Reassessment of Long-Term Tsunami Hazards in Samoa Based on Sedimentary Signatures

Abstract

Investigating tsunamis and cyclones from depositional records enables an understanding of the long-term hazards to coastal communities. In Samoa, whilst a long-term record of tsunamis and cyclones spanning the last few millennia has been previously suggested based on preliminary sediment core/trench studies, a detailed assessment of the characteristics distinguishing these events has not been presented. This study reevaluates the depositional evidence available for Samoa and offers a more robust interpretation of the temporal and spatial records of tsunami events preserved in the Samoan sedimentary record. Tsunami inundation and runup records of the 2009 South Pacific tsunami along with differences in depositional settings, and sedimentary and geochemical characteristics of the associated deposits provide modern analogies for interpreting comparable older event-type deposits deeper in the Samoan geological record. These are aided by the 1990/1991 Cyclones Ofa and Val deposits previously suggested at some sites, which provides a modern analogy for interpreting cyclone-related deposits. Available radiocarbon and radiometric dates for the core/trench sites provide time-indicators to identify contemporaneous events, which we use to interpret the long-term record of tsunamis in this island region.

Reconstructing the 26th June, 1917 Samoa Tsunami Disaster and Implications on Present-day Hazard Exposure

“…The lesser known predecessor to the 2009 event, the 26 th June, 1917 UTC (local time in 1917 = UTC-11) earthquake and tsunami which originated in a proximal source region northwest of the 2009 epicentre (Figure 2), is arguably considered the largest earthquake to have occurred in this region in terms of magnitude (i.e., Mw 8.3 compared with Mw 8.1 for the 2009 earthquake sequence) [22][23][24]. However, the scale of impacts from the resulting tsunami appear to have paled in comparison with the devastation observed in the 2009 event (e.g., [25,26]). While anecdotal records indicate that the 1917 tsunami inundation had flooded several villages and caused damage to buildings and infrastructure (e.g., Satupaitea in southeast Savai'i and Lotofaga in southeast Upolu) [4,27,28], there are virtually no accounts of any casualties.…”

“…Runup observations derived from historical records of eyewitness accounts documented in [4,26,27], provided benchmarks to infer the extent of wave runup onto land. These information were digitized to help validate the tsunami runup modelling.…”

Multiscale Quantification of Tsunami Hazard Exposure in a Pacific Small Island Developing State: The Case of Samoa

“…Recent studies have revealed an apparent long-term local tsunami hazard in Samoa associated with the NTT source region [31]. Indeed, the most recent predecessor to the 2009 SPT was the 1917 Mw 8.3 Samoa-Tonga earthquake and tsunami, which is known to have inundated the southern coasts of Savaii and Upolu [28,32] (Figure 1).…”

“…Indeed, the most recent predecessor to the 2009 SPT was the 1917 Mw 8.3 Samoa-Tonga earthquake and tsunami, which is known to have inundated the southern coasts of Savaii and Upolu [28,32] (Figure 1). Detailed historical records on the built environment's damage and loss of life from the 1917 event are scant (e.g., [28,31]). Available sedimentary and anecdotal evidence suggests that inundation was more geographically widespread than the 2009 SPT, extending west from Falealupo on Savaii eastward to Lalomanu on Upolu (e.g., [31]).…”

“…Detailed historical records on the built environment's damage and loss of life from the 1917 event are scant (e.g., [28,31]). Available sedimentary and anecdotal evidence suggests that inundation was more geographically widespread than the 2009 SPT, extending west from Falealupo on Savaii eastward to Lalomanu on Upolu (e.g., [31]). However, there does not appear to be evidence of impacts comparable to the magnitude of the devastation experienced due to the 2009 event, which was focused on southeast Upolu.…”