The Great Sumatra-Andaman Earthquake: Why did the rupture propagate north not the southeast of the epicenter?

HR: 0800h
AN: U11A-0830 [Abstracts]
TI: The Great Sumatra-Andaman Earthquake: Why did the rupture propagate north not the southeast of the epicenter?
AU: Malod, J
U11A-0830 AF: UBO, UMR 6538 Place Nicolas Copernic, Brest, 29280 France
AU: * Singh, S C
EM: singh@ipgp.jussieu.fr
AF: IPGP, Geosciences Marines 4, Place Jussieu, Paris, 75005 France
AU: Sibuet, J C
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Rangin, C
U11A-0830 AF: College de France, Hall D- BP 80, AIX EN PROVENCE, 13545 France
AU: Apprioual, R
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Begot, J
U11A-0830 AF: UBO, UMR 6538 Place Nicolas Copernic, Brest, 29280 France
AU: Cattaneo, A
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Chauhan, A
U11A-0830 AF: IPGP, Geosciences Marines 4, Place Jussieu, Paris, 75005 France
AU: Creach, R
U11A-0830 AF: SHOM, MOA, Brest, 29240 France
AU: Crozon, J
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Domzig, A
U11A-0830 AF: UBO, UMR 6538 Place Nicolas Copernic, Brest, 29280 France
AU: Falleau, N
U11A-0830 AF: IPGP, Geosciences Marines 4, Place Jussieu, Paris, 75005 France
AU: Graindorge, D
U11A-0830 AF: UBO, UMR 6538 Place Nicolas Copernic, Brest, 29280 France
AU: Harmegnies, F
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Klingelhoefer, F
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Kolloru, S K
U11A-0830 AF: NIO, Dona Paula, Goa, 403004 India
AU: Landure, J Y
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Lann, C L
U11A-0830 AF: SHOM, MOA, Brest, 29240 France
AU: Normand, A
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Oggian, G
U11A-0830 AF: Universite Bordeaux I, UMR 5805 EPOC, Bourdeaux, 33405 France
AU: Schneider, J L
U11A-0830 AF: SHOM, MOA, Brest, 29240 France
AU: Sultan, N
U11A-0830 AF: Ifremer, DRO/GM BP 70, Brest, 29280 France
AU: Umber, M
U11A-0830 AF: ULP, 4 rue Blaise Pascal, Strasbourg, F-67070 France
AU: Yamaguchi, H
U11A-0830 AF: JAMSTEC, Natsushima-cho2-15, Yokosuka, 237-0061 Japan
AB: The Great Sumatra-Andaman Earthquake of December 26, 2004 initiated around 3 N near Simeulue and the rupture propagated for about 1250 km up to Andaman Islands. These results were further confirmed by the aftershocks that are located in the same region between 3 N and 12 N. The epicenter of second large mega-thrust earthquake of March 28, 2005 was about 150 km further SE. The aftershocks and modeling study suggest that the rupture of this earthquake was confined SE of Simeulue Island. These observations require a lithosphere-scale boundary near Simeulue, which could be either in the lower plate or in the upper plate. The occurrence of a rupture in the lower plate has been evidenced from slab earthquakes pattern and tomography. However, the distribution of earthquakes for 30 years before the great Sumatra earthquake shows that the locked zone, the area with very few events, is located beneath the accretionary prism west of Simeulue Island and of the Aceh fore-arc basin. Then we have to consider the role of an upper plate major boundary in the propagation of the rupture. In July-August 2005, we carried out a marine survey on the French R/V Marion Dufresne. We recorded swath bathymetry, imaging and 3.5 kHz echo-sounder surveys in a 380*80 km area that covers the trench in the Indian Ocean to the north of the Sumatra fault. We found an active feature on the western flank of the Aceh fore-arc basin that seems to be connected with West Andaman Fault (WAF) in the north. It is very clearly visible for about 200 km, but we were able to follow it southwards for further 200 km, close to the trench SW of Simeulue Island.This feature is segmented on a small scale from tens of km to 50-60 km into restraining and releasing bends leading to compressive and extensive regime along the fault, suggestive of right lateral strike slip motion. Its azimuth varies from 345° in north to 325° close to Simeulue Island as it approaches the trench. We suggest that this wrench lineament named the Aceh Wrench Fault (AWF) is connected to WAF and is a major lithospheric boundary between subduction wedge in the west and Sumatra fore-arc sliver plate in the east It might have acted as the south-eastern extremity of the 26th December earthquake rupture. A part of it might be the northeastern boundary of the seismogenic zone (locked zone) that guided the propagation of rupture northward within a corridor bounded by the trench in the west and WAF in the east. High heat flow and the occurrence of several morphological scarps in the accretionary prism between the trench and WAF may indicate transpressional tectonics in this region. The AWF-WAF feature might be accommodating the extra one-third right-lateral motion required by the GPS studies. The strong aftershocks activity east of Nicobar Islands might be associated with the interaction between the WAF and Sumatra Fault. These results should have important implication for seismic and tsunami risks and modeling studies, and they suggest that the upper plate plays fundamental role on the size and nature of the earthquake.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Union [U]
MN: Fall Meeting 2005