Tsunami risk seems low, say seismologists

PARIS—The risk of a major tsunami being generated by the giant earthquake off Sumatra in Indonesia on Wednesday appears to be low, British and Australian experts said.

The earthquake’s movement was horizontal, not vertical, and caused no apparent movement of the sea floor, which is what triggers tsunamis, Susanne Sargeant, a seismologist with the British Geological Survey (BGS) told Agence France-Presse.

“We’ve had two blocks rubbing together, it’s called a strike-slip earthquake,” Sargeant said.

“That means there hasn’t been any displacement of the sea floor. Although an earthquake of this magnitude has the potential to cause a large tsunami, the fact that we haven’t seen any drop of the sea floor, which is what generates the wave, it looks like the possibility of a tsunami being generated is low.”

Kevin McCue, director of the Australian Seismological Centre, said the “predominantly strike-slip” movement suggested “any tsunami would be small and local.”

The US Geological Survey (USGS) said an 8.6-magnitude earthquake struck off the coast of Sumatra at 2:38 p.m. (0838 GMT) at a depth of 33 kilometers (14.2 miles).

Tsunami alerts or evacuation orders were issued for parts of Malaysia, India, Indonesia, Sri Lanka and Thailand.

Mok Hing-yim, senior scientific officer at the Hong Kong Observatory, said a “minor tsunami” of 31 centimeters, or about one foot, in height had been reported at Sabang, on the coast of Indonesia.

“It’s not very significant, but we cannot ignore that there will be some higher tsunami along the coast of Indonesia. The reading indicated that a tsunami has been generated,” Mok said.

Geoscience Australia specialist Daniel Jaksa said “there’s definitely tsunami signals reaching the Sumatran coast.”

“Off the coast off Indonesia at the moment there’s 50 centimeters,” or 20 inches, he said.

“Fifty centimeters can be quite dangerous. You only need about 20 centimeters to lift up a car and make it move, so this is not insignificant,” Jaksa said.

On December 26, 2004, nearly a quarter of a million people were killed on coasts around the Indian Ocean by a tsunami unleashed by a 9.2-magnitude quake, which had hit about 200 kilometers (120 miles) away from the latest event.

James Goff, head of the Australia-Pacific Tsunami Research Centre, agreed the impact would be significantly smaller than in 2004.

“Of course such a large shake could generate submarine landslides, like in Haiti, which can also generate tsunamis. We continue to watch,” he said.

The experts said the quake occurred in a notoriously seismic area, where the Indian tectonic plate descends into the Earth beneath the Eurasian plate.

Kevin Furlong, working with the USGS, warned the tremor could trigger seismic tensions in the region.

“This is an extremely large magnitude earthquake for this style of faulting, meaning that it likely involved substantial fault movement, and the fault likely extends for 200-plus kilometers,” he said.

“Any earthquake of this size will change the stress regimes acting on the nearby plate boundaries,” said Furlong.

Just over two hours after the giant quake, a massive aftershock, measuring 8.2, occurred 615 km (382 miles) from the Sumatran city of Banda Aceh.

Sargeant said: “We can certainly expect aftershocks after an event of this magnitude. There is always the potential for large earthquakes in this region, but it’s hard to know whether this is setting the scene for a larger event in the near future, it’s not possible to say at this stage.”