Strong Earth Tides Can Trigger Earthquakes, UCLA Scientists Report(Forwarded)

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Date: October 21, 2004

Strong Earth Tides Can Trigger Earthquakes, UCLA Scientists Report

Earthquakes can be triggered by the Earth's tides, UCLA scientists confirmed
Oct. 21 in Science Express, the online journal of Science. Earth tides are
produced by the gravitational pull of the moon and the sun on the Earth, causing
the ocean's waters to slosh, which in turn raise and lower stress on faults
roughly twice a day. Scientists have wondered about the effects of Earth tides
for more than 100 years. (The research will be published in the print version of
Science in November.)

"Large tides have a significant effect in triggering earthquakes," said
Elizabeth Cochran, a UCLA graduate student in Earth and space sciences and lead
author of the Science paper. "The earthquakes would have happened anyway, but
they can be pushed sooner or later by the stress fluctuations of the tides."

"Scientists have long suspected the tides played a role, but no one has been
able to prove that for earthquakes worldwide until now," said John Vidale, UCLA
professor of Earth and space sciences, interim director of UCLA's Institute of
Geophysics and Planetary Physics, and co-author of the paper. "Earthquakes have
shown such clear correlations in only a few special settings, such as just below
the sea-floor or near volcanoes."

"There are many mysteries about how earthquakes occur, and this clears up one of
them," Vidale said. "We find that it takes about the force arising from changing
the sea level by a couple of meters of water to noticeably affect the rate of
earthquakes. This is a concrete step in understanding what it takes to set off
an earthquake."

Cochran, Vidale and co-author Sachiko Tanaka are the first researchers to factor
in both the phase of the tides and the size of the tides, and are using
calculations of the effects of the tides more accurate than were available just
three years ago. Tanaka is a seismologist with Japan's National Research
Institute for Earth Science and Disaster Prevention.

Cochran and Vidale analyzed more than 2,000 earthquakes worldwide, magnitude 5.5
and higher, which struck from 1977 to 2000. They studied earthquakes in
"subduction zones" where one tectonic plate dives under another, such as near
the coasts of Alaska, Japan, New Zealand and western South America. "These
earthquakes show a correlation with tides because along continent edges ocean
tides are strong," Vidale said, "and the orientation of the fault plane is
better known than for faults elsewhere."

Cochran conducted a statistical analysis of the earthquakes and tidal stress
data, using state-of-the-science tide calculations from Tanaka and the best
global earthquake data, which came from Harvard seismologists. This research
follows up on a 2002 study by Tanaka. The current research was funded by the
National Science Foundation and the Laurence Livermore National Laboratory.

Cochran and Vidale found a strong correlation between when earthquakes strike
and when tidal stress on fault planes is high, and the likelihood of these
results occurring by chance is less than one in 10,000, Cochran said. They found
that strong tides impose enough stress on shallow faults to trigger earthquakes.
If the tides are very large, more than two meters, three-quarters of the
earthquakes occur when tidal stress acts to encourage triggering, she found.
Fewer earthquakes are triggered when the tides are smaller.

In California, and in fact in most places in the world, the correlation between
earthquakes and tides is considerably smaller, Vidale said. In California, tides
may vary the rate of earthquakes at most one or two percent; the overall effect
of the tides is smaller, he said, because the faults studied are many miles
inland from the coast and the tides are not particularly large.