July 5, 2006

“Silent quakes” may signal big temblors: study

SAN FRANCISCO (Reuters) - Locating so-called "silent
earthquakes" may help scientists better predict the probability
of severe temblors, a Stanford University professor said on

Silent earthquakes -- slow-moving events tracked by
satellites measuring subtle changes on the earth's surface
because they do not broadcast shock waves -- appear to build
pressure on fault zones, contributing to weak magnitude-two and
magnitude-three earthquakes, said Stanford geophysicist Paul

Locating silent earthquakes could help better understand
how small temblor activity develops and that could help
scientists better gauge the likelihood of more powerful quakes,
Segall said, referring to findings his research team has
published that will appear on Thursday in the journal Nature.

Data from silent earthquakes may help scientists develop a
"stress gauge" for particular fault zones, Segall said.

"Each time you have a silent event it's like you are
tweaking the system a little, pushing on it a little harder,"
he said. "If we could see an increase in the rate of little
earthquakes in response to these silent events then we could
calibrate the gauge."

"That would be potentially a way of making a probabilistic
forecast" for large earthquakes, Segall said. "We could say
that during this few-week period the probability is higher ...
and hopefully be able to put some numbers on the probability."

Seismologists estimate severe earthquakes strike Japan
every 200 years and the Pacific Northwest region of the United
States every 500 years. Scientists in the past six years have
found southwestern Japan and parts of the Pacific Northwest to
be experiencing silent earthquake activity, Segall said.

"It's possible that each time a slow event occurs, and as
we get later and later in the cycle, or closer and closer to
the really big one, these slow events should start to get
bigger, because the area that's getting closer to failure will
have grown larger," Segall said.