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  PlanetArk 8 Dec 05
Tsunami Researchers Find Bigger Seafloor Shifts

BBC 7 Dec 05
Scientists voice tsunami concern
By Jonathan Amos BBC News science reporter, San Francisco

A US scientist studying the islands off southern Sumatra says it is very clear the region can expect more big quakes and tsunami in the coming decades.

Prof Kerry Sieh is using a GPS network to monitor land movements close to the great fault line that ruptured to produce last December's awful events. His work indicates there is still huge strain bound up in the fault, and that this could let go in the near future.

He believes the cities of Pedang and Bengkulu may be at greatest risk.

"The time is now to start mitigating for such an event," said Kerry Sieh, who is attached to the California Institute of Technology's Tectonics Observatory. "I don't know with certainty that it's going to happen but our team is telling people on the coast that they have to expect that this will happen in the lifetime of their children."

To the south The 26 December quake of magnitude 9.2 stemmed from a rupture along the line where the Indian/Australian and Eurasian tectonic plates grind over each other.

The associated tsunami wrought destruction throughout the Bay of Bengal, from Northern Sumatra to Thailand, Sri Lanka, and India. It was followed by a magnitude 8.7 in March - with the rupture occurring further to south along the plate boundary.

Prof Sieh, speaking here at the American Geophysical Union Fall Meeting, says the concern of scientists is now focused on events further south still, to a region known as the Mentawai islands patch.

This zone has experienced giant earthquakes about every two centuries, and is nearing the end of its earthquake cycle.

Coral rise

Prof Sieh says the strain building up in the region is evident from the behaviour of island coastlines - some are becoming submerged. It is in the nature of tectonic plates that they do not glide smoothly past each other, as one dives under the other.

In fact, the plates move in "stick-slip" fashion, which means land at the leading edge of the overriding plate is pulled down briefly before suddenly slipping back up, generating a large earthquake.

Surveys along northern Sumatra following the 26 December and 28 March events have revealed coral reefs that have come out of the water as land has thrust back up.

"When you look further to the south, the groves of trees and other coastal features are still sitting out in the water; they have not yet risen. So, we know the strain is still accumulating; our GPS network is telling us it is still accumulating," Prof Sieh explained.

"Our concern is that the next thing to happen will be ruptures."

Wave model

Prof Sieh says historical and coral records show the Mentawai islands patch experiences magnitude 8-plus quakes on a roughly 200-year cycle, and are accompanied but large tsunami.

"It appears that these giant earthquakes either occur singly or in couplets: singly in the 1300s and late 1500s, and as a couplet in the late 18th Century and early 19th Century; and the average time between those three sets is about 240 years."

Prof Stephan Grilli, from the University of Rhode Island, has modelled the tsunami that would result in the area from a magnitude 9.2 quake, the same as 26 December.

"Our prediction for Pedang and Bengkulu further south would be up to 10m waves hitting the cities," he told the AGU meeting.

Both Pedang and Bengkulu are bigger cities than Banda Aceh which was destroyed in the 26 December tsunami. Like Banda Aceh, Pedang is very low-lying.

PlanetArk 8 Dec 05
Tsunami Researchers Find Bigger Seafloor Shifts

SAN FRANCISCO - Scientists who studied the seafloor near Sumatra with remotely operated vehicles after a tsunami devastated the Indonesian island last year said on Tuesday they found parts displaced by up to 12 meters (39 feet), nearly twice as much as they had expected.

The tsunami a year ago killed more than 200,000 people on coastlines along the Indian Ocean.

The discovery will help refine tsunami forecasting and improve coastal warning systems for areas with faultlines, including the US Pacific Northwest, said tsunami modeler Stephan Grilli of the University of Rhode Island.

"Communities in Oregon and Washington have been anticipating waves of only 10 to 12 meters or so, but now they need to be even better prepared," Grilli said, pointing to the potential for much bigger waves.

Grilli, at a meeting of the American Geophysical Union, noted that if a 9.2 magnitude earthquake struck off the coasts of Oregon and Washington, the level of the quake that triggered the tsunami, parts of the coastlines of the two states could be hit by waves up to 30 meters (98 feet) high, or three times more than predicted previously.
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