2004 Tsunami survey in Comoros and Tanzania with inferences on tsunami risk in the Western Indian Ocean during future mega-thrust 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 2006-2008 by International Tsunami Survey Teams. Run-up at the Northeastern tip of Grande Comore (6.8 m) is comparable to Socotra (6.1 m), and surpassed only in the Western Indian Ocean by the catastrophic values in Somalia (run-up 9 m; inundation 700 m). Run-up in Mayotte, and to a lesser extent Zanzibar, show considerable variations (from 1 to 5 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 to at least 20. 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 a probable event releasing the strain left over from the 1833 rupture after 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 17 offshore virtual gauges strategically located from Oman to South Africa. We confirm more quantitatively the results of Okal and Synolakis (2008), namely that the change of directivity characteristics results in an increase of tsunami amplitude (with respect to 2004) at all sites South of Kenya (including Madagascar and the Mascarenes), while amplitudes at the Horn of Africa (Socotra, Somalia) remain large, due to focusing by individual bathymetric features. In short, potential earthquake sources along the Southern coast of Sumatra could result in higher wave heights than in 2004, along most of the Eastern shores of Africa, Madagascar and the Mascarenes.