THE rise and fall of the tides could help us to predict major earthquakes like the magnitude 9 quake that triggered Japan's tsunami last year.
Sachiko Tanaka of the National
Research Institute for Earth Science and Disaster Prevention in Tsukuba,
Japan, says that as stresses build up in the Earth's crust, it becomes
more susceptible to minor earthquakes triggered by tidal forces, a sign
of major quakes to come. She has spent over a decade amassing evidence
for her theory. According to her latest results, tidally triggered
earthquakes were rife off the north-east coast of Japan for several
years before 2011's massive earthquake.
"I read hundreds of earthquake prediction papers," says Ross Stein
of the United States Geological Survey in Menlo Park, California. "The
vast majority is dry rot. Tanaka's stands out. It could be very
important."
At high tide, more water is piled up
on top of geological faults, adding to the stress acting on the rocks.
If the fault is already close to rupture, the effect can trigger small
tremors.
Tanaka compared records of tides and
submarine earthquakes from 1976 to 2011 for 100,000 square kilometres
surrounding the epicentre of last year's quake. For the first 25 years
of records, there was no sign of tidally triggered earthquakes, but
after 2000 the number of these quakes gradually increased, reaching a
peak just before last year's megaquake. Afterwards, the effect
disappeared again (Geophysical Research Letters, DOI: 10.1029/2012GL051179).
Tanaka's findings show that stress was
building up in the region for a decade before the main quake struck.
Her earlier research had uncovered a similar effect in the run-up to three Sumatran earthquakes, including the 2004 quake that caused the Boxing Day tsunami. Tidal quakes happened more frequently in the run-up to bigger quakes, suggesting stress had built up year-on-year.
Despite decades of effort, seismologists still cannot reliably predict earthquakes.
Tanaka's approach is promising, but Harold Tobin of the University of
Wisconsin-Madison points out that her analysis was done after the quake
happened.
Tanaka is cautious, but will now try
predicting quakes. "I am planning to monitor tidal triggering in the
north-eastern Japan and Sumatra subduction zones," she says.
"Just for something to have this level of promise is unique," says Sateesh
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