HR: 16:20h
AN: T24A-01    [Abstracts]
TI: Hunting for Ancient Tsunamis in the Tropics
AU: * Atwater, B F
EM: atwater@usgs.gov
AF: USGS at ESS Univ of Washington, Box 351310, Seattle, WA 98195, United States
AB: Paleotsunami deposits may prove harder to find in tidal wetlands and beach-ridge plains around the tropical Indian Ocean than in temperate but otherwise comparable settings on the Pacific Rim. The reasons for this challenge are probably unrelated to tsunami size or recurrence. Estuarine marshes and grassy beach-ridge plains provide widespread opportunities for tsunamis to lay down preservable sand sheets in northeastern Japan, Kamchatka, the northwestern United States, and south-central Chile. The small plants of these lowlands offered little resistance to tsunami flow. Coseismic subsidence and net late Holocene submergence provided caps of tidal mud that help preserve the sand. By contrast in tidal wetlands overrun by the 2004 Indian Ocean tsunami, mangrove swamps and their inhibit the formation and preservation of tsunami sand sheets. For example, in mangrove swamps along tidal inlets near Ban Nam Kem and Tab Lamu, Thailand, sandy deposits of the 2004 tsunami were probably limited to feather- shaped channel-margin areas where the tsunami lost much of its momentum to the toppling of leafy trees. When these deposits were examined in July 2006, deposit-feeding crabs were busily mixing the sand into muddy, peaty mangrove soils. Such limitations of mangrove swamps as paleotsunami recorders may help explain why a reconnaissance in May 2006 turned up no sand sheets in the soils of mangroves near Cilacap, on the south coast of Java. Thus far it is unclear whether this coast, which faces the Sunda Trench, lacks potential for tsunamis as enormous as Aceh's in 2004, or whether it has a history of enormous tsunamis that simply failed to leave a long-lasting record in the Cilacap mangroves. Disturbance by humans limits the paleotsunami targets on beach-ridge plains facing the Indian Ocean. Thai coastal plains, though apparently grassy where undisturbed, have been extensively modified by placer mining for tin. In Java and southeastern India, most coastal plains have been under the plow for centuries. The outlook for paleotsunami hunting in South and Southeast Asia probably depends on new targets that include coral boulders and scarcely disturbed beach-ridge plains in Thailand; archaeological sites that provide cultural timelines in India; lagoons of Sri Lanka; coastal rivers that offer cutbanks and oxbows on beach-ridge plains of Java; delicately laminated deposits of salt flats routinely overrun by storm surges on the arid northern shores of the Arabian Sea; and records of prehistoric land-level change close to fault-rupture areas along the Sunda Trench.
DE: 1817 Extreme events
DE: 3022 Marine sediments: processes and transport
DE: 4217 Coastal processes
DE: 7221 Paleoseismology (8036)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly