Source of the great tsunami of 1 April 1946: a landslide in the upper Aleutian forearc

The Unimak (eastern Aleutians) earthquake of April 1, 1946 is an enigma. The earthquake (MS=7.1) produced a disproportionately large tsunami (Mt=9.3) which killed 167 people. The tsunami was highly directional, and projected its largest waves along a beam perpendicular to the Aleutian arc. Those waves passed just east of the Hawaiian Islands, ran the length of the Pacific, and were still large when they ran ashore in Antarctica. In the near field, the tsunami had very high runup (42 m at Scotch Cap) but rapid lateral decay (6 m at Sanak Village, 120 km to the east). No earthquake source can simultaneously explain the narrow beam of large waves in the far field and the rapid variation in near-source runup. The slow rupture, the tsunami directivity, the rapid variation in near-source wave heights, the period of the waves, and the strong T-phase generation, together suggest an earthquake-triggered landslide rather than a purely tectonic source. From USGS GLORIA imagery we have identified a candidate landslide within the aftershock zone of 1946. The slide bites into the Aleutian shelf at a depth of only 120 m, is 25 km across, 65 km long, and has a volume of 200–300 km3. A slide with these dimensions would produce a tsunami matching the observations while still satisfying the seismic data. Such slope failures appear to be common along the Aleutian forearc, which has serious implications for tsunami warning.