Tsunami Impact Computed from Offshore Modeling and Coastal Amplification Laws: Insights from the 2004 Indian Ocean Tsunami

The 2004 Indian Ocean tsunami gave the opportunity to gather unprecedented tsunami observation databases for various coastlines. We present here an analysis of such databases gathered for 3 coastlines, among the most impacted in 2004 in the intermediate- and far field: Thailand–Myanmar, SE India–Sri Lanka, and SE Madagascar. Non-linear shallow water tsunami modeling performed on a single 4′ coarse bathymetric grid is compared to these observations, in order to check to which extent a simple approach based on the usual energy conservation laws (either Green’s or Synolakis laws) can explain the data. The idea is to fit tsunami data with numerical modeling carried out without any refined coastal bathymetry/topography. To this end several parameters are discussed, namely the bathymetric depth to which model results must be extrapolated (using the Green’s law), or the mean bathymetric slope to consider near the studied coast (when using the Synolakis law). Using extrapolation depths from 1 to 10 m generally allows a good fit; however, a 0.1 m is required for some others, especially in the far field (Madagascar) possibly due to enhanced numerical dispersion. Such a method also allows describing the tsunami impact variability along a given coastline. Then, using a series of scenarios, we propose a preliminary statistical assessment of tsunami impact for a given earthquake magnitude along the Indonesian subduction. Conversely, the sources mostly contributing to a specific hazard can also be mapped onto the sources, providing a first order definition of which sources are threatening the 3 studied coastlines.