Inferring relative tsunami magnitudes from inverse and forward sediment transport modeling of tsunami deposits in the Eastern Aleutian Islands.

Abstract:

Tsunami recurrence intervals can be determined by age dating paleotsunami deposits, but relative tsunami magnitude is more difficult to infer from deposit characteristics alone. Deposit thickness, grain size, and certain sedimentary structures are used to infer hydrodynamic conditions during deposition, which can be used as proxies for tsunami magnitude. Recent field studies in the eastern Alaska-Aleutian subduction zone have identified sequences of tsunami deposits from the 1957 Andreanof Islands earthquake (M_W 8.6) and at least five other pre-historic tsunami events from the last 2,400 years. At Stardust Bay on the Pacific Coast of Sedanka Island, a sand-rich deposit attributed to the 1957 tsunami is 1-13 cm thick and is found at elevations up to 18.5 m. Older sand units are 6-50 cm thick and often have rounded gravel at the base of multiple, normally-graded sand beds. At Driftwood Bay on the south side of Umnak Island, about 200 km to the southeast of Stardust Bay, the 1957 deposit is 1 - 5.5 cm thick, underlain by a sequence of peat with up to 8 sandy deposits, some of which exhibit normally-graded beds up to 14 cm thick. Relatively thick deposits that exhibit suspension grading, a type of grading created by sediment falling out of suspension that is often observed in modern tsunami deposits, are typically formed under steady and uniform flow and are therefore good candidates for reconstructing flow conditions using inverse sediment transport models. By applying forward models of sediment transport, we will test how different tsunami waveforms, wave heights, sediment source distributions, roughness, and local slopes affect patterns of deposition. This will help us assess which deposits have characteristics that scale with tsunami wave heights used as initial conditions in the forward model, and are therefore more indicative of relative tsunami magnitude. Here, we attempt to determine if the tsunamis that created the pre-historic deposits found at Stardust and Driftwood Bays were larger than the 1957 tsunami. Several of the deposit ages can be correlated between Stardust and Driftwood Bay, suggesting common sources in large subduction zone earthquakes. We may ultimately be able to use the deposit characteristics and modeling results to estimate minimum magnitudes of these pre-historic great earthquakes.