Tsunami inundation maps for the city of Sand Point, Alaska

Abstract

Staff from Alaska Earthquake Center, Geophysical Institute and Alaska Division of Geological & Geophysical Surveys evaluated potential tsunami hazards for the city of Sand Point, on Popof Island in the Shumagin Islands archipelago. We numerically modeled the extent of inundation from tsunami waves generated by local and distant earthquake sources. We considered the results in light of historical observations. The worst-case scenarios are defined by analyzing results of the sensitivity study of the tsunami dynamics with respect to different slip distributions along the Aleutian megathrust. For the Sand Point area, the worst-case scenarios are thought to be thrust earthquakes in the Shumagin Islands region with magnitudes ranging from Mw 8.8 to Mw 9.0. We additionally consider a Mw 9.0 rupture between Shumagin Islands and Kodiak Island, a Mw 9.0 rupture of the Cascadia subduction zone, and a Mw 8.6 outer-rise rupture in the area of the Shumagin Islands. Conducted numerical experiments reveal that a tsunami might start to arrive to Sand Point in about 15 minutes after the earthquake with a strong positive wave reaching in 1 hour the height of 7-8 m (23-26 ft) with respect to the pre-earthquake sea level. Consecutive waves might have the same of height or even be higher. The highest predicted wave height is 12-14 m (39-46 ft); a vertical difference between the trough and crest could be as much as 16 m (53 ft) and a time period between the water withdrawal and runup could be as short as 15 minutes. At least three devastating waves can reach the community in the first 2.5 hours after the earthquake. The first wave may not be the highest; the later waves might be more damaging and produce larger inundation. Results from the numerical modeling are intended to provide guidance to local emergency management agencies in tsunami inundation assessment, evacuation planning, and public education to mitigate future tsunami hazards. Users can access the complete report and digital data from the DGGS website: http://doi.org/10.14509/29706.