Geologic Signatures of the September 2009 South Pacific Tsunami (Invited)<i/>
Abstract
The September 29th 2009 tsunami caused widespread destruction along the shorelines of Samoa in the South Pacific. Preliminary measurements indicate maximum runup values of around 12 m (average ~3-6 m) and shore-normal inundation distances of up to ~ 400m. Geological field reconnaissance studies were conducted within three weeks of the event in order to document the erosion, transport, and deposition of sediment by the tsunami. Types of data collected included: a) general morphology and characteristics of the coast, b) tsunami water level measurements (inundation, flow depth and direction, and wave height, c) surficial and subsurface sediment samples, d) topographic mapping, and e) boulder size and location measurements. Where available for transport, mud, sand, and gravel size material was moved by the tsunami and formed distinct sedimentary deposits. Four main sedimentary deposits were identified: 1) Gravel deposits that typically occurred as either isolated coral boulders derived from the adjacent reef system and deposited on the lower beach face, or, fields of basalt boulders derived from coastal engineering structures and deposited inland on the coastal plain. In both cases the boulders were found either on the surface or partially buried by sand. Patchy accumulations of staghorn corals (Acropora sp?) occurred along some shorelines, presumably where there was a nearby reef source. 2) Sand deposits that ranged from very thin patches (< 1 cm) to broad sand sheets up to 10’s of cm thick. Localized thick sand accumulations were common in the lee of structures, such as low walls, and in topographic depressions. The thicker sand deposits had multiple laminations with varying degrees of particle segregation. In some localities clusters of the green alga Halimeda were incorporated in sand deposits. The sand appeared to be derived from the reef flats, beaches, and in some cases erosion of the land surface (i.e. from sandy soil). 3) Organic debris of over 10 cm thickness found further inland (at the forest edge) in topographic lows. 4) Surface mud deposits ranged from thin mud drapes (< 1 cm thick) to multi-layered, thick (4+ cm) mud caps that showed pronounced desiccation cracks two weeks after the tsunami. Documenting the geologic characteristics of recent tsunami deposits is an important tool used in the identification of paleotsunamis in the geologic record. The spatial distribution of the deposits, combined with sediment texture and composition attributes, will provide valuable criteria in the discrimination between tsunami deposits and those formed by other processes such as extreme storms or variations in sea level.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.U24A..05R
- Keywords:
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- 4564 OCEANOGRAPHY: PHYSICAL / Tsunamis and storm surges;
- 9355 GEOGRAPHIC LOCATION / Pacific Ocean