Quake puzzles tsunami experts

Tsunami experts could not understand why Monday's forceful earthquake off Indonesia failed to produce massive waves similar to those generated by the December 26 quake.

A magnitude 8.7 quake shook Indonesia's west coast, killing hundreds of people and spreading panic that another devastating tsunami was on the way.

There was no tsunami, but a small wave was detected by a tide gauge on Cocos Island near Australia, about 2 400 kilometres south of the epicentre, according to the Pacific Tsunami Warning Centre on Oahu.

"I'm baffled an earthquake this size didn't trigger a tsunami near the epicentre," said Robert Cessaro, a geophysicist at the centre, which is operated by the National Oceanic and Atmospheric Administration.

Centre Director Charles McCreery said earthquakes of at least 8.0 magnitude usually generate major tsunamis.

"We expected some destructive tsunami with some distant destructive effects. It was surprising," he said.

The latest event also demonstrated "there's a whole world of uncertainty about trying to judge a tsunami based on the earthquake data", he said.

The warning centre initially estimated the December 26 earthquake to have a magnitude of 8.0, but it turned out to be larger, with a magnitude of 9.0.

Depth of the quake
Monday's preliminary estimate was magnitude 8.5 but had no destructive tsunami.

"The one we initially thought was bigger turns out to have no effect," McCreery said. "The one we initially thought was smaller had a huge effect. This is the challenge of tsunami warning."

Some scientists believe the depth of the quake was the reason no tsunami was generated.

The US Geological Survey said Monday's quake struck about 30 kilometres under the seabed. The December 26 quake was closer to the surface.

"What causes a tsunami is if the ocean floor heaves, so if it's a very shallow tsunami, it's apt to heave the floor more than a deeper one. If it's very deep, it sort of gets absorbed on its way up," said Allen Clark, director of the Pacific Disaster Centre on Maui.

The warning centre, established in 1949, came under heavy criticism following the December tsunami for not being more aggressive about warning Asian nations and possibly saving thousands of lives.

Earlier this month, a group of 58 European tsunami survivors and relatives of victims sued NOAA and other agencies, alleging the centre did not do enough to warn people about the disaster.

"Although we certainly wish that somehow the event unfolded in a way that we could've done more for the region, we really did all we could under the circumstances," McCreery said.

Since then, several Indian Ocean nations have established communications with the centre and are now on its alert list. On Monday, the facility was able to alert those nations.