U.S. Department of the Interior
U.S. Geological Survey

Press Release

Release Date: April 27, 2005

Contacts: Catherine Puckett 707-442-1329
Eric Geist 650-814-5458

USGS Research Sheds Light on Why the March Tsunami was Smaller than the
December Tsunami

As soon as the 8.7 magnitude March 2005 earthquake occurred in the Indian
Ocean, the world anxiously awaited word of yet another devastating tsunami
similar to the one that occurred on December 26, 2004, as the result of the
9.0 Indian Ocean earthquake.

But although the tsunami that resulted was certainly large 13 to 16 feet
is not a tsunami to dismiss it paled in comparison to the enormous
December event, which had run-ups of 100 feet or higher in Sumatra.

Why such a big difference?

Eric Geist, a U.S. Geological Survey tsunami expert, has been researching
probable reasons for the differences between the two tsunamis. He will
present his preliminary findings at the 2005 Annual Meeting of the
Seismological Society of America meetings in Incline Village, Nevada on
April 27 (8:30 p.m., Lakeside Ballroom).

To Geist, unearthing the answer to the "why the difference" question is
vital because of the many implications such an answer has for designing the
most effective tsunami warning systems.

Geist and his colleagues Vasily Titov and Diego Arcas at NOAA and Susan
Bilek at New Mexico Tech are focusing their scientific scrutiny on four
factors that they believe explain why the March tsunami was so weak in
comparison to the December one. Much of the information scientists have to
determine how these tsunami waves were generated comes directly from seismic
recordings of the earthquake itself, Geist says.

The first factor, obviously, is magnitude in March, it was 8.7, in
December 9.0. As Geist reminded, the magnitude of an earthquake is a function of the
rupture area and the average amount of slip throughout the rupture. The
December earthquake had a higher average amount of slip and ruptured a
longer segment of the fault than the March earthquake.

"Most earthquakes," said Geist, "start with the greatest amount of slip near
the epicenter. The exception to this 'rule' is what happened in December,
where it started with a little slip, then a big burst occurred later
farther away from the epicenter."

The second factor is water depth and is probably the biggest contributor
to the difference in the two tsunamis, Geist said. "In December, a lot of
the energy in the initial earthquake rupture occurred in deep water (about 1
to 2.5 miles), whereas the March earthquake occurred beneath the shelf just
offshore mainland Sumatra at water depths of less than .6 miles or even
beneath Sumatra itself. This difference in water depth over areas where the
greatest energy was released from the earthquake produced a difference in
amplification as the tsunami traveled from the source region to shore.

Hence, said Geist, the more amplification of the wave, the bigger the
run-ups. Of course, if much of the movement of the surface of the earth
caused by the earthquake is on land, it will not go into generating a
tsunami because there is no water to displace.

The third factor is the depth below the surface of the earth where most of
the fault slip occurred. In March, most of slip occurred at depths of 12 to
25 miles below the surface, whereas for the December earthquake, slip may
have extended all the way to the sea floor at the oceanic trench. This
factor, along with the difference in magnitude, resulted in greater vertical
movement of the sea floor in December. The peak vertical displacement of
the sea floor for the two earthquakes was approximately 16 feet for the
December earthquake and about 10 feet for the March earthquake.

The last factor is that there was a significant difference between the two
events in the primary direction of tsunami-wave "focusing," which affects
the distant or "far-field" impact of the tsunami. In December, the tsunami
energy was focused to the west, toward Sri Lanka and India, and to the east,
toward Thailand. In March, the tsunami energy was focused to the southwest,
away from any nearby land masses (excluding Sumatra itself). Because the
March earthquake occurred beneath the island shelf of Sumatra, the island
itself blocked significant wave activity toward Thailand and Malaysia.

Geist emphasizes that the knowledge researchers gain about these two
important earthquakes will help scientists design and build more precise
models of tsunami propagation under different conditions -- even in the same
geographic area. Also, he noted, by analyzing these two earthquakes and
comparing them with historical earthquakes of similar magnitude (more than
8.5), the end result will be a better ability to forecast future variations
in tsunami runup. "This improved understanding will greatly aid global
efforts to provide accurate tsunami warnings and hazard assessments. The
ultimate goal, of course, is to save lives and reduce property damage from
future tsunami disasters."

For more information and updates on field activities, see the USGS Tsunami &
Earthquakes Web Site:

The USGS serves the nation by providing reliable scientific information to
describe and understand the Earth; minimize loss of life and property from
natural disasters; manage water, biological, energy, and mineral resources;
and enhance and protect our quality of life.

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