Wed. Mar. 30 2005 8:55 AM ET
Latest earthquakes surprise seismologists
Associated Press
The latest deadly earthquake off the coast of Indonesia wasn't unexpected
but may have arrived earlier than experts anticipated.
After the December 26 quake that sent out a devastating tsunami, every
seismologist knew that the earthquake potential of nearby faults had
increased, Yale University seismologist Jeffrey Park said in a telephone
interview.
"But I don't think any one of us would have predicted it would have
occurred in three months, at this magnitude," he said.
Dave Oppenheimer, a seismologist with the U.S. Geological Survey in Menlo
Park, Calif., noted that the December quake, to the north, released a lot
of stress, but also increased the stress on nearby fault zones.
The area of Monday's quake had a major tremor in 1861 and had been storing
up tension since then, he said, so the December quake dumped stress onto
an area that was ready to go.
Another section, southeast of Monday's quake, last shook in 1833,
Oppenheimer added. "Will it go tomorrow, will it go in two months, two
years, two decades ... we don't know, but it will occur," he said.
Aftershocks are common following large quakes. Oppenheimer said, and he
called Monday's tremor a large aftershock from the December quake. But
Park declined to call it an aftershock, since it wasn't located in the
same fault section.
"That doesn't mean that the two aren't connected; they very likely are
connected," he said.
But what seismologists don't understand is the time lag, he said, noting
that in the Anatolian fault zone in Turkey and in California, the time
scale can be decades.
A 1971 quake in California loaded extra stress on a nearby fault that
ruptured in the 1994 Northridge quake, Park said.
"So the correlation is pretty clear but the cause, in terms of knowing the
cause well enough to predict when the next one is going to occur, that's
still mysterious," he said.
Asked about the likelihood of another powerful Indonesian quake, he
responded: "If you would ask me what the odds are in the next three
months, I'd say they are low. In the next 15 years I'd say there is reason
to be concerned. I'd say let's get that tsunami warning system out there,"
he said.
Indeed, a new tsunami warning system for the Indian Ocean was scheduled to
begin service on Friday, but got an initial tryout Monday, relaying
warnings from Japan and the United States that the quake had the potential
to cause another great wave.
Ultimately, no serious tsunami was reported, but one could arrive with any
future quake, and Oppenheimer noted that there may have been a tsunami
that knocked out communications in rural areas and the damage will only be
discovered later. The full Indian Ocean tsunami warning system is expected
to go into service by 2006.
Why the December quake generated a devastating tsunami and Monday's didn't
isn't yet understood, but there are several possibilities, he said.
To generate those great waves, there has to be vertical movement of the
sea floor, he said, and Monday's quake was deeper in the Earth than
December's, so there may have been less direct effect on the ocean bottom.
Also, he said, it occurred beneath an island, reducing the sea floor
effects.
In addition, the quake orientation was different. In December the quake
energy went east and west, toward Sri Lanka, Thailand and Indonesia.
Monday the orientation was south-southeast, directing the energy into open
ocean and Antarctica.
Park said in some ways the Sumatra quakes are a wake-up call after a long
period of relative seismic quiet following a series of major quakes in the
1950s and 1960s.
"There's probably nothing ominous or portentous in that, by itself," he
said. "The largest quakes are relatively rare."
Indeed, Monday's 8.7 quake was the second most powerful since 1964, he
said, following the December Sumatra quake which had a magnitude estimated
at 9.0 or more.
The December quake was unusual, he added, noting that it persisted for a
relatively long time, 400 to 500 seconds -- roughly 6 1/2 to 8 1/2
minutes.
In the 20th century there were only about a half-dozen quakes as large as
Monday's and four occurred along one boundary where the giant plates that
make up the surface of the planet grind together. That boundary stretches
from Russia's Kamchatka peninsula along the Aleutian Islands to Alaska.
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