NASA Tsunami Research Makes Waves in Science Community

http://www.jpl.nasa.gov/news/news.cfm?release=2008-007

NASA Tsunami Research Makes Waves in Science Community
Jet Propulsion Laboratory
January 17, 2008

PASADENA, Calif. - A wave of new NASA research on tsunamis has yielded
an innovative method to improve existing tsunami warning systems, and
a
potentially groundbreaking new theory on the source of the December
2004
Indian Ocean tsunami.

In one study, published last fall in Geophysical Research Letters,
researcher Y. Tony Song of NASA's Jet Propulsion Laboratory, Pasadena,
Calif., demonstrated that real-time data from NASA's network of global
positioning system (GPS) stations can detect ground motions preceding
tsunamis and reliably estimate a tsunami's destructive potential
within
minutes, well before it reaches coastal areas. The method could lead
to
development of more reliable global tsunami warning systems, saving
lives and reducing false alarms.

Conventional tsunami warning systems rely on estimates of an
earthquake's magnitude to determine whether a large tsunami will be
generated. Earthquake magnitude is not always a reliable indicator of
tsunami potential, however. The 2004 Indian Ocean quake generated a
huge
tsunami, while the 2005 Nias (Indonesia) quake did not, even though
both
had almost the same magnitude from initial estimates. Between 2005 and
2007, five false tsunami alarms were issued worldwide. Such alarms
have
negative societal and economic effects.

Song's method estimates the energy an undersea earthquake transfers to
the ocean to generate a tsunami by using data from coastal GPS
stations
near the epicenter. With these data, ocean floor displacements caused
by
the earthquake can be inferred. Tsunamis typically originate at
undersea
boundaries of tectonic plates near the edges of continents.

"Tsunamis can travel as fast as jet planes, so rapid assessment
following quakes is vital to mitigate their hazard," said Ichiro
Fukumori, a JPL oceanographer not involved in the study. "Song and his
colleagues have demonstrated that GPS technology can help improve both
the speed and accuracy of such analyses."

Song's method works as follows: an earthquake's epicenter is located
using seismometer data. GPS displacement data from stations near the
epicenter are then gathered to derive seafloor motions. Based upon
these
data, local topography data and new theoretical developments, a new
"tsunami scale" measurement from one to 10 is generated, much like the
Richter Scale used for earthquakes. Song proposes using the scale to
make a distinction between earthquakes capable of generating
destructive
tsunamis from those unlikely to do so.

To demonstrate his methodology on real earthquake-tsunamis, Song
examined three historical tsunamis with well-documented ground motion
measurements and tsunami observations: Alaska in 1964; the Indian
Ocean
in 2004; and Nias Island, Indonesia in 2005. His method successfully
replicated all three. The data compared favorably with conventional
seismic solutions that usually take hours or days to calculate.

Song said many coastal GPS stations are already in operation,
measuring
ground motions near earthquake faults in real time once every few
seconds. "A coastal GPS network established and combined with the
existing International GPS Service global sites could provide a more
reliable global tsunami warning system than those available today," he
said.

The theory behind the GPS study was published in the December 20 issue
of Ocean Modelling. Song and his team from JPL; the California
Institute
of Technology, Pasadena, Calif.; University of California, Santa
Barbara; and Ohio State University, Columbus, Ohio, theorized most of
the height and energy generated by the 2004 Indian Ocean tsunami
resulted from horizontal, not vertical, faulting motions. The study
uses
a 3-D earthquake-tsunami model based on seismograph and GPS data to
explain how the fault's horizontal motions might be the major cause of
the tsunami's genesis.

Scientists have long believed tsunamis form from vertical deformation
of
seafloor during undersea earthquakes. However, seismograph and GPS
data
show such deformation from the 2004 Sumatra earthquake was too small
to
generate the powerful tsunami that ensued. Song's team found
horizontal
forces were responsible for two-thirds of the tsunami's height, as
observed by three satellites (NASA's Jason, the U.S. Navy's Geosat
Follow-on and the European Space Agency's Environmental Satellite),
and
generated five times more energy than the earthquake's vertical
displacements. The horizontal forces also best explain the way the
tsunami spread out across the Indian Ocean. The same mechanism was
also
found to explain the data observed from the 2005 Nias earthquake and
tsunami.

Co-author C.K. Shum of Ohio State University said the study suggests
horizontal faulting motions play a much more important role in tsunami
generation than previously believed. "If this is found to be true for
other tsunamis, we may have to revise some early views on how tsunamis
are formed and where mega tsunamis are likely to happen in the
future,"
he said.

JPL is managed for NASA by the California Institute of Technology in
Pasadena.

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Media contact: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@xxxxxxxxxxxx

2008-007

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