Reevaluation of tsunami formation by debris avalanche at Augustine Volcano, Alaska

Debris avalanches entering the sea at Augustine Volcano, Alaska have been p roposed as a mechanism for generating tsunamis. Historical accounts of the 1883 eruption of the volcano describe 6- to 9-meter-high waves that struck the coastline at English Bay (Nanwalek), Alaska about 80 kilometers east of Augustine Island. These accounts are often cited as proof that volcanigeni c tsunamis from Augustine Volcano are significant hazards to the coastal zo ne of lower Cook Inlet. This claim is disputed because deposits of unequivo cal tsunami origin are not evident at more than 50 sites along the lower Co ok Inlet coastline where they might be preserved. Shallow water (< 25 m) ar ound Augustine Island, in the run-out zone for debris avalanches, limits th e size of an avalanche-caused wave. If the two most recent debris avalanche s, Burr Point (A.D. 1883) and West Island (< 500 yr. B.P.) were traveling a t velocities in the range of 50 to 100 meters per second, the kinetic energ y of the avalanches at the point of impact with the ocean would have been b etween 10(14) and 10(15) joules. Although some of this energy would be diss ipated through boundary interactions and momentum transfer between the aval anche and the sea, the initial wave should have possessed sufficient kineti c energy to do geomorphic work (erosion, sediment transport, formation of w ave-cut features) on the coastline of lower Cook Inlet. Because widespread evidence of the effects of large waves cannot be found, it appears that the debris avalanches could not have been traveling very fast when they entere d the sea, or they happened during low tide and displaced only small volume s of water. In light of these results, the hazard from volcanigenic tsunami s from Augustine Volcano appears minor, unless a very large debris avalanch e occurs at high tide.