Major quake feared near Dec. 26 site: Scientists

Major quake feared near Dec. 26 site: Scientists

Richard Ingham, Agence France-Presse/Paris

Seismologists say there is a heightened risk that a major
earthquake may soon strike the western coast of Sumatra as a
result of the monster quake that generated the Dec. 26 tsunami.

The Indonesian city of Bandar Aceh, which was already badly
hit by the killer wave, could be at risk from a quake measuring
up to 7.5 on the Richter scale and there is a potential for a
tsunami-making 8.5 quake offshore, they warn.

"There is no doubt -- our calculations show a very significant
increase on stress on two major active faults in the Sumatra
region" since Dec. 26, seismologist John McCloskey at Britain's
University of Ulster told AFP.

McCloskey noted that in so-called subduction zones, an
earthquake can be swiftly followed by another one if certain
geological conditions are met.

"There is a very well established link between these stresses
and following earthquakes," he said.

Energy released by the Dec. 26 quake has boosted stress in
adjoining parts of two dangerous faults, he said.

One fault runs under land to the east of the Dec. 26 quake and
crosses the northwestern tip of Sumatra. The other fault, known
as the Sunda Trench, runs under the sea to the south, parallel to
the coast, where two fatal tsunamis occurred in 1833 and 1861.

"We are not trying to cry wolf," said McCloskey. "We can point
to many other quakes where the stresses like the one we've
measured have resulted in a following earthquake, and we are
suggesting there is a significantly increased risk.

"But we are also pointing out deficiencies in our knowledge,
and we cannot say there will be an earthquake in the next year or
whatever. At the moment, this science I believe doesn't allow us
to make that statement."

In a study published on Thursday in the British weekly science
journal Nature, McCloskey's team redraw the geological map of one
of Earth's seismic hotspots after the 9.0 Dec. 26 quake.

The massive movement ruptured 250,000 square kilometers on a
stretch of the Burma microplate, a narrow tongue of the Earth's
crust that is jostled by the neighboring Indian, Australian and
Sunda plates.

That seabed plunge, by as much as 20 meters, triggered the
tsunami, killing more than 273,000 people in 11 nations on the
northern rim of the Indian Ocean.

Part of the energy released by the quake was transferred to
the contiguous fault sections. It distorted, compressed and
deformed the rock, adding to the burden at known stress points
and creating new ones.

Several known episodes in seismic history point to the danger
of an imminent follow-on earthquake in subduction zones when the
interplay between two vast forces, of sliding and vertical
stresses, is right.

Just a very small increase in pressure on these tensed parts
of the Earth's crust can trigger a catastrophic rupture.

In part of the Nankai Trough southeast of Japan, five of the
seven large earthquakes of the past 1,500 years unleashed
earthquakes in the fault's next section within the following five
years, the study says.

Another example is what happened in Turkey in 1999. A 7.4
earthquake in Izmit, southeast of Istanbul, was caused by the
stresses of previous temblors on the Anatolian fault. In turn,
this placed stress on the adjoining section of the same fault,
unleashing a 7.1 quake at Duzce three months later.

Both of the Turkish quakes were the result of stresses that
were far lower than the energy imparted by the Dec. 26 event, the
study points out.

McCloskey said that, even though scientific gaps remain about
when the next big quake could strike, the heightened risk
underscored the need to swiftly set up a tsunami early-warning
system for the Indian Ocean.

"When it comes to earthquakes, lightning does strike twice,"
he said.