Abstract
AbstractWe deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.
Highlights
The Sumatra-Andaman earthquake of 26 December 2004 revealed an historically large seismic moment released beneath the seafloor 1200 km from offshore Simeulue Island south of Sumatra Island to the north of Andaman Island
All globally determined earthquakes were relocated along the dipping plate that subducted from the Indian Ocean
The magnitudes of earthquakes determined by ocean bottom seismographs (OBSs) during the period of 20 February–1 March were distributed from −0.5 to 5
Summary
The Sumatra-Andaman earthquake of 26 December 2004 revealed an historically large seismic moment released beneath the seafloor 1200 km from offshore Simeulue Island south of Sumatra Island to the north of Andaman Island. All analyses above are based on observed data that were taken remotely from the rupture area With such observed data, it is difficult to infer the precise geometry of the earthquake faults that slipped during the earthquake. It is difficult to infer the precise geometry of the earthquake faults that slipped during the earthquake Many of those analyses subsumed the geometry of a plate boundary in which the rupture took place. They relied on results of Harvard CMT analyses or geometry of background seismicity The former assumes that the entire rupture took place in a simple planar surface. The latter data (from such a catalog as Engdahl et al, 1998) has great limitations that are unavoidable from the scarce number of earthquakes observed during 40 years or so. It is especially difficult above 30 km to find an appropriate depth and dipping angle of the plate interface from the seismicity
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