because faults in the prism are steeper than the megathrust down-dip and because slip may be larger in the prism following prior blind ruptures. Rare but large coseismic deformation of the prism can generally account for the erosional morphology of trench slopes and for al least some ‘tsunami earthquakes’. On the landward side of the forearc high, the West Andaman fault borders the Aceh Basin and branches southward into a fault crossing obliquely the forearc high. Seismic profiles at the bifurcation image a flower structure with strain partitioning and an active dextral pull-apart. This fault system, however, did not release significant strain in 2004.

Introduction

The Mw9.2 earthquake and tsunami of 26 December 2004 caused a human disaster of historic proportions. The rupture was huge, more than 1/4 million km2 and 1300 km long (Fig. 1; Lay, 2005, Ammon et al., 2005, Ni et al., 2005, Stein and Okal, 2005, Krueger and Ohrnberger, 2005, Ishii et al., 2005, Tolstoy and Bohnenstiehl, 2005, Hirata et al., 2006, Chlieh et al., 2007). Fortunately, only few ruptures of that size occur in a century. As a result, these major geological events are still poorly understood. Adding to the paucity of data, ruptures of this size occur only along subduction zones and their near-field effects are mostly submarine. High-resolution techniques for submarine observations have greatly improved in the 4 decades since the 1964 earthquake in the Gulf of Alaska, the penultimate event in this class size. Thus the 2004 event affords new opportunities for understanding these large earthquakes, the subduction-accretion process they punctuate, and the signal they leave in the geomorphic and geologic record (e.g., Plafker, 1969, Plafker, 1972, Atwater, 1987, Itou et al., 2000, Goldfinger et al., 2003, Natawidjaja et al., 2003, McAdoo et al., 2004, Ikehara, 2004, Henstock et al., 2006).

This study is based on a suite of submarine geophysical observations taken in an international cruise organized as a rapid response by JAMSTEC (Japan) and BPPT (Indonesia) after the 26 December 2004 mainshock (Soh et al., 2005a, Soh, 2005b). Our data include single-channel seismic profiles, multibeam (SeaBat) bathymetry, side-scan sonar, and visual observations by the remotely operated vehicle (ROV) Hyper Dolphin. They cover a relatively narrow segment (≈ 40 km) along the Sunda subduction zone west of Aceh, northern Sumatra. This is the southern part of the 2004 rupture where slip was at a maximum, as much as 20 m or more (Fig. 1; e.g., Ammon et al., 2005, Hirata et al., 2006). Across strike, the study area includes the buried trench, the accretionary prism, the outer high, the Aceh forearc basin, and the West Andaman fault (Fig. 2). Our data contribute new insight on the long-term tectonic role of these major forearc structures. In addition, our observations were sufficiently hi-resolution, for example by ROV, and sufficiently soon (1.5–2 months) after the 2004 mainshock, to clearly distinguish fresh disruption of the seafloor and to observe the transient suspension of sediment. The main purpose of this paper is to examine these effects in terms of the causative earthquake processes, such as shaking and coseismic fold growth, and in terms of their long-term accumulation by repeated ruptures into the morphology and structure of the subduction orogen. Whether the observed disruption manifests shaking or tectonic strain is important for tsunami generation and for strategies to recover evidence of past earthquakes.

Where and how often can we expect M9-class earthquakes? The dramatic difference between the societal effects of the 2004 M9.2 and 2005 M8.6 earthquakes offshore Sumatra highlights the practical import of this issue. Some of the results in this paper suggest that subduction boundaries capable of M9 + earthquakes might exhibit unique geologic and geomorphic characteristics. Coseismic fracturing, sea-floor disruption, and rapid erosion may be highly dependent on the amount of deformation in individual ruptures. Rapid erosion of the trench wall recycles sediment, increases the accretion rate, and requires rapid structural growth to maintain critical taper of the accretionary prism (Davis et al., 1983). Our high-resolution data collected soon after the 2004 mainshock combined with other post-event investigations begin to address these issues. They highlight a sharp contrast between the dramatic geomorphic expressions of tectonic activity in the accretionary prism versus relative stability of the more internal parts of the forearc. They also point to some of the structures that may have absorbed strain in the great earthquake. The discussion focuses on the relation between coseismic phenomena, as observed at the outcrop scale, and the accumulated effects of many such events.

