HR: 17:30h
AN: U14A-06    [Abstracts]
TI: 26th December 2004 Great Sumatra Earthquake: first insights from the summer 2005 Marion Dufresne cruise
AU: * Sibuet, J
EM: jcsibuet@ifremer.fr
AF: Ifremer Centre de Brest, B.P. 70, Plouzane, 20280 France
AB: The 26th December 2004 Great Sumatra Earthquake is the second biggest earthquake (Mw=9.3) recorded during the past century. It initiated at a depth of 20-30 km, close to an indentation of the Indonesian fore-arc. The rupture propagated about 1200 km northward and terminated north of Andaman Islands. The "Sumatra Aftershocks" cruise performed on the French R/V Marion Dufresne started July 15 in Jakarta and ended August 9, 2005 in Colombo. We carried out a complete swath-bathymetric survey in a 370*75 km stripe located between northern Sumatra and the Indonesia/India boundary and from the trench to northeast of the Sumatra fault. 20 OBSs were deployed in the area and recorded about 2000 earthquakes during that period. Coring, heat-flow and piezometer measurements were also carried out. Offshore northern Sumatra, the Australia/Sundaland motion is 5 cm/yr in the N008° direction (Bock et al., J. Geophys. Res., 2003), which means that both strike-slip and normal components might occur along features parallel to the trench direction. However, focal mechanisms of the 26th December 2004 earthquake as well as co-seismic motions and 5-days post-seismic displacement field determined from GPS sites are essentially perpendicular to the trench direction (Vigny et al., Nature, 2005). There is consequently an apparent mismatch between deformations derived from plate motions and from co-seismic and immediate after-slip motions. This might suggest that long-term post-seismic motions might have also a significant component of strike-slip motion along features parallel to the trench direction. Between the trench and the backstop located southwest of the Aceh forearc basin, numerous trench parallel piggy back basins were mapped, suggesting the existence of numerous thrust faults. On the basis of morphology, coring and heat flow measurements, a major active thrust fault, which might correspond to an active splay fault, was identified southwest of the Aceh forearc basin. The identification of pull-apart basins along its trend also suggests the presence of a strike-slip component. Other parallel splay faults located in direction of the trench might have been also active during and since the 26th December 2004 earthquake but this will be established from the distribution of the recorded seismicity. In addition, we have demonstrated that 1) close to the trench, the slab was essentially inactive (no significant active tectonic features and presence of minor recent land slides); 2) the southwestern Aceh basin boundary was inactive; 3) the Sumatra fault and its offshore prolongation was also inactive suggesting that no faults were significantly active northeast of the backstop. In conclusion, the co-seismic motion was probably partitioned between a major splay fault located just southwest of the backstop and other potential splay faults located within the piggy back basins, between this feature and the trench.
DE: 8104 Continental margins: convergent
SC: Union [U]
MN: Fall Meeting 2005