HR: 16:45h
AN: S24A-04    [Abstracts]
TI: Thai Evidence for Recurrent Indian Ocean Tsunamis of the Last 3,000 Years
AU: * Jankaew, K
EM: kjankaew@yahoo.co.uk
AF: Dept of Geology, Chulalongkorn Univ, Bangkok, 10330, Thailand
AU: Atwater, B F
EM: atwater@usgs.gov
AF: USGS at Dept of Earth & Space Sciences, Univ of Washington, Seattle, WA 98195-1310, United States
AU: Charoentitirat, T
EM: ctitima@geo.sc.chula.ac.th
AF: Dept of Geology, Chulalongkorn Univ, Bangkok, 10330, Thailand
AU: Choowong, M
EM: montri@geo.sc.chula.ac.th
AF: Dept of Geology, Chulalongkorn Univ, Bangkok, 10330, Thailand
AU: Martin, M E
EM: memartin@u.washington.edu
AF: Dept of Earth & Space Sciences, Univ of Washington, Seattle, WA 98195-1310, United States
AU: Rajeshwara Rao, N
EM: raonandamuri@gmail.com
AF: Dept of Applied Geology, University of Madras, Chennai, 600-005, India
AU: Seshachalam, S
EM: ponmozhisrini2001@yahoo.com
AF: Dept of Geology, Anna Univ, Chennai, 600-025, India
AU: Sawai, Y
EM: yuki.sawai@aist.go.jp
AF: Active Fault Research Center AIST, Univ of Washington, Seattle, WA 98195-1310, United States
AB: Late Holocene tsunamis probably like the one in 2004 have attacked Thailand's Andaman Sea coast at irregular intervals of less than 1,000 years on average. The 2004 tsunami ended a recurrence interval ~600 years long. Such geology could have foretold the 2004 Indian Ocean tsunami. These interpretations, all preliminary, are based on stratigraphic reconnaissance of marshy swales between grassy beach ridges of Ko Phra Thong, an island 125 km north of Phuket. Pre-2004 tsunamis best explain several properties of sand lentils that are intercalated with freshwater peaty soils of the swales: SETTING. The island's beach-ridge plain, which extends 3 km inland and 15 km coastwise, was mostly overrun by the 2004 tsunami, which coated much of it with sand. By contrast, river floods do not reach the plain because tidal inlets separate Ko Phra Thong from the mainland. Storm surges of recent decades scarcely penetrated the beach-ridge plain. Storm runoff from the beach-ridge crests, which gently rise just 1-2 m above the swales, is limited by permeable soils; the ridges lack rills and sheetwash fans. THICKNESS. Like the 2004 tsunami deposit, three of the sand lentils maintain thicknesses of 5-20 cm along a cross-swale extent of 20-40 m, as seen in correlated pits and a trench 0.5 km inland from the present beach. ARCHITECTURE. Along this cross section and another nearby, each of the sand lentils extends farther up the seaward flank of the swale than up the landward side. The 2004 sand sheet is thick on the seaward flank and thin on the landward flank. FREQUENCY. In a sequence of three sand lentils, the youngest is close to 550-700 years old and the two earlier ones probably formed between 2,000 and 2,800 years ago. These estimates are based on radiocarbon dating of leaves and bark in buried soils and of marine shells below the lowest peat. RECYCLING. Two of the lentils contain leaf fragments centuries older than the time of deposition. The discordance is consistent with tsunami-induced scour of the beds of tidal inlets. Puzzles about the sand lentils include: MICROFOSSILS. Sand lentils examined thus far contain few calcareous foraminiferal tests and no diatom valves. Both these fossil types are abundant in Ko Phra Thong's 2004 tsunami deposits. In addition, the fossils of freshwater diatoms have been found in the peaty soils of the swales, both beneath the 2004 deposits and beneath the underlying sand lentils as well. Did tropical diagenesis erase fossil diatoms from the sand lentils? INUNDATION. Along the cross sections studied thus far, the maximum flow depth in 2004 probably exceeded 10 m. Do the sand lentils represent similar inundation? TSUNAMI SOURCES. In one of the cross sections, traces of a sand bed 1-2 cm thick are preserved between two of the sand lentils. Does the thin bed signify a tsunami source little larger than that of 1881, while the thicker sand lentils signify full-length Sumatra-Andaman ruptures?
DE: 3030 Micropaleontology (0459, 4944)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 4564 Tsunamis and storm surges
DE: 8036 Paleoseismology (7221)
SC: Seismology [S]
MN: 2007 Fall Meeting