Abstract

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 MW 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 which of the published source models in the Tonga Trench region most suitably represents the 2009 event for use in hazard assessment around Samoa. We show that only a few of the published sources are suitable to reproduce large inundation observed in Samoa and none reproduces runup as high as observed in areas that were most severely impacted on the southeast Upolu coast. The distribution and intensity of inundation is dependent on local topographic and bathymetric features, configuration of coastal geomorphology, and trapping of short-period waves over the reef flats. For one of the sources, comparison of the relative contributions of the normal and thrust faulting components of 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.