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Strong Earth tides can trigger earthquakes, UCLA scientists report
October 22, 2004 - 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.
University of California - Los Angeles
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