U. SOUTHAMPTON (UK)—Researchers have uncovered clues as to why some undersea earthquakes generate huge tsunamis. Their findings, published recently in the journal Science, may help explain why the 2004 Sumatra “Boxing Day Tsunami” was so devastating.

Early in the morning of December 26, 2004, a powerful undersea earthquake started close to Simeulue Island off the west coast of Sumatra, Indonesia, and extended more than 1,200 kilometers to the north.

The resulting tsunami caused devastation along the coastlines bordering the Indian Ocean, with tsunami waves up to 30 meters high inundating coastal communities. With very little warning of impending disaster, more than 230,000 people lost their lives and countless others were made homeless.

Nearly four months later, on March 28, 2005, another strong earthquake (although significantly smaller than the one that caused the Boxing Day Tsunami) occurred immediately to the south, but triggered only a relatively small tsunami that claimed far fewer lives, most of them on the island of Nias.

“Both earthquakes occurred on the same fault system, initiating 30-40 kilometers below the seabed,” says Simon Dean of the University of Southampton.

“Our results will help us understand why different parts of the fault system behave differently during earthquake slip which then influences tsunami generation. This is critical for adequate hazard assessment and mitigation.”

The largest undersea earthquakes occur at “subduction zones” where one tectonic plate is forced (or subducts) under another. Large sections of plate can get stuck, causing deformation, and eventual slippage or rupture with the release of vast amounts of stored energy.

Seismic reflection profiles collected by Dean and colleagues crossing the Sumatra margin from northwest of Simeulue Island to Nias Island revealed differences between the plate boundaries of the 2004 and 2005 Sumatra earthquake ruptures.

Between the deep ocean sediments and the subducting oceanic basement of the Indian tectonic plate there is a surface, called the “décollement” that forms the plane of slippage. At the plate boundary, the front edge of the overlying Eurasian plate acts rather like a bulldozer, scraping the material above the décollement up to form an accretionary prism.

The researchers discovered a number of unusual features at the rupture zone of the 2004 earthquake, such as the seabed topography, how the sediments are deformed in the accretionary prism, and the locations of small earthquakes (aftershocks) following the main earthquake.

They also found that the décollement surface has different properties in the two earthquake rupture regions. These differences resulted in slip during the 2004 earthquake continuing further seaward and much closer to the seabed, potentially one of the factors causing a larger tsunami.

“By comparing our results with other subduction zones around the world, we believe that the region of the 2004 Sumatra earthquake is very unusual, suggesting that tsunami hazards may be particularly high in this region,” says Lisa McNeill, also of the University of Southampton.

Researchers from the University of Texas at Austin, Agency for the Assessment and Application of Technology in Indonesia, and the Indonesia Institute for Sciences contributed to the work, which was conducted as part of a Natural Environment Research Council–funded UK Sumatra Consortium research project.

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