March 5, 2002,
Tuesday
SCIENCE
DESK
Early Warnings When the Volcano Starts to Slip
By KENNETH CHANG (NYT) 747 words
Every 300,000 years or so, a piece of earth about the size of Rhode Island
slides off the side of a Hawaiian volcano and falls into the ocean. The splash
-- a disastrous ocean-size version of tossing a pebble in a pond -- sends
rings of waves 100 feet and higher radiating outward until they crash,
catastrophically, against shorelines around the Pacific Rim, from California
to South America to Australia.
Writing in the current issue of Nature, scientists at the United States
Geological Survey's Hawaiian Volcano Observatory report that they have, for
the first time, directly observed a sudden slip of a volcano slope.
This particular slip moved only a few inches and does not presage a
collapse, the scientists said, and it demonstrated that satellite monitoring
systems could provide weeks or months of warning of a collapse.
''I like to say these flank collapses are like meteor impacts,'' said Dr.
Peter Cervelli, a research geophysicist at the observatory and an author of
the paper. ''It is indisputable they occur. It is indisputable when they
occur, they cause catastrophic damage, but we also know their occurrence is
very rare.''
The southern flank of the Kilauea volcano on Hawaii, a possible location of
a future giant landslide, is continually in motion without posing any danger,
sliding at a glacial rate of three to four inches a year.
On Nov. 8, 2000, the motion suddenly accelerated by factor of several
hundred, with some parts of the slope moving as much as half an inch over the
course of one day.
A person standing on the ground there would not have noticed anything, but
sensors on the volcano communicating with global positioning satellites made
note of the shifts.
The movements were similar to what scientists would expect for the
beginnings of a catastrophic flank collapse, but in this case, the accelerated
motion lasted only a day and a half before returning to its normal crawl.
Scientists at the volcano observatory say that unlike earthquakes, where a
swarm of small temblors may signal a large quake to follow, the small slip is
not a sign of larger slips to come.
''It's not a priming,'' said Dr. Asta Miklius, a geophysicist at the
observatory and another author of the Nature paper.
But the scientists were interested in understanding what had moved deep
underground to cause the surface shifts. Their computer models indicate that a
9-mile-by-3-mile slab located about 2 1/2 miles underground slipped about 3
1/2 inches, pulling along the ground above it. The event was equivalent to a
magnitude 5.7 earthquake, they said.
The scientists hypothesize that heavy rainfall nine days earlier -- three
feet of rain in 36 hours -- may have been the cause, either by lubricating the
underground fault or by adding weight to the slope.
In an accompanying commentary, Dr. Steven N. Ward, a research geophysicist
at the University of California at Santa Cruz, describes in detail the
devastating destruction a flank collapse would unleash with up to 1,200 cubic
miles of rock sliding at speeds of up to 100 miles an hour for 20 to 40 miles.
The eruption of Mount St. Helens, by contrast, blew away only three-quarters
of a cubic mile of material, Dr. Ward said.
The slide would be the trigger for giant waves known as tsunami. ''It's
going to stir up a pretty big wave,'' Dr. Ward said. ''You can visualize the
large wave getting started and propagating across the Pacific.''
Dr. Ward's computer simulations indicate that a series of about 20 waves
about 100 feet tall would scour the California coast. Smaller, but still
devastating, waves would wash up along South America, Australia and islands in
the Pacific.
But because flank collapses are so rare, ''I don't lose any sleep over
it,'' Dr. Ward said. He estimates that one occurs somewhere in the world once
every 10,000 years.
Debris from the last collapse of Kilauea's flank has been dated at more
than 200,000 years old.
The paper by the scientists in Hawaii, however, shows that the monitoring
system on Kilauea provides a useful model for watching other oceanic volcanoes
should the unlikely calamity occur.
''These things ought to be at least kept an eye on,'' Dr. Ward said.
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