The 29th September Samoa Islands tsunami: preliminary simulations based on the first focal mechanisms hypotheses and implications of uncertainties in tsunami early warning strategies
Abstract
At 6:48 AM local time (17:48 UTC time) a strong earthquake of magnitude Mw=8.0 occurred less than 200 km south of the Samoa Islands (Western Samoa and American Samoa), triggering a tsunami that was detected by several tide gauges located all around the source area. The areas most affected were the south coasts of Western and American Samoa, where almost 200 persons were killed and run-up heights were measured in excess of 5 meters on several locations along the coast and and the tide gauges reached a maximum peak-to-peak height of about 3 meters near Pago-Pago (American Samoa) and 1.5 meters in front of Apia (Western Samoa) The existence of many tide gauge records is important to support the investigation of the source mechanism. The epicenter of this earthquake is located very close to the point where the Tonga trench turns its direction from northward to westward. Here the Pacific plate moves westward beneath the Australia plate, determining a subduction zone along the north-oriented segment of the trench and a transform zone along the west-oriented segment. The epicenter location in this complex tectonic context makes identifying the fault mechanism responsible for the tsunami generation a non-trivial task. The goal of this preliminary work is testing different fault models based on the focal mechanism solution proposed by USGS, CMT and EMSC for this earthquake, through the comparison between the tide gauge records and the synthetic signals provided by the numerical simulations, and possibly suggesting new source solutions trying to reproduce as better as possible the tsunami recordings. The numerical simulations are computed by means of the UBO-TSUFD code, developed and maintained by the Tsunami Research Team of the University of Bologna, Italy. The code solves the linear and non-linear shallow water equations and can compute inundation inland. Furthermore the computational domain can be split in grids of different space resolution in order to have more detailed results in specific areas. The objective difficulties in the identification of the tsunami source, due to the quite complex tectonic setting of the Tonga region in the epicentral area introduce uncertainties in the fault determination that maybe relevant a posteriori, and are a fortiori much more relevant in the real-time data processing practice. This reflects in uncertainties in the possibility of accurately forecasting tsunami propagation and arrival, which poses problems concerning the best strategy to adopt for tsunami early warning.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.U21E2185T
- Keywords:
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- 4255 OCEANOGRAPHY: GENERAL / Numerical modeling;
- 4564 OCEANOGRAPHY: PHYSICAL / Tsunamis and storm surges