CONSTRAIN · Constrained heterogeneous loading of complex fault networks

Final Activity Report Summary - CONSTRAIN (Constrained heterogeneous loading of complex fault networks)

Several independent indicators imply a high probability of a great, i.e. of magnitude greater than eight, earthquake rupture of the subduction megathrust under the Mentawai Islands of West Sumatra. The Sunda megathrust is advanced in its seismic cycle and, additionally, has experienced stress increases due to March 2005 earthquake. The human consequences of such an event depend crucially on its tsunamigenic potential, which in turn depends on unpredictable details of the slip distribution of the future earthquake and on how the resulting seafloor movements and the propagating tsunami waves shall interact with bathymetry. A lack of detailed knowledge of the possible future earthquake in the region was overcome by modelling about 1 000 possible complex earthquake ruptures and calculating the seafloor displacements and tsunami wave height distributions that would result from the most likely 100 or so. The likelihood of the events was judged by reference to paleogeodetic and global positioning system (GPS) data and stress accumulation studies. The vertical seabed displacement was calculated by means of a fully three-dimensional numerical model in which the rheological and topographic complexities of the subduction zone were included. The performed simulations showed the great importance of lateral and vertical geological complexities in the seafloor deformation and its effects in the maximum produced tsunami height. Through digitising nautical charts, we adopted a highly detailed seafloor bathymetry that was understood to strongly influence the wave height close to the coast. A robust, and general, observation was that Indian Ocean coasts, hit hard by the December 2004 tsunami, would experience much smaller waves from future rupture of the Mentawai patch, mainly because of the geometry of the trench that would cause dissipation of most of the energy along southwest Indian Ocean. The results also indicated a generally smaller regional tsunami hazard than that which was realised in Aceh during the December 2004 event, although more than 20 % of simulations resulted in tsunami wave heights of more than 5 m for the southern Sumatran cities of Padang and Bengkulu, indicating a great threat for the cities along the coast. Remarkably, the results showed that, for any near-field location, the timing of tsunami inundation was independent of the slip-distribution of the earthquake or even of its magnitude. This knowledge would be of paramount importance for developing tsunami preparedness strategies. All project results were of great importance in developing tsunami preparedness strategies around the Indian Ocean and in particular along the coasts of western Sumatra.

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