Section snippets

Tectonic setting

The Sunda arc is one of the largest subduction structures on Earth, with a length of 5000 km and 90° of curvature. The system has been active at least since the time of Himalayan collision in the Paleogene, judging from ages of accreted sediment (Hamilton, 1979, Moore and Karig, 1980). The forearc between the volcanic chain and the deformation front is consistently about 300 km wide and is remarkably uniform in its overall geomorphic and structural profile, despite wide variation in subduction

Data and methods

This study is based on a suite of data collected during February–March 2005 in the maximum-displacement area of the 26 December 2004 mainshock with the R/V Natsushima, which is operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC; Soh et al., 2005a, Soh, 2005b, Araki et al., 2006; Fig. 2). The remotely operated vehicle (ROV) Hyper Dolphin was deployed in several areas of the forearc and provided some of the key observations. Its depth limit of 3000 m was not a

Coseismic accretion and mass wasting on the trench slope

At the seaward tip of the overriding plate, the accretionary prism accounts for convergence by internal thrusting and folding. It is thus expected to exhibit the most intense tectonic deformation of the seafloor associated with a complete rupture of the subduction boundary in a great earthquake. Observations sufficiently detailed to recognize fresh deformation of the sea floor after the 2004 mainshock are consistent with this notion. Disruption of the sea floor inferred to stem from coseismic

The forearc trough and the West Andaman fault

The landward part of the forearc in our study area (Fig. 2) displays major active faults, related to basin formation and absorbing the dextral component of plate motion. Results described below provide new specificity to this tectonic system. This area is directly above the seismogenic 2004 rupture (e.g., Araki et al., 2006). Yet, a similar search in this area as the one carried out along the accretionary prism, including two ROV dives and acoustic back-scatter from a multibeam survey, revealed

Discussion: disruption by shaking or strain?

An analogy can be drawn between the disruption caused on the accretionary prism by the 2004 earthquake with the ‘slump belt’ caused by the 1934 M8.7 Bihar earthquake along the Himalayan thrust front. This slump belt was a 300 km long relatively narrow strip of fractured and tilted blocks of foredeep sediment along the Gangetic Plain. Another more widespread manifestation of that earthquake was dewatering from the near-surface sediment causing geyser-like fountains reaching several meters above

Discussion: partitioning of strain in an earthquake sequence

The slip distribution in the 2004 rupture does not fully account for plate kinematics and omits some of the arc-parallel component in the northern Sunda arc (e.g., Fig. 1). The possible contribution of aftershocks to this arc-parallel component is insignificant, given the size of the mainshock. Most of the missing motion is expected in future earthquakes on the arc-parallel dextral faults, which were brought closer to failure by the 2004 mainshock (ten Brink and Lin, 2004, Lin and Stein, 2004,

Conclusions

Along the segment with the greatest slip offshore Aceh, northern Sumatra, the December 2004 rupture caused seafloor disruption along discrete patches of the trench slope, which marks the zone of active accretion. Similar visual ROV observations elsewhere on the same segment, however, found no evidence of seafloor disturbance. At the scale of an ROV dive (≈ 1 km), the disruption was either intense and pervasive, or absent. Fracturing, slope failure, and mass wasting were continuous along the 2 km

Acknowledgments

JAMSTEC conducted the rapid-response expedition with an international research team. The Ministries of Foreign Affairs of Indonesia and Japan generously helped with permits and logistics for our scientific research in the Indonesian EEZ. We heartily express our gratitude to all parties concerned. Our survey was conducted in accordance with the United Nations Law of the Sea (UNCLOS). We are indebted to Kiyoshi Suyehiro and other colleagues for their help and encouragement. We are grateful for

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