Seismologists warn Sumatra at risk of more quakes, tsunami

Seismologists warn Sumatra at risk of more quakes, tsunami

Richard Ingham, Agence France-Presse/Paris

The Indonesian island of Sumatra, smashed by a tsunami and shaken
by an enormous earthquake in the past six months, is now at risk
from two potentially major quakes, one of which could generate
waves 10 meters high, seismologists warn on Thursday.

The research team is headed by the same expert who predicted
with uncanny accuracy a quake that struck Sumatra on March 28,
barely three months after the Dec. 26 mega-temblor, the second
biggest earthquake ever recorded.

He fears the next quakes may be as high as 7.5 and 9
respectively on the Richter scale -- and in the latter case,
cities along much of Sumatra's west coast would be exposed to a
tsunami.

"I think it would be irresponsible for those in charge of
preparing people in this area to ignore the possibility that the
earthquake could happen in a year," lead author John McCloskey, a
professor of environmental sciences at Britain's University of
Ulster, told AFP.

The study takes a fresh look at Sumatra's seismic mosaic in
the light of the last two great quakes, focussing on the two
biggest faultlines.

One faultline runs on the land down the western side of
Sumatra, and has lateral friction, with one side trying to head
northwest and the other trying to move southeast.

Stress on this so-called Sumatra fault, especially in the
northwest, in the region of Banda Aceh, remains high, the
researchers warn.

"The threat of an earthquake of magnitude 7.0-7.5 on the
Sumatra fault north of four degrees north (of the equator) has
not receded," they say in the study, which appears in the British
science journal Nature.

An even greater threat lies in the second faultline, the so-
called Sunda Trench, a notorious seabed crack that runs about 200
kilometers to the west of Sumatra.

This area has a different and more perilous profile than the
Sumatra fault, for it has vertical movement -- the kind capable
of creating big waves by thrusting up sections of the sea bed.

It lies in a so-called "megathrust" area, in which the
Australian plate is trying to push its way under the Eurasian
plate to the northeast.

The Sunda Trench has been a flashpoint of seismic activity for
centuries.

Part of its northern section, at the conjunction with the
tongue-shaped Burma microplate, was the epicenter for the Dec.
26, 2004 quake, at 9.3 the second highest ever recorded.

The ocean floor ruptured along 1,200 kms, creating a wave in
which 217,000 people around the Indian Ocean's rim were killed or
left unaccounted for.

That massive event had a domino effect, placing further stress
to the south.

On March 28, just 160 kilometers to the south, an 8.7-
magnitude earthquake killed more than 900 people.

In turn, this quake has created a new spot of high
vulnerability about 500-600 kms farther south underneath the
Mentawai Islands, between 0.7 and 5.5 degrees south of the
equator, according to the computer modeling.

On average, the Mentawai Islands produce a very big quake
every 230 years, says McCloskey's team.

The last big one in the southern part of this section was in
1833, with an 8.5 quake that unleashed a damaging tsunami up to
10 meters high.

By comparison, the wave that scoured the coastline of the
northern Indian Ocean on Dec. 26 varied in height from 10 to 15
meters.

In the northern part of this section, there has not been any
big quake since 1797, when there was a small slip of only a few
meters under the main island of Siberut.

"Slip on the northern part of this section could be greater
than 10 meters depending on the timing of the last rupture, (and)
slip on the southern portion could be as great as in 1833," the
researchers say.

When could this happen?

McCloskey says no one knows for sure. But he says time was
already short before the next expected event and may now be even
shorter because of the cascade of recent thrusts on this highly
tensed region.

Two "minute... tiny" changes in stress, of one-tenth of an
atmosphere were sufficient to unleash the Dec. 24 and March 28
events, he notes.

"If this earthquake (under the Mentawai Islands) were to
happen within a year, it would not surprise me," he said.

"Even if the earthquake doesn't happen for 10 years, it's
still better to start moving toward a culture of preparedness for
these things, because they will happen.

"An earthquake under the Mentawai Islands which produced a
tsunami would be felt strongly along the cities along the west
coast of Sumatra.

"The tsunami will be generated 200 kms offshore, moving at 750
kms an hour, which gives you 15-20 minutes for people to get on
to higher ground."