Deep-Sea Tremors May Provide Early Warning System for Larger Earthquakes

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March 24, 2005

Predicting when large earthquakes might occur may be a step closer to
reality, thanks to a new study of undersea earthquakes in the eastern
Pacific Ocean. The study, reported in today’s Nature,
is the first to suggest that small seismic shocks or foreshocks
preceding a major earthquake can be used in some cases to predict the
main tremors.

Scientists from the Woods Hole Oceanographic Institution (WHOI) and the
University of Southern California (USC) report that some types of large
undersea earthquakes may be predictable on time scales of hours or
less. Earthquakes on land are generally not preceded by
systematic immediate foreshocks and hence cannot be predicted as easily
with the same methods.

The research team, led by Jeffrey McGuire of WHOI, studied past
earthquakes along five transform faults on the East Pacific Rise, where
tectonic plates are spreading apart at a rate of more than ten
centimeters or five inches a year. The team used data from
sensors deployed by the National Oceanic and Atmospheric
Administration’s Pacific Marine Environmental Laboratory that
pinpointed the time and location of foreshocks and the mainshocks or
larger earthquakes.

“This is the first demonstration of good short-term predictability for
big earthquakes,” study coauthor Thomas Jordan of the USC Southern
California Earthquake Center said. “Some scientists believe that
earthquakes come on suddenly with no warning signs, and the big ones
are therefore unpredictable. In other parts of the oceans, they may
be.”

The researchers defined a foreshock as any tremor of at least a
magnitude 2.5 on the Richter scale, and a mainshock as a tremor of
magnitude 5.4 or greater. Using earthquakes that occurred in the last
ten years as a test case, they set up a hypothetical “alert” for an
hour within a 15-kilometer (about ten-mile) radius of the epicenter of
every potential foreshock. This “early-warning system” would have
successfully predicted six of the nine major earthquakes that occurred
along two of transform faults, Discovery and Gofar, between 1996 and
2001, despite having alerts in a very small percentage of the total
time period.

The team’s findings suggest that short-term prediction – the ability to
forecast an earthquake in the hours or minutes before it hits – may be
feasible under certain circumstances. McGuire says that although
mid-ocean ridges and associated transform faults are far from main
population centers on land, the fact that a degree of short-term
predictability exists in the deep sea should help seismologists better
understand the earthquake process in general.

McGuire says a new generation of ocean bottom instrumentation, a major
technical challenge, will help scientists improve understanding of the
earthquake process. “If both foreshocks and mainshocks are triggered by
an earlier event, which could be a gradual slipping along a fault line,
technically known as an aseismic slow slip transient that doesn’t
create seismic waves, then it could be detected with the right
instruments.”

Such slow events can be detected on land at places like the San Andreas
Fault, where movement is recorded by an extensive array of sensors. In
the deep sea, slow slip transients have been detected in subduction
zones, where one tectonic plate is thrust under another. Such events
have been detected off Japan and along the Cascadia Fault off the
Pacific Northwest. However ,the detected events did not
trigger major earthquakes.

The researchers note that subduction zones have higher foreshock rates
than continental regions, so the ability to detect foreshocks, even on
a short-term basis, has significance in earthquake prediction.
Moreover, a slow slip transient was detected 15 minutes before the 1960
Chilean earthquake, at magnitude 9.5 the largest in recorded history.

Most major earthquakes occur along subduction zones, but whether
earthquakes on subduction zones can be predicted systematically remains
controversial and will require improved seafloor-based observations.

The researchers say that if an extensive array of sensors like that
along the San Andreas Fault were placed on the sea floor, seismologists
would likely see the earthquake coming. McGuire will lead an expedition
in 2007 to deploy sensors along the East Pacific Rise and begin testing
that idea.

The study was supported by the Frank and Lisina Hoch Endowed Fund and the Deep
Ocean Exploration Institute at WHOI, the National Science Foundation,
the Southern California Earthquake Center, and the U.S. Geological
Survey.