Tsunami inundation maps for Skagway and Haines, Alaska

In this report we evaluate potential tsunami hazards for the southeastern Alaska communities of Skagway and Haines and numerically model the extent of inundation from tsunami waves generated by tectonic and submarine landslide sources. We calibrate our tsunami model by numerically simulating the 2011 Tohoku, Japan tsunami at Skagway and comparing our results to instrument records. Analysis of calculated and observed water level dynamics for the 2011 event in Skagway reveals that the model underestimates the observed wave heights in the city by a factor of 1.5, likely due to complex tsunami-tide interactions. We compensate for this underestimation numerically by increasing the coseismic slip of the hypothetical tsunami sources in our models. Potential hypothetical maximum credible tsunami sources include variations of the extended 1964 rupture and megathrust earthquakes in the Prince William Sound and Alaska Peninsula regions. Local underwater landslide events in Taiya, Chilkoot, and Chilkat inlets are also considered as possible tsunamigenic scenarios. The results show that the maximum predicted wave height resulting from a tectonic tsunami is 2–3 m (7–10 ft) in Skagway and Haines, while the maximum landslide-generated tsunami may cause a runup of 15–16 m (49–52 ft). Results presented here are intended to provide guidance to local emergency management agencies in tsunami inundation assessment, evacuation planning, and public education to mitigate future tsunami hazards. INTRODUCTION Subduction of the Pacific plate under the North American plate has resulted in numerous great earthquakes and is the source of locally-generated tsunamis in Alaska (Dunbar and Weaver, 2008). Several historic earthquakes along the Alaska–Aleutian subduction zone (fig. 1) have generated tsunamis resulting in widespread damage and loss of life in exposed coastal communities in Alaska and throughout the Pacific (Lander, 1996). However, tsunamis originating in the vicinity of the Aleutian Islands, Alaska Peninsula, and the Gulf of Alaska are of particular concern to Alaskans because, as local hazards, waves can reach coastal communities within minutes of the earthquake that caused them. The vulnerability of Alaska’s coast to tsunami waves was demonstrated by the March 27, 1964, Great Alaska earthquake. This Mw 9.2 megathrust earthquake (fig. 1) was the largest recorded earthquake in North America and generated the most destructive tsunami in Alaska history (Johnson and others, 1996; Kanamori, 1970; Lander, 1996; Plafker and others, 1969). In addition to the major tectonic tsunami triggered by ocean-floor displacement, multiple local tsunamis were generated by submarine landslides. The landslide-generated tsunamis arrived almost immediately after shaking was felt, leaving virtually no time for warning or evacuation. Of the 131 fatalities associated with the 1964 earthquake, 122 were caused by tectonic and landslide-generated tsunamis (Lander, 1996). The local tsunamis in the Gulf of Alaska region caused most of the damage and accounted for 76 percent of the aforementioned 122 tsunami deaths (Lander, 1996). The 1964 tsunami did not uniformly impact the vast Alaska coastline. Waves did not cause significant damage in Skagway and Haines. The 1964 tsunami arrived in Skagway approximately three hours after the earthquake on a rising tide and its height was estimated to be about 5 m (16 ft). 2 Report of Investigation 2018-2 Service Layer Credits: Esri, DeLorme, GEBCO, NOAA NGDC, and other contributors 135°W 140°W 145°W 150°W 155°W 62 °N