Reconstructing the 26 June 1917 Samoa Tsunami Disaster

Abstract

The 26 June 1917 tsunamigenic earthquake in Samoa is considered the largest historical event on record to have impacted this region in terms of earthquake magnitude and intensity. Yet, very little is known about the scale and distribution of tsunami impacts compared with the recent 2009 event which originated about 150 km east along the subduction zone bend of the Northern Tonga Trench (NTT). In this study we set out to: 1) reconstruct the 1917 tsunami from source to inundation to understand its hazard risk characteristics in the Samoan islands of Savai’i and Upolu; and 2) assess the hazard implications of tsunamis sourced from different locations along the subduction zone bend of the NTT on present-day exposure of coastal assets relative to the 2009 tsunami benchmark. We use the BG-Flood numerical modelling suite to produce model outputs representing inundation extent and hazard depth intensities at spatially flexible grid resolution (10 m, 20 m and 40 m). These are validated using available tide gauge records in Apia harbour and limited observations of runup that were derived from historical records. We then combine the inundation model with available digital distributions of buildings in the RiskScape multi-hazard risk analysis software, to produce exposure metrics for understanding the likely impacts on present-day coastal asset and population distributions if a similar tsunami were to occur. Results of the tsunami modelling indicate variable modelled-to-observed consistency using available source models, wave and runup validation data. Discrepancies in recorded vs modelled wave arrival time at Apia of between 30—40 mins are observed, with modelled runup underestimated in southeast Upolu Island compared with the rest of the country where runup observations are available (e.g., Savai’i Island). These differences likely reflect complexities in the tsunami source mechanism which might not currently be represented in our modelling. Nevertheless, our results suggest that a larger proportion of people would be exposed in Savai’i island (71% of exposure total), compared with Upolu island if a characteristic 1917-type event were to occur. While this study provides the first detailed inundation model of the 1917 tsunami in the Samoan region, the observed discrepancies suggest that further investigation is required to constrain potential tsunami source complexities which might not be accounted for in this study. Notwithstanding these limitations, our findings help to reinforce an appreciation of the risk to the greater Samoan region faced by local tsunamis sourced at different locations along the subduction zone bend of the NTT.