Sydney Morning Herald
Scientists puzzled over quake ripples
March 29, 2005 - 2:54PM
Tsunami experts can't understand why a forceful earthquake off Indonesia
overnight failed to produce massive waves similar to those generated by the
December 26 quake that killed at least 174,000 people in the same region.
A magnitude 8.7 quake shook Indonesia's west coast, killing hundreds of
people and spreading panic another devastating tsunami was on the way.
There was no tsunami, but a small wave was detected by a tide gauge on Cocos
Island off Western 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, operated by
the National Oceanic and Atmospheric Administration.
The centre is responsible for monitoring seismic and ocean conditions in the
Pacific and alerting Pacific Rim nations and United States agencies.
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.
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."
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.
The Indian Ocean has no warning centre similar to the one in Hawaii.
© 2005 AP
Sydney Morning Herald
Why this earthly lightning struck twice
Date: March 30 2005
Two weeks ago an expert warned Sumatra to expect another quake. Yesterday
he was proved right. Richard Macey and Deborah Smith report.
It was 2.33am when Mark Leonard's mobile phone rang. The seismologist
with Geoscience Australia was woken by a computer-generated message
announcing that the west coast of Sumatra had been rocked by another massive
earthquake, bringing back horrific memories of the Boxing Day tsunami.
A dismayed Dr Leonard studied the details relayed from automated
seismology stations around the country.
The culprit, just as it had been on December 26, was the Indo-Australian
tectonic plate that has been steadily forcing its way north at up to seven
centimetres a year, for centuries. Where it collides with its neighbouring
Eurasian plate, on which the island of Sumatra rides, the Indo-Australian
plate is driven deep into the Earth's mantle.
Tremendous forces build up where the opposing plates jam together. The
inevitable result is that sooner or later one section, unable to contain the
mounting stress, must slip, triggering a powerful earthquake. It happened in
one section of this region, known as the Sunda trench, off Sumatra's south
coast in 1833. Twenty-eight years later, in 1861, it slipped again, just to
the north. Then there was a lull until, on Boxing Day last year, yet another
section of the Sunda trench ruptured near the northern tip of Sumatra,
creating the tsunami that claimed almost 300,000 lives.
Two weeks ago, John McCloskey, of the University of Ulster, went public
with his prediction that the region was a prime target for another major
quake - and soon. People tended to think lightning didn't strike twice in
the same place, he told the Herald. "But with earthquakes it's
exactly the opposite."
His team had calculated that the Boxing Day earthquake had increased
stresses in the adjacent area of the Sunda trench, which had not experienced
a quake since 1861. At 2.09 yesterday morning, Sydney time, the inevitable
happened. The plates ruptured, almost exactly where Professor McCloskey had
predicted, triggering a massive 8.7 magnitude earthquake, the world's eighth
biggest since 1900.
The Pacific Tsunami Warning Centre, based in Hawaii, was soon beaming out
alerts, advising the "evacuation of coasts within 1000 kilometres of the
epicentre". Thousands of people were evacuated in Thailand, Sri Lanka and
Malaysia.
"At 4am [Sydney time] we were very concerned there would be another
tsunami," Dr Leonard said. But as the hours passed there were no reports of
coast beyond Sumatra being swamped.
At 4.35am tidal gauges on the Cocos Islands off Western Australian merely
noted a wave 25 centimetres high, from peak to trough, passing harmlessly
by. Several more waves followed in the next hour. At 8.30am the sea level at
Hillarys, about 30 kilometres north of Fremantle, rose from 10 to 20
centimetres, and then settled back.
About the same time, said Dr Leonard, observers in the Maldives, in the
Indian Ocean, were reporting that the sea level had risen just 20 to 40
centimetres. Late yesterday, however, news did emerge of a three-metre
tsunami wreaking havoc of Simeulue. Nevertheless, as Dr Leonard studied the
data, he gradually realised this was no repeat of the Boxing Day calamity,
which killed 300,000 people across 11 Indian Ocean nations.
Although the quake had registered 8.7, and was still "a very, very big
earthquake" by any measure, it was on the order of eight to 10 times weaker
than the magnitude 9 quake that triggered the December 26 tsunami. While the
numbers appear close, the force of earthquakes is measured on an exponential
scale. The area of crust that shifted was also much smaller - about 50
kilometres wide and 550 kilometres long - compared with the slab of earth
1000 by 250 kilometres that slipped last year. The plate's sideway slip
probably averaged only two or three metres, a quarter of the average shift
in the Boxing Day rupture.
Last year's quake, noted Dr Leonard, happened on a section of faultline
running north to south. The result was a tsunami that rolled out across the
Indian Ocean, heading due west, slamming into Sri Lanka, India and finally
Africa. The faultline that shifted yesterday runs from the north-west to the
south-east, parallel with Sumatra's coast. This time the tsunami headed
south-west, out into a largely empty ocean. "This quake was probably due to
happen sometime in the next decade," said Mr Leonard, confessing, "I was not
expecting another one so soon." He speculated the Boxing Day event had
"probably nudged it forward ... it must have been ready to go".
The seismologist declined to punt on what would happen next, saying he
did not believe earthquakes could be predicted. "I would not be surprised if
there is another 9 magnitude quake off south-western Java ... some time in
the next couple of centuries."
Professor McCloskey was not so cautious. "Unfortunately, I'm not at all
surprised by today's news," he told ABC radio. He said the next stretch of
Sumatra's undersea Sunda trench would now be under additional stress from
yesterday's quake. It was too early to say whether this would trigger
another quake in the region that has not ruptured since 1833.
He also predicted this quake could increase stress on a faultline running
down the centre of Sumatra and under the city of Banda Aceh, already
destabilised by the Boxing Day quake.
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