A new review of tsunami hazards concludes that the 2004 catastrophe was far from the worst possible in many Indian Ocean borderlands - and notes that warning systems to guard at-risk populations are still lagging.
Costas Synolakis, director of the University of Southern California Tsunami Research Center is co-author of ‘Far-Field Tsunami
Wave patterns generated by an earthquake just west of the Indonesian island of SumatraHazard From Mega-Thrust Earthquakes in the Indian Ocean,’ just published in the Geophysical Journal International.
Synolakis and co-author Emile Okal of Northwestern University evaluated all known potential tsunami-generating sources in the vast area between Africa, Asia, Australia and Antarctica, and then calculated the impact of the tsunamis they can generate, should they rupture. Their paper presents the geographical distribution of risk.
The pair examined eight scenarios, two along Southern Sumatra (in Indonesia), two in the North Andaman segment of the Sumatra Subduction Zone, two sources along the Makran Subduction Zone (south of western Pakistan) and two sources south of Java. (Indonesia)
According to Synolakis, a professor in the USC Viterbi School of Engineering's Sonny Astani Department of Civil and Environmental Engineering, "the most important lesson from the scenarios we investigated is that the patterns of far-field maximum amplitudes predicted by our simulations will not be a repeat of those observed in 2004." The differences result from differences in the directions in which the disturbances propagate, "and in many instances the results are counterintuitive."
Synolakis expressed high confidence in the reliability of the projections. "Even if the earthquakes, as they materialize in the future, have geometric characteristics that are slightly different from our hypothetical scenarios, the far field impact projections are robust to small initial perturbations arising from uncertainty in the rupture characteristics."
Among the paper's conclusions:
1. The impact in the mid-ocean Maldive Islands from all scenarios appears to be similar or less than what was observed in 2004 - however the low-lying structure of the islands makes them more difficult to evacuate than other risk sites.
2. The impact in Madagascar and the Mascarene Islands (Mauritius, Rodrigues and Réunion) and the Seychelles could be far greater than in 2004, particularly from earthquakes in Southern Sumatra and in South Java. Madagascar is found particularly vulnerable from South Sumatran tsunamis.
3. Africa suffered in excess of 300 deaths in 2004, 300 of them in Somalia. Its east coast is vulnerable from south Sumatran tsunamis and in particular, Somalia remains at high risk due to the focusing effect of the Maldives ridge. The Comoro islands located between Tanzania and Madagascar would probably be affected more severely than in 2004.
4. Large earthquakes in south Java would generate substantial levels of destruction in Northern Australia, despite the sparse level of development there.
5. The Strait of Malacca area appears more vulnerable than in 2004, from earthquakes in the North Andaman. Bali and Lombok and could be severely affected by large events in south Java. In fact Bali was affected by the 1994 tsunami, whose trigger was smaller than the ones envisioned here.
6. The Kerguelen Islands, part of the French Southern and Antarctic Territories, are highly vulnerable. Other than the North Andaman scenarios, practically all other events affect the Kerguelens, where apparently the 2004 tsunami did not cause damage. The much larger offshore heights the simulations predict would put the scientific base there (60-100 persons) at risk.
Many of these scenarios have never been examined before. Synolakis' USC colleague Jose Borrero and others examined the local impact from south Sumatran scenarios in a 2006 paper in the Proceedings of the National Academy of Sciences. Synolakis and Okal concentrated on basin-wide impacts not studied earlier.
According to that paper, the Makran Coast of Baluchistan constitutes a subduction zone along which the Arabian plate sinks under the Eurasian one. This was the site of a major earthquake on 1945 November 27, which was accompanied by a significant regional tsunami, with run-up in the five to ten meter range.
In the paper Synolakis and Okal raise concerns about the current warning system. "It is quite clear that a tested and true tsunami early warning system as now works in the Pacific by the Pacific Tsunami Warning Center needs to be urgently implemented in the Indian Ocean," said Synolakis. "This system should include hundreds of pre-computed detailed scenarios of inundation for all Indian Ocean nations to facilitate emergency planning for evacuation should any of these scenarios materialize. Public education is a must and local people and visitors should be made aware of tsunami hazards, no matter how unlikely they may be, just as Hawaii and Oregon are already doing."
The National Science Foundation and the European Union supported the research.
•Date: 29th January 2008• Region: Various •Type: Article •Topic: Emergency planning
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