2004 Sumatra-Andaman tsunami surveys in the Comoro islands and Tanzania and regional tsunami hazard from future Sumatra events

We present a total of 59 new data points of run-up of the 2004 Sumatra tsunami in the Comoros and Tanzania, surveyed in 2006 to 2008 by International Tsunami Survey Teams. Run-up along the northeastern coast of Grande Comore reached 6.9 m, surpassed only in the western Indian Ocean by the catastrophic values in Somalia (9 m). Tsunami inundation in Mayotte, and to a lesser extent Zanzibar, show considerable variations (from 1 to 4 m), attributed to the influence of the local structure of the reef surrounding these islands. By contrast, the unreefed islands of Anjouan and Moheli, and the mainland coast of Tanzania around Dar-es-Salaam, feature more consistent values in the 2 to 3 m range. The death toll in Tanzania is revised upwards to an estimate of at least 20. This new dataset complements the ones previously published for other western Indian Ocean shores, from Oman to South Africa. We then use the MOST code to simulate the propagation on the high seas of both the 2004 tsunami, and of potential future tsunamis under scenarios of mega-earthquakes rupturing in the South Sumatra region; in particular, we consider the case of the great 1833 Mentawai earthquake, and of a probable future event releasing the strain accumulated on the 1833 rupture area but not released during the 2007 Bengkulu earthquake. While these studies are not carried to the full extent of run-up calculations at individual sites, they give a general estimate of expectable hazard, relative to 2004, under the relevant scenarios, at 19 offshore virtual gauges strategically located from Oman to South Africa. In general, the fragmentation of rupture expressed by the relatively moderate 2007 Bengkulu event leads to wave amplitudes smaller than in 2004 under the most probable future scenario, but at specific sites, including Port Elizabeth, unfavourable conditions such as a high tide could bring run-up to higher levels, with potential for serious destruction. Under the worst-case scenario of a rupture extending southeast of the 1833 fault zone, run-up greater than in 2004 could be expected at all sites south of the Comoros.