An international team of researchers suggests that sediment from the Himalayas and Tibetan plateau might have increased the severity of the catastrophic 2004 Sumatra earthquake.

     Eroded over millions of years and transported thousands of kilometers by rivers and in the Indian Ocean, the sediment became sufficiently thick over time to generate temperatures warm enough to strengthen itself.

     The magnitude 9.2 earthquake on December 26, 2004, generated a massive tsunami that devastated coastal regions of the Indian Ocean, killing more than 250,000 people and making it one of the deadliest natural disasters in history.

     In a paper to be published on Friday in the journal Science, the research warned that the same mechanism could be in place in the Cascadia Subduction Zone off the Pacific Northwest coast of North America, also off Iran, Pakistan and in the Caribbean.

     The team sampled for the first time sediment and rocks from the tectonic plate that feeds the Sumatra subduction zone. From the research vessel JOIDES Resolution, the team drilled down 1.5 kilometers below the seabed, measured different properties of the sediments, and ran simulations to calculate how the sediment and rock behaves as it piles up and travels eastward 250 kilometers toward the subduction zone.

     "The 2004 Indian Ocean tsunami was triggered by an unusually strong earthquake with an extensive rupture area," expedition co-leader Lisa McNeill, an Oregon State University (OSU) graduate now at the University of Southampton, was quoted as saying in a news release. "We wanted to find out what caused such a large earthquake and tsunami, and what it might mean for other regions with similar geological properties."

     "We discovered that in some areas where the sediments are especially thick, dehydration of the sediments occurred before they were subducted," noted Marta Torres, an OSU geochemist and co-author on the study. "Previous earthquake models assumed that dehydration occurred after the material was subducted, but we had suspected that it might be happening earlier in some margins. The earlier dehydration creates stronger, more rigid material prior to subduction, resulting in a very large fault area that is prone to rupture and can lead to a bigger and more dangerous earthquake."

     Explaining that when the researchers examined the sediments, they found water between the sediment grains that was less salty than seawater only within a zone where the plate boundary fault develops, some 1.2 to 1.4 kilometers below the seafloor, Torres said "this along with some other chemical changes are clear signals that it was an increase in temperature from the thick accumulation of sediment that was dehydrating the minerals."

     The discovery, according to Lead author Andre Hüpers of the University of Bremen in Germany, will generate new interest in other subduction zone sites that also have thick, hot sediment and rock, especially those areas where the hazard potential is unknown.  Enditem