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原文自NATURE網頁
http://www.nature.com/news/2009/090610/full/news.2009.561.html
Typhoons trigger gentler tremors
Small quakes may act as a release valve that prevents catastrophic convulsions.
Amanda Leigh Mascarelli
Typhoons pack enough punch to trigger slow earthquakes in eastern Taiwan,
scientists have found. Slow earthquakes occur as a result of slippage
or rupture of a geologic fault — the same process
that leads to violent earthquakes. But unlike the sudden release of energy
in an ordinary earthquake, slow quakes take place over the course of hours
or even one or two days.
Now, scientists in the United States and Taiwan
have examined slow earthquake events in eastern Taiwan
that occurred between 2002 and 2007. They found that 11 out of 20 slow quakes
coincided with typhoons — tropical cyclones that originate in the northwest
Pacific Ocean. During typhoons, the atmospheric pressure on land is reduced,
and at least in the case of eastern Taiwan,
this pressure change seems to be enough to unclamp a fault
that is under strain and to cause a fault failure.
"The typhoon is nothing but a little hair trigger," says Alan Linde,
a geophysicist at the Carnegie Institution of Washington DC.
Linde is one of the authors of the study,
which is published in Nature this week1.
"It requires just the slightest touch of that trigger and the fault will fail."
Tiny tremors
Slow earthquakes do not emit seismic waves that are strong enough
to be detected by seismometers, so to detect the quakes' activity,
the researchers placed highly sensitive 'strainmeters'
inside boreholes some 200–270 metres deep.
Taiwan sits at the boundary of the Philippine Sea plate and the Eurasian plate,
and is considered to be one of the most seismically active regions
in the world. The authors speculate that these frequent,
slow-quake events may be acting as a kind of pressure-release valve,
releasing stress along short segments of the fault and preventing
the build-up of violent earthquakes in this region.
"We do not get stress built up continuously over a long fault segment,
which is necessary in order to generate a large earthquake," says Linde.
The most powerful slow quake that the team studied in this region
corresponds to an earthquake with a magnitude of 5.4.
In comparison, the nearby Nankai Trough area in southwestern Japan experiences
magnitude-8 earthquakes approximately every 100–150 years.
"So we speculate that these slow events prevent the generation of
a magnitude-8-class event" in eastern Taiwan, says Linde.
Undetected quakes?
It is not yet clear whether similar phenomena are occurring elsewhere
in the world. In some regions of the Cascadia fault,
which reaches from northern Vancouver Island in Canada to northern California,
tremors associated with small seismic slips — known as 'episodic tremor
and slip' (ETS) events — occur approximately every 15 months2.
Previous studies have shown that ETS events can be triggered
by remote earthquakes and by tidal variations.
Although the Cascadia region is not affected by typhoons,
it does experience extensive low-pressure atmospheric systems
that could produce small stress changes within the crust.
These might be similar to those occurring in eastern Taiwan, says Herb Dragert,
a geophysicist with the Geological Survey of Canada in Sidney,
British Columbia, who was not involved in the study.
To date, no one has investigated whether atmospheric pressure changes
could be triggering ETS events, says Dragert. But the similarity
between this latest study and previous ones, he notes,
is that "very small stress changes can initiate this kind of slow slip
and slow earthquake phenomenon".
The study focuses on one localized segment of a fault in eastern Taiwan,
but Gregory Beroza, a seismologist at Stanford University in California,
says that ultimately,
"it might give us some deeper understanding into how earthquakes work".
It is difficult to say how widespread slow-quake events are,
adds Beroza, because there are so few monitoring instruments across the globe.
"This sort of thing could be happening all over the place
but we just don't have the instruments in place to see it."
References
Liu, C., Linde, A. T. & Sacks, I. S. Nature 459, 833-836 (2009).
Rogers, G. & Dragert, H. Science 300, 1942-1943 (2003).
Gomberg, J. et al. Science 319, 173 (2008).
Rubinstein, J. L., La Rocca, M., Vidale, J. E., Creager, K. C. & Wech,
A. G. Science 319, 186-189 (2008).
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