Prehistoric record improves forecast of large West Sumatran earthquake and tsunami
For at least the past 700 years, clusters of large quakes have occurred about every two centuries. Last year’s 8.4 is the first in a series that will play out over the next few decades.
An international team of scientists has deciphered a geological record of ancient earthquakes from the coral reefs of the Mentawai islands, off the coast of West Sumatra. Their study, published today in the journal Science, shows cycles of tectonic strain accumulation and relief that have been about two centuries long and that commonly end in a spate of two or more large earthquakes.
To develop such a long history, the researchers used ancient corals that record in their annual growth bands histories of sea-level fluctuations. These changes in water level indicate that the islands subside during the long, quiet periods between earthquakes. When they pop up suddenly, during earthquakes, the corals rise above sea level and die.
Three localities, along about a 110 kilometers length of the Mentawai island chain, contain broadly similar sea level histories. At each of the three “paleoseismic” sites – on South Pagai, North Pagai and Sipora islands – sudden coral deaths occurred in the mid- to late 1300s, the late 1500s to early 1600s, and in 1797 and 1833. These broadly similar histories imply that large earthquakes occur about every two centuries.
However, the researchers found that each site’s seismic history differs significantly from its neighbours’. For example, at the South Pagai site the earliest recorded coral deaths occurred in about 1350, whereas at the other two sites, farther north, the first coral deaths occurred in about 1375. These differences show that, at least for the past 700 years, failure of the underlying active fault, the Sunda megathrust, occurs incrementally, in a series of earthquakes, rather than all at once.
This 700-year record of ancient West Sumatran earthquakes helped the scientists interpret recent seismic happenings in West Sumatra. During the September 2007 magnitude 8.4 earthquake and its aftershocks, the South Pagai site rose about 73 cm, while the other two paleo-seismic sites remained stable. Although the details differ, this mimics what happened in about 1350.
The upshot of the 700-year-long history is that the September earthquake appears to be the beginning of the next big failure sequence for the 700-km-long section of the Sunda megathrust. The researchers refer to as the Mentawai patch. “If previous cycles are a reliable guide,” explains Professor Kerry Sieh, Director of the Earth Observatory of Singapore, Nanyang Technological University (NTU), “we can expect one or more very large West Sumatran earthquakes as the rest of the Mentawai patch fails within the next few decades.”
Prof Sieh adds, “We know that the amount of potential slip that was not relieved in September 2007 is enough to generate an Mw 8.8 earthquake, in a region with more than a million people living along the coast.”
Adds Danny H. Natawidjaja from the Research Center for Geotechnology, Indonesian Institute of Sciences; “This study shows that the next great earthquake and tsunami are very likely within the lifetimes of children and young adults currently living along the coast of Western Sumatra. We hope that our results will encourage governments and various humanitarian agencies to continue and even accelerate their preparations.”
The work detailed in the Science paper, "Bicentennial earthquake supercycles inferred from sea-level changes recorded in the corals of West Sumatra," was supported by grants from the US National Science Foundation, the Gordon and Betty Moore Foundation, and the Indonesian Institute of Sciences. Copies of the paper can be obtained at www.sciencemag.org.
Authors of the paper are Kerry Sieh 1,5, Danny H. Natawidjaja2, Aron J. Meltzner1, Chuan-Chou Shen3, Hai Cheng4, Kuei-Shu Li3, Bambang W. Suwargadi2, John Galetzka1, Belle Philibosian1 and R. Lawrence Edwards4.
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1 Tectonics Observatory, California Institute of Technology, Pasadena, California, USA
2 Research Center for Geotechnology, Indonesian Institute of Sciences, Bandung, Indonesia
3 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC
4 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA
5 Now at Earth Observatory of Singapore, Nanyang Technological University, Singapore
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