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[SANTIAGO] Even moderate earthquakes on fault lines like those near the coast of Haiti could cause potentially catastrophic tsunamis, a study says, making the risk of tsunamis an order of magnitude greater than current global estimates suggest.

Most Caribbean nations are located near strike–slip faults, where two tectonic plates slide past each other laterally. Earthquakes at these faults are estimated to cause only three per cent of tsunamis — since most tsunamis are caused by the vertical displacement of tectonic plates.

But small tsunamis observed during January's magnitude 7 Haiti earthquake, which killed an estimated 200,000 people, were caused by weak sediment at the shore collapsing and sliding along the seafloor, displacing the overlying water, according to the study, published in Nature Geoscience this month (10 October).

Using field surveys and historical data, the geologists found that of the nine documented tsunamis in the region over the last 300 years, at least three — 33 per cent — were triggered by such underwater landslides caused by earthquakes.

"With slide-generated tsunamis the size and shape of the wave can be quite variable," said Matthew Hornbach of the University of Texas Austin, United States, the study's lead author. "It may pose a significant hazard to areas near the actual slide but may not be a problem for regions hundreds of kilometres away," he added. "It's very hard to predict the size, location and strike zone."

Tsunami warnings are usually based on earthquakes at thrust faults where there is vertical movement, Hornbach said, but earthquakes at strike–slip faults can also cause tsunamis.

Although strike–slip faults cause only a small percentage of tsunamis, John Orcutt, director of the Center for Earth Observations and Applications at the Scripps Institution of Oceanography, United States, told SciDev.Net that in the Caribbean "it is quite likely that strike–slip earthquakes will generate significant, life-threatening tsunamis close to major island populations".

Costas Synolakis, director of the University of Southern California's Tsunami Research Center, told SciDev.Net that if landslide tsunamis are more frequent than previously thought, "this is really bad news".

But he said the paper does not provide enough data to show that tsunamis in Haiti are 10 times more likely than previously estimated. Finding evidence of landslides does not necessarily imply that they triggered tsunamis, he said.

Public education about tsunamis is vital, he said. "All coastal residents living in seismic zones should know that if they feel an earthquake tremor that lasts over 30 seconds, they should evacuate inland or to high ground immediately."


Nature Geoscience doi: 10.1038/ngeo975 (2010)