Forearc Backthrusting as a Source of 26 December 2004 Sumatra-Andaman Tsunami
Abstract
The massive tsunami generated during the Great Sumatra-Andaman earthquake of 26 December 2004 killed over 250000 around the coastlines of Southeast Asia, with the biggest loss in northern Sumatra of about 150000 lives where runup heights reached up to 30 metres. The source of the 26 December 2004 tsunami has been related to vertical coseismic uplift near the subduction toe and possible splay faults within the accretionary wedge, but these modelled sources do not completely reconcile with the observed near field characteristics of the tsunami in north Sumatra. The maximum coseismic slip estimated seismologically and even higher slip values postulated by tsunami modelling results do not explain the large runup heights reported, if the vertical uplifting of the water column was just localized near the subduction front. Using high- resolution seismic tomography results here we demarcate the presence of a backstop buttress structure beneath the outer arc high. On coincident deep seismic reflection profile we show the presence of an active backthrust that is imaged down to at least 15 km depth and reaches the surface near the western edge of the Aceh forearc basin. We suggest that coseismic slip along the forearc backthrust boundary might provide a reasonable explanation for the generation of this massive tsunami, and can explain the incongruities in the observed large runup heights and arrival times in the near field. Modelling results obtained by solving shallow water wave equations for tsunami propogation, from the proposed backthrust tsunami source further support our hypothesis and corroborate with near field tsunami arrival times recorded. The dip of backthrust being nearly orthogonal to that of the main thrust, it would be difficult to separate the rupture along the two thrusts seismologically. These results imply that slip along backthrust faults during a mega-thrust earthquake could prove destructive, and should be considered during tsunami modelling and mitigation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.U51A0004C
- Keywords:
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- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 7240 Subduction zones (1207;
- 1219;
- 1240);
- 7270 Tomography (6982;
- 8180);
- 8104 Continental margins: convergent;
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)