Effects of Source Faulting and Fringing Reefs on the 2009 South Pacific Tsunami Inundation in Southeast Upolu, Samoa

The subduction zone along the northern Tonga Trench has the highest plate convergence rate in the world, but limited records of its seismic and tsunamigenic activities. In 2009, a tsunami generated by an M-W 8.1 earthquake doublet caused severe impacts in the region including damage and loss of life on the south shores of Upolu and Savaii Islands, Samoa. Here, we use numerical modeling aided by recorded data and eyewitness accounts to evaluate and identify the published source models in the Tonga Trench region that most suitably represent the 2009 event for use in hazard assessment around Samoa. We show that only a few of the published sources are suitable to reproduce the large inundation observed in Samoa, and none reproduces runup as high as observed in the areas that were most severely impacted on the southeast Upolu coast. The distribution and intensity of runup and inundation are dependent on source model, local topographic and bathymetric features, regional coastal geomorphology, and trapping of short-period waves over the reef flats. The computed results are also influenced by model approximation and grid resolution especially for areas with extreme runup records. Comparison of the relative contributions from the doublet to the southeast Upolu inundation indicates that the initial intraplate normal faulting dominated the east-northeastward tsunami propagation and inundation compared with the subsequent interplate thrust faulting. Overall, two key source models are discussed and identified for future refinement.