Earth's movement prevented tsunami

The latest big earthquake to strike Southeast Asia did not cause a tsunami because the earth must have moved downwards rather than upwards, a British seismologist said on Tuesday.

David Booth of the British Geological Survey said he was "very surprised" that Monday's quake had not created a killer wave because it was almost similar in force and depth to the December 26 temblor that caused so much devastation.

"It appears Monday's quake caused a downward movement of the earth rather than the upwards jolt of the Boxing Day quake which caused a vertical displacement of the ocean floor, which creates the tsunami," Booth said.

"On Boxing Day, the quake pushed the earth beneath the ocean shelf upwards, wedging the Indian oceanic plate beneath the front edge of Sumatra," he said.

"This caused enormous pressure which, when released, made the front edge of Sumatra leap upwards by several metres."

"It generates the energy of tens of Hiroshimas, making the earth ring like a bell and squashing it like a football."

Booth added: "I'm very surprised there was no tsunami yesterday. The people of Asia are extremely fortunate the quake movement appears to have been downwards."

The seismologist, who is based in Scotland's capital Edinburgh, said predicting such quakes was an imprecise science but the one certainty was they would continue.

"Indonesia is one of the most seismic parts of the world and this movement has been happening, and is likely to go on, for millions of years," he said.

Officials say at least 400 people were killed in northwest Indonesia as a result of Monday's earthquake which prompted tsunami warnings and panic spread across Indian Ocean rim states.

The epicentre of the temblor, which measured 8.7 on the Richter scale, was only 320 kilometres south of the December 26 quake that sent giant waves crashing into 12 nations.