1946 Unimak Tsunami Earthquake Area: Revised Tectonic Structure in Reprocessed Seismic Images and a Suspect Near-Field Tsunami Source

摘要

A tsunami that was generated off Unimak Island at the western end of the Alaskan Peninsula in 1946 damaged Pacific coastal areas from Alaska to Antarctica. Its 42-meter (m) runup at Scotch Cap, Unimak Island, destroyed the lighthouse there. In the Hawaiian Islands, it took 159 lives and caused 26 million dollars of property damage; the tsunami similarly damaged island facilities across the south Pacific and destroyed a hut on the coast of Antarctica (Fryer and Tryon, 2005). The tsunami magnitude was 9.3 (Abe, 1979) which is comparable to the magnitude 9.1 tsunami (Ide and others, 2011) that devastated the populated Tohoku coast of Japan in 2011. The epicenter of the causative 1946 earthquake occurred in the shallow reaches of the Aleutian subduction zone, and its surrounding aftershocks were mostly beneath the slope between Unimak Pass and Sanak Island (Lopez and Okal, 2006). As pointed out by Kanamori (1972), the earthquake was a charter member in the tsunami earthquake category that produced an outsized wave compared to its quake magnitude. Despite several expeditions to the area and multiple seismological studies, the mechanics of this event are still controversial (Kanamori, 1972; Johnson and Satake, 1997; Okal and others, 2002, 2003; Fryer and others, 2004; Lopez and Okal, 2006). Conceivably, the 1946 earthquake produced the far-field tsunami by tectonic seafloor displacement, but the near-field tsunami is inferred to be sourced by a large landslide (Okal and Herbert, 2007). However, a causative landslide was not identified (Rathburn and others, 2009) during the latest expedition there, and if a proposed accretionary prism is located in the epicenter area (Lewis and others, 1988), this would be an unlikely location for an Mw 8.6 earthquake. The most controversial issue revolves around a possible source large enough to generate the high runup observed along the Unimak coast (Okal and others, 2003).