PRELIMINARY RESULTS OF PALEOTSUNAMI DEPOSITS ALONG THE SUNDA MEGATHRUST, SIMEULUE ISLAND, SUMATRA

Although great megathrust earthquakes have been previously documented by coral microatoll studies, the timing of past tsunamis on Simeulue Island, west Sumatra, are poorly documented. According to local residents and our recent reconnaissance, a large megathrust earthquake along the Sunda arc occurred in 1907 and generated a tsunami along the west coast of Sumatra. Our recent studies have identified candidate paleotsunami deposits at three different locations on Simeulue Island between 2o and 3o N at Inor, Busong Bay, and Langi Bay. Shallow excavations at each site exposed candidate tsunami deposits. At Inor, excavations made parallel to a river that runs perpendicular to the ocean document the inland extent of past tsunamis. At both Busong Bay and Langi Bay, excavations were made parallel to the shoreline. Diatom and grain size analyses of candidate tsunami deposits will help to determine if the sand layers are marine or terrestrially derived. Coral blocks at Busong Bay and Langi Bay may have been deposited by recent (2004) and past tsunamis.

U/Th dating of a buried coral boulder found on a candidate tsunami sand in a trench exposure at Busong Bay will constrain the age of the Simeulue Island paleotsunami deposit. AMS radiocarbon dating of woody fragments from peat layers will also help to constrain the age of the candidate paleotsunami deposit. By constraining the age of the paleotsunami deposit, we will determine recurrence intervals for the portion of the Sumatran subduction zone that broke in 2004. Characterizing depositional settings and the origin of candidate tsunami deposits (marine or terrestrial) will help to further understand inundation of tsunamis and their preservation in the geologic record.