HR: 14:10h
AN: OS13B-03    [Abstracts]
TI: Tsunami Deposits on Simeulue Island, Indonesia—A tale of two tsunamis
AU: * Jaffe, B E
EM: bjaffe@usgs.gov
AF: U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060, United States
AU: Higman, B
EM: hig314@gmail.com
AF: University of Washington, Earth and Space Sciences Johnson Hall 070 Box 351310 4000 15th Ave. NE, Seattle, WA 98195-1310, United States
AB: As tsunami deposits become more widely used for evaluating tsunami risk, it has become increasingly valuable to improve the ability to interpret deposits to determine tsunami characteristics such as size and flow speed. A team of U.S. and Indonesian scientists went to Simeulue Island 125 km east of Sumatra in April 2005 to learn more about the relation between tsunami deposition and flow. Busong, on the southeast coast of Simeulue Island, was inundated twice in a three-months period by tsunamis. The 26 December 2004 tsunami inundated 130 m inland to an elevation of approximately 4 m. The 28 March 2005 tsunami inundated less than 100 m to an elevation of approximately 2 m. Both tsunamis created deposits that were observed to be an amalgamated 20- cm thick, predominately fine to medium sand overlying a sandy soil. The contact between 2004 and 2005 tsunami deposits is at 13 cm above the top of the sandy soil and is clearly marked by vegetation that grew on the 2004 deposit in the 3 months between tsunamis. Grass roots are present in the upper half of the 2004 deposit and absent both below that level and in the 2005 deposit. We analyzed the fine-scale sedimentary structures and vertical variation in grain size of the deposits to search for diagnostic criteria for unequivocally identifying deposits formed by multiple tsunamis. At Busung, we expected there to be differences between each tsunami's deposits because the tsunami height, period, and direction of the 2004 and 2005 tsunamis were different. Both the 2004 and 2005 deposits were predominately normally graded, although each had inversely graded and massive sections. Faint laminations, which became more defined in a peel of the deposit, were discontinuous and predominately quasi-parallel. Knowing where the contact between the two tsunamis was, subtle sedimentary differences were identified that may be used to tell that it is composed of two separate tsunamis. We will present quantitative analyses of the variations in the deposits that arise from the different flow speeds, direction of inundation, and wave heights of the 2004 and 2005 tsunamis and assess whether it is possible to apply the criteria we develop to interpret whether paleotsunami deposits were formed by multiple tsunamis.
UR: http://walrus.wr.usgs.gov/news/reports.html
DE: 1862 Sediment transport (4558)
DE: 4558 Sediment transport (1862)
DE: 4564 Tsunamis and storm surges
DE: 4863 Sedimentation (1861)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting