SEATOS 2005 Summary

In May 2005, an international, interdisciplinary team of scientists conducted a detailed survey of the seafloor in the vicinity of the epicenter of the Great Sumatra earthquake of December 26, 2004-dubbed Sumatra Earthquake and Tsunami Offshore Survey (SEATOS 2005). The overall goal of the survey was to gather data to improve models of seafloor displacement and the resulting tsunami waves.

To accomplish this goal, SEATOS built upon the results from the February 2005 HMS Scott multibeam data. Seven sites
(see full summary at www.oce.uri.edu/seatos_summary.pdf) were selected from the HMS Scott data for SEATOS detailed investigations: a large underwater landslide, initially though to be the result of the December 26th earthquake: a ~20km long trench feature at the deformation front that was interpreted to represent recent seafloor displacement; seafloor locations on the three sites on the forearc high, and a steep mound north of Sumatra, interpreted as volcanic.
 
Operations began after a 30-hour steam to the northeastern edge of the margin offshore Sumatra where high-resolution seismic gear was deployed to collect the cross-margin seismc data.  Following this transect, detailed surveys including seismic, and remotely operated vehicles (ROV) were conducted at each site. ROV surveys included high definition television quality video imaging of the seafloor, as well as core sampling for biological and geotechnical analyses.

SEATOS 2005 collected seismic data, ROV imagery, biological core data and sedimentological core data at sites along the accretionary complex deformation front, at one site in the forearc fold regions, and at sites along the forearc high. Early results suggest seafloor disturbances (from the Dec. 26th 2004 earthquake) and the formation of new seafloor features occurred at the deformation front and stable seafloor conditions prevailed at the forearc high. In addition, on the return to port, SEATOS investigated an unusual seafloor feature 200 m high, north of Sumatra that is a volcano, that is most likely part of the island arc. These data are currently being analyzed, along with recently published seismic inversion analyses, to develop a seafloor displacement model that best represents the Dec. 26th event.

During SEATOS, tsunami simulations were conducted for the Bay of Bengal using a wave model based on fully nonlinear Boussinesq equations. Such a model better represents the oscillatory wave trains that were typically observed during the event. Initially the model used the ETOPO2 bathymetry and tsunami sources were defined using information from seismic inversion and hydroacoustic models. The comparison of model results with observations at tide gages, one satellite track, and runup measured in recent field surveys is quite good. A more detailed bathymetry was used to conduct case studies near the Northern tip of Sumatra and the coast of Thailand. These simulations are still in progress.

Measured bottom displacements during SEATOS are being analyzed to better define the co-seismic tsunami source(s) used in the tsunami wave model and, hopefully, provide an even better understanding of the 12/26/04 tsunami event.

Contacts: Kate Moran, kate.moran@uri.edu; Stephan Grilli, grilli@oce.uri.edu; and Dave Tappin, DRTA@exchange.keyworth.bgs.ac.uk

To view full text with images, go to: www.oce.uri.edu/seatos_summary

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