Published online 11 December 2008 | Nature | doi:10.1038/news.2008.1296

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Earthquake warning for Sumatra

Ancient corals reveal cycles of seismic activity.

Dead coralA coral reef that died as a result of the earthquake that hit Sumatra in September 2007.Science

Another huge earthquake may hit the Sumatra region within the next few decades, geologists have warned. An analysis of fossilized coral beds in the region has revealed that the magnitude 8.4 and 7.9 quakes that hit the island in September 2007 could be harbingers of further, and potentially more destructive, ruptures.

Kerry Sieh, an earthquake geologist at the California Institute of Technology in Pasadena, and his colleagues found evidence for recurring changes in the local sea level, which they attribute to tectonic plates shifting upwards and subsidence of the sea floor — sure signs of a quake.

Coral clock

Coral can't grow out of water so if sea levels are falling, it tends to grow outwards rather than upwards — a pattern of growth that's recorded in their fossilized remains. The scientists found evidence in the coral record for three past ruptures of the Mentawai section — a 700-kilometre stretch of a 6,000-kilometre-long fault known as the Sunda megathrust. One of these ruptures occurred around 1350, one around 1600, and the other from 1797 to 1833 — each causing two or more earthquakes.

The 2007 quakes, they suggest, may mark the beginning of a new earthquake 'supercycle' in the region, which, if the previous cycles are indicative, might last for a few decades to a century. The next big earthquake, possibly strong enough to spawn a deadly tsunami that could reach Sumatra's coasts within minutes, will probably occur in the lifetime of the island's children and young adults, they report in Science1.

Worryingly, Sieh and his team also found that the tremors that initiate a cycle are often soon followed by much stronger quakes. Given the amount of unreleased pressure that has accumulated since 1833 in the plates off Sumatra, the new cycle could culminate in a magnitude 8.8 quake, they believe.

"For Sumatra, the worst is still to come," says Costas Synolakis, an environmental engineer and director of the Tsunami Research Center at the University of Southern California in Los Angeles, who was not involved in the study.

Dire warning

Geologists including Sieh had reported earlier in December that the 2007 Sumatra quakes had ruptured only a fraction of the area that had been ruptured in 18332. Sieh fears the patch will re-rupture in just a few years or decades, when the stress and strain that has accumulated in stronger parts of the Mentawai section is released.

The repetitiveness of the cycles, says Synolakis, provides the most compelling evidence to date for 'elastic rebound' of previously stored energy, an earthquake theory first proposed by the American geologist Henry Fielding Reid after the big 1906 San Francisco quake.

Models of a possible mega-tsunami hitting the region's low-lying coastal stretches suggest that losses could be as great as those on 26 December 2004, when around 200,000 were killed in Aceh, Sumatra's northernmost province3.

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An early-warning system has been set up by Indonesia to safeguard people living along west-Sumatran coastlines, including the roughly 800,000 people in the provincial capital Padang. But even so, time for reaching higher ground will be scarce if a major quake just off the coast triggers a tsunami.

The deadly waves could also quickly propagate eastwards across the Indian Ocean as they did in 2004.

Synolakis, who recently modelled Indian Ocean tsunami hazards based on ten earthquake scenarios at seismic zones surrounding the basin 4, warns that a tsunami generated by a megathrust failure of the Mentawai section off Sumatra would threaten coastal communities as far away as Oman, Kenya, Madagascar and South Africa. "This paper by Sieh and colleagues is the last wake-up call for public education efforts," he says. 

  • References

    1. Sieh, K. et al. Science 322, 1674-1678 (2008). | ChemPort |
    2. Ozgun Konca, A. et al. Nature 456, 631-635 (2008). | Article | PubMed | ChemPort |
    3. Borrero, J. C., Sieh, K., Chlieh, M. & Synolakis, C. E. Proc. Natl Acad. Sci. USA 103, 19673–19677 (2006).
    4. Okal, E. A. & Synolakis, C. E. Geophys. J. Int. 172, 995–1015 (2008).
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