Population vulnerability to tsunami hazards informed by previous and projected disasters: a case study of American Samoa

Abstract

Population vulnerability from tsunamis is a function of the number and location of individuals in hazard zones and their ability to reach safety before wave arrival. Previous tsunami disasters can provide insight on likely evacuation behavior, but post-disaster assessments have not been used extensively in evacuation modeling. We demonstrate the utility of postdisaster assessments in pedestrian evacuation modeling for tsunami hazards and use the US territory of American Samoa as our case study. We model pedestrian travel times out of tsunami inundation zones recreated for the 2009 M w 8.1 Samoa earthquake, as well as for a probable maximum tsunami zone for future threats. Modeling assumptions are guided by fatality trends and observations of 2009 evacuation behavior, including insights on departure delays, environmental cues, transportation mode, and demographic characteristics. Differences in actual fatalities from the 2009 disaster and modeled population vulnerability suggest that a single set of estimated travel times to safety does not fully characterize evacuation potential of a dispersed, at-risk population. Efforts to prepare coastal communities in American Samoa for future tsunamis may be challenging given substantial differences in wave characteristics and evacuation potential of the probable maximum hazard compared to the 2009 event.

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

“…At DART 51426 (southeast of the epicenter), a leading crest followed by a deep trough from the combined thrust and normal faulting was recorded. The comparison with the DART and Apia tide gauge records corroborates the work of Dilmen et al (2018) and Wood et al (2018) in which a leading trough was observed at the Pago Pago Tide Gauge in Tutuila and at DART 51425 in the north, and a leading crest at DART buoys 51426 and 54401 south of the epicenter.…”

“…(2018) and Wood et al. (2018) in which a leading trough was observed at the Pago Pago Tide Gauge in Tutuila and at DART 51425 in the north, and a leading crest at DART buoys 51426 and 54401 south of the epicenter.…”

“…Wood et al. (2018) later showed that multiple waves with similar amplitudes impacted most locations in American Samoa rather than a single large wave. Nevertheless, fringing reef morphology and narrow embayments are important factors in explaining the variations in runup and flow depth on Tutuila (Dilmen et al., 2015, 2018; Gelfenbaum et al., 2011; Roeber et al., 2010).…”

“…The near-shore bathymetry, generally characterized by steep slopes and narrow reef morphology on the insular shelf, was suggested to modulate the incident tsunami waves resulting in varying flow conditions around the island. Wood et al (2018) later showed that multiple waves with similar amplitudes impacted most locations in American Samoa rather than a single large wave. Nevertheless, fringing reef morphology and narrow embayments are important factors in explaining the variations in runup and flow depth on Tutuila (Dilmen et al, 2015(Dilmen et al, , 2018Gelfenbaum et al, 2011;Roeber et al, 2010).…”

“…In the aftermath of the event, extensive investigation of the tsunami source and wave characteristics were undertaken based on seismic and GPS records, field surveys, and numerical modeling (e.g., Beavan et al., 2010; Dilmen et al., 2018; Fan et al., 2016; Goff & Dominey‐Howes, 2011; Han et al., 2019; Hossen et al., 2017; Lay et al., 2010; E. A. Okal et al., 2010; Roeber et al., 2010; Williams, 2019; Wood et al., 2018; Yamazaki et al., 2011; Zhou et al., 2012). Previous efforts to reconstruct tsunami characteristics in Samoa were limited to simulating wave initiation and propagation up to the reef edge (e.g., Franchello & Annunziato, 2012; Fritz et al., 2011; E. Okal et al., 2011; Zhou et al., 2012).…”