Wednesday, June 10,
2009
Scientists have made the surprising finding that typhoons trigger
slow earthquakes, at least in eastern Taiwan. Slow earthquakes are
non-violent fault slippage events that take hours or days instead
of a few brutal seconds to minutes to release their potent energy.
The researchers discuss their data in a study published the June
11, issue of Nature.
"From 2002 to 2007 we monitored deformation in eastern Taiwan using
three highly sensitive borehole strainmeters installed 650 to 870
feet (200-270 meters) deep. These devices detect otherwise
imperceptible movements and distortions of rock," explained
coauthor Selwyn Sacks of Carnegie's Department of Terrestrial
Magnetism. "We also measured atmospheric pressure changes, because
they usually produce proportional changes in strain, which we can
then remove."
Taiwan has frequent typhoons in the second half of each year but is
typhoon free during the first 4 months. During the five-year study
period, the researchers, including lead author Chiching Liu
(Academia Sinica, Taiwan), identified 20 slow earthquakes that each
lasted from hours to more than a day. The scientists did not detect
any slow events during the typhoon-free season. Eleven of the 20
slow earthquakes coincided with typhoons. Those 11 were also
stronger and characterized by more complex waveforms than the other
slow events.
"These data are unequivocal in identifying typhoons as triggers of
these slow quakes. The probability that they coincide by chance is
vanishingly small," remarked coauthor Alan Linde, also of
Carnegie.
How does the low pressure trigger the slow quakes? The typhoon
reduces atmospheric pressure on land in this region, but does not
affect conditions at the ocean bottom, because water moves into the
area and equalizes pressure. The reduction in pressure above one
side of an obliquely dipping fault tends to unclamp it. "This fault
experiences more or less constant strain and stress buildup," said
Linde. "If it's close to failure, the small perturbation due to the
low pressure of the typhoon can push it over the failure limit; if
there is no typhoon, stress will continue to accumulate until it
fails without the need for a trigger."
"It's surprising that this area of the globe has had no great
earthquakes and relatively few large earthquakes," Linde remarked.
"By comparison, the Nankai Trough in southwestern Japan, has a
plate convergence rate about 4 centimeters per year, and this
causes a magnitude 8 earthquake every 100 to 150 years. But the
activity in southern Taiwan comes from the convergence of same two
plates, and there the Philippine Sea Plate pushes against the
Eurasian Plate at a rate twice that for Nankai."
The researchers speculate that the reason devastating earthquakes
are rare in eastern Taiwan is because the slow quakes act as
valves, releasing the stress frequently along a small section of
the fault, eliminating the situation where a long segment sustains
continuous high stresses until it ruptures in a single great
earthquake. The group is now expanding their instrumentation and
monitoring for this research.
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Carnegie Institution
Thanks to Carnegie Institution for
this article.