Pacific Basin Tsunami Hazards Associated with Mass Flows in the Aleutian Islands of Alaska

The Aleutian Islands are a chain of volcanic islands formed by an intra-oceanic subduction zone. This area consists of a submerged chain of mountains, volcanic islands, and submarine canyons, surrounded by a low- relief continental shelf above about 1000-2000 m water depth. Part of the island chain is fragmented into a series of fault-bounded blocks, tens to hundreds of km in length, and separated from one another by distinctive fault- controlled canyons that are roughly normal to the arc axis. The canyons are geomorphically low areas between the higher relief blocks and are natural regions for the accumulation and conveyance of sediment derived from glacial and volcanic processes. The volcanic islands in the region include a number of historically active volcanoes and some possess geological evidence for large-scale sector collapse into the sea. The physical setting of the Aleutian Islands indicates that mass flows of unconsolidated debris that originate either as submarine mass flows or as subaerial debris avalanches entering the sea may be potential tsunami sources. Large scale mass-flow deposits have not been identified on the seafloor south of the Aleutian Islands, primarily because the area has never been mapped or examined at the resolution required to identify such features. Extensive submarine landslide deposits and debris flows are known on the north side of the arc and are common in similar settings elsewhere and thus they likely exist on the trench slope south of the Aleutian Islands. We suggest that tsunamigenic mass flows are a plausible geologic process in the Aleutian Islands and that the tsunamis produced by such flows may be large enough to cross the Pacific Ocean basin. To test this hypothesis we present a series of numerical simulations of submarine mass-flow initiated tsunamis from eight different source areas. We consider four submarine mass flows originating in submarine canyons and four flows that evolve from submarine landslides. The flows have lengths that range from 40-80 km, maximum thicknesses of 400-800 m, and maximum widths of 10-40 km. Although some of these hypothetical flows are large, they are not unprecedented and flows of similar dimensions are known in other continental slope settings. We calculate tsunami sources using the numerical model TOPICS and simulate wave propagation across the Pacific using a spherical Boussinesq model which is a modified version of the public domain code FUNWAVE. Our numerical simulations indicate that geologically plausible submarine mass flows originating in the North Pacific near the Aleutian Islands can indeed generate large tsunamis. These waves may be several meters in amplitude at distal locations, such as Japan, Hawaii, and along the South American coastline where they would constitute significant hazards.