Even in the absence of earthquakes,
the venerable seismograph still records tiny deflections of the needle as that
familiar roll of paper glides through. Once considered random fluctuations,
geophysicists now say that atmospheric disturbances at sea are the cause of
this noise, enabling seismic records to be used to expand our historical
knowledge of past storms, potentially settling the global warming debate.
"Microseismic noise is
generated from atmospheric storms located over oceans," said geophysicist
Carl Ebeling from Northwestern University (Evanston) at the recent Geological Society
of America annual meeting (Portland, Ore., Oct.
18-21, 2009).
"The energy from these storms couples to the water column and ultimately
to the earth under the water where it travels as seismic waves."
Ebeling and Northwestern professor
Seth Stein are examining past records trying to discern the "seismic
signature" of storms. If they are successful in correlating storms with
microseismic noise, then they will be able to extend our knowledge about how
frequent and how intense past storms have been, giving climatologists hope in
answering questions like whether today's storms are intensifying in response to
global warming.
"The reason this is important
is that rising sea surface temperatures are driving a debate as to whether
global warming is causing storms to become more frequent and more
powerful," said Ebeling. "There's one group out there that says
'yes,' there is another group that says 'no' and there is a group that says we
don't have enough data because our observations only started in the mid-1960s
when satellite observations began to be recorded."
What Ebeling wants to do is extend
the database of storm observations back to the end of the 19th century by
correlating the microseismic signature of storms to the historical seismic
records which began with the invention of the seismograph circa 1880. Since
then, paper rolls all over the world have been recording microseismic noise.
"These seismic records offer an
independent way of looking back in time to answer these questions about whether
storms are getting more powerful today or not," said Ebeling.
The National Science Foundation
(NSF) agrees, giving Ebeling a three-year grant to program his microseismic
signatures into a pattern-recognition algorithm that can be used to deduce the
frequency and intensity of storms from seismic recordings.
"So far we've proven the
concept with historical data from hurricane Andrew in 1992," said Ebeling.
"We were able to find statistically significant correlations between
microseismic noise and the atmospheric pressure and wind speed associated with
the storm."
Over the
next three years, Ebeling plans to create a computer model that can recognize the
signature of storms in seismic data, allowing all those paper rolls to be
converted into a history of storm frequency and intensity. By extending our
knowledge base of past storms all the way back to the 19th century,
climatologists should be able to settle the global warming debate.
