Why no tsunami this time? Answer lies in location
Why no tsunami this time? Answer lies in location
Maggie Fox
Reuters/Washington, D.C.
The powerful quake that struck off Indonesia on Monday probably
did not generate a killer tsunami because it was deep and in a
fortuitous location, experts said on Tuesday.
But they said the uncertainty over whether there would be a
tsunami after the 8.7 magnitude quake under the Indian Ocean
floor showed just how little is known about earthquakes and their
effects.
"That is why a tsunami early warning system is still badly,
badly needed for this area," said Jian Lin, a marine geophysicist
at the Woods Hole Oceanographic Institution in Massachusetts.
Thailand, Sri Lanka, Mauritius and India all called tsunami
alerts when Monday's quake hit, but canceled them after learning
it generated only a very small tsunami that posed little danger
to coastlines.
More than 1,000 people were killed in Indonesia, and the
government expects the toll to rise.
Officials in the Indian Ocean region were on hair-trigger
readiness after the Dec. 26 magnitude 9 quake in which more than
280,000 people were killed or disappeared.
But there is a big difference between a magnitude 8.7 and a
magnitude 9 quake on the logarithmic scale used by geologists,
said Lin.
"This earthquake was significantly smaller, even though it was
a great earthquake," he said in a telephone interview.
"And second, we do not know yet how deep this earthquake was.
If it turns out to have been deeper, that would fit one of the
explanations" of why there was no big tsunami.
It will take a few days to analyze the data from the
seismographs and accurately calculate how deep beneath the ocean
floor the quake was, Lin said.
The U.S. Geological Survey has initially set the depth of the
quake at 30 km, or about 20 miles, but this is a default depth
that has not yet been accurately calculated, Lin said.
"I would say another potentially important factor here is
where the earthquake occurred," Lin added.
The Indian Ocean epicenter of Monday's quake was about 100
miles (160 km) southeast of the temblor three months earlier.
The December quake on Dec. 26 was centered north of the
Indonesia island of Sumatra, with nothing but open water between
the rupture point and the beaches of Thailand, India and Sri
Lanka.
But Monday's earthquake was further south and the island of
Sumatra blocked much potential for a tsunami, he said.
Like the December event, Monday's was a vertical "thrust
fault" earthquake in which part of the ocean floor was pushed
upward by another tectonic plate pushing beneath it.
"Whenever there is a thrust fault like yesterday's of that
magnitude, it always has the potential to generate a tsunami,"
Lin said.
"In fact there was tsunami yesterday. It was just small."
Seismologists had been warning of a second earthquake off
Sumatra due to the increased geological stress caused by last
December's quake. But of course there was no way to predict when,
Lin said.
The science of quake and tsunami prediction had been badly
neglected because, for decades, there were no major quakes
affecting areas that rich governments care about, seismologists
have complained.
They hope the disasters in Indonesia will generate enough
ongoing interest to fund such research.
REUTERS
GetRTR 3.00 -- MAR 30, 2005 03:00:13