The 1917 Samoa tsunamigenic earthquake is the largest historical event to impact this region. Over a century later, little is known about the tsunami magnitude and its implications for modern society. This study reconstructs the 1917 tsunami to understand its hazard characteristics in the Samoan region and assesses the risk implications of tsunamis sourced from different locations along the subduction zone bend of the Northern Tonga Trench (NTT). We model the event from its origin to produce outputs of tsunami inundation extent and depth at spatially flexible grid resolution, which are validated using available runup observations and Apia harbour tide gauge records. We then combine the inundation model with digital distributions of buildings to produce exposure metrics for evaluating the likely impacts on present-day coastal assets and populations if a similar tsunami were to occur. Results exhibit recorded and modelled wave arrival time discrepancies in Apia harbour of between 30–40 min, with runup underestimated in southeast Upolu Island compared with the rest of the country. These differences could reflect complexities in the tsunami source mechanism that are not represented in our modelling and require further investigation. Nevertheless, our findings suggest that if a characteristic 1917-type event were to occur again, approximately 71% of exposed people would reside in Savai’i. Overall, this study provides the first detailed inundation model of the 1917 tsunami that supports an appreciation of the regional risk to local tsunamis sourced at the subduction zone bend of the NTT in Samoa.

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.

The 26th 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 for this event compared with the recent 2009 tsunamigenic earthquake which originated in a proximal source region at the Northern Tonga Trench. In this paper, we reconstruct the 1917 tsunami from source to inundation using the BG-Flood numerical modelling suite to understand the magnitude of inundation for this event. Model outputs representing inundation extent and hazard depth intensities at spatially flexible grid resolution (10 m, 20 m and 40 m), are validated using available tide gauge records in Apia harbour and limited observations of runup that were derived from historical records. Results indicate variable modelled-to-observed consistency using available source models, wave and runup validation data. Significant 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., west Savai’i Island). We combine the inundation model with available digital distributions of buildings and roads 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. A comparison between the distribution of hazard risk exposure for the 1917 and 2009 events is discussed along with the uncertainties in our results, with suggestions for future work offered.

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