The destructive 1946 Unimak near-field tsunami: New evidence for a submarine slide source from reprocessed marine geophysical data

Abstract

The Mw 8.6 earthquake in 1946 off the Pacific shore of Unimak Island at the end of the Alaska Peninsula generated a far-field tsunami that crossed the Pacific to Antarctica. Its tsunami magnitude, 9.3, is comparable to the 9.1 magnitude of the 2011 Tohoku tsunami. On Unimak Island's Pacific shore, a runup of 42 m destroyed the lighthouse at Scotch Cap. Elsewhere, localized tsunamis with such high runups have been interpreted as caused by large submarine landslides. However, previous to this study, no landslide large enough to generate this runup was found in the area that is limited by the time interval between earthquake shaking and tsunami inundation at Scotch Cap. Reworking of a seismic reflection transect and colocated multibeam bathymetric surveys reveal a landslide block that may explain the 1946 high runup. It is seaward of Scotch Cap on the midslope terrace and within the time-limited area. Key Points A 10 × 10 km landslide block on the Alaska trench midslope terrace was imagedThis block is a most likely source for the 1946 42 m tsunami runup at Scotch CapReworking legacy data advanced an understanding of tectonic and slide history