Himalayan sediments made 2004 tsunami more severe: study

Sediments from the Himalayas may have aggravated the severity of the catastrophic 2004 Sumatra earthquake that caused a massive tsunami to claim over 250,000 lives in the Indian coastal regions and other countries, a new study has found.
Researchers found that the sediment that eroded from the Himalayas and Tibetan plateau over millions of years was transported thousands of kilometres by rivers and in to the Indian Ocean.
It became sufficiently thick over time to generate temperatures warm enough to strengthen the sediment and increase the severity of the Sumatra earthquake in on December 26 in 2004.
The 9.2 magnitude earthquake, generated a massive tsunami that devastated coastal regions of the Indian Ocean.

The earthquake and tsunami together killed more than 250,000 people making it one of the deadliest natural disasters in history.
According to the researchers, the same mechanism could be in place in the Cascadia Subduction Zone off the Pacific Northwest coast of North America, as well as off Iran, Pakistan and in the Caribbean.

"The 2004 Indian Ocean tsunami was triggered by an unusually strong earthquake with an extensive rupture area," said Lisa McNeill, a former graduate student at the Oregon State University (OSU) in the US.
"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," said McNeill, now at the University of Southampton in the UK.

The research team sampled sediment and rocks from the tectonic plate that feeds the Sumatra subduction zone for the first time.
From a research vessel, they drilled down 1.5 kilometres 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 kilometres toward the subduction zone.
"We discovered that in some areas where the sediments are especially thick, dehydration of the sediments occurred before they were subducted," said Marta Torres, from OSU.
"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," said Torres.

"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," she said.
Scientists found that water between the sediment grains was less salty than seawater only within a zone where the plate boundary fault develops, some 1.2 to 1.4 kilometres below the seafloor.
"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," Torres said.
The discovery 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, said Andre Hupers of the University of Bremen in Germany.

The research was published in the journal Science.