http://www.terradaily.com/reports/2004_Tsunami_Forces_Rethinking_Theory_Of_Giant_Earthquakes.html

by Staff Writers
Pasadena CA (SPX) Mar 03, 2006
The Sumatra-Andaman earthquake of December 26, 2004, was one of the worst
natural disasters in recent memory, mostly on account of the devastating
tsunami that followed it. A group of geologists and geophysicists,
including scientists at the California Institute of Technology, has
delineated the full dimensions of the fault rupture that caused the
earthquake.
Their findings, reported in the March 2 issue of the journal Nature,
suggest that previous ideas about where giant earthquakes are likely to
occur need to be revised. Regions of the earth previously thought to be
immune to such events may actually be at high risk of experiencing them.

Like all giant earthquakes, the 2004 event occurred on a subduction
megathrust-in this case, the Sunda megathrust, a giant earthquake fault,
along which the Indian and Australian tectonic plates are diving beneath
the margin of southeast Asia. The fault surface that ruptured cannot be
seen directly because it lies several kilometers deep in the Earth's
crust, largely beneath the sea.

Nevertheless, the rupture of the fault caused movements at the surface as
long-accumulating elastic strain was suddenly released. The researchers
measured these surface motions by three different techniques. In one, they
measured the shift in position of GPS stations whose locations had been
accurately determined prior to the earthquake.

In the second method, they studied giant coral heads on island reefs: the
top surfaces of these corals normally lie right at the water surface, so
the presence of corals with tops above or below the water level indicated
that the Earth's crust rose or fell by that amount during the earthquake.

Finally, the researchers compared satellite images of island lagoons and
reefs taken before and after the earthquake: changes in the color of the
seawater or reefs indicated a change in the water's depth and hence a rise
or fall of the crust at that location.

On the basis of these measurements the researchers found that the 2004
earthquake was caused by rupture of a 1,600-kilometer-long stretch of the
megathrust-by far the longest of any recorded earthquake. The breadth of
the contact surface that ruptured ranged up to 150 kilometers. Over this
huge contact area, the surfaces of the two plates slid against each other
by up to 18 meters.

On the basis of these data, the researchers calculated that the so-called
moment-magnitude of the earthquake (a measure of the total energy
released) was 9.15, making it the third largest earthquake of the past 100
years and the largest yet recorded in the few decades of modern
instrumentation.

"This earthquake didn't just break all the records, it also broke some of
the rules," says Kerry Sieh, who is the Sharp Professor of Geology at
Caltech and one of the authors of the Nature paper.

According to previous understanding, subduction megathrusts can only
produce giant earthquakes if the oceanic plate is young and buoyant, so
that it locks tightly against the overriding continental plate and resists
rupture until an enormous amount of strain has accumulated.

Another commonly accepted idea is that the rate of relative motion between
the colliding plates must be high for a giant earthquake to occur. Both
these conditions are true off the southern coast of Chile, where the
largest earthquake of the past century occurred in 1960. They are also
true off the Pacific Northwest of the United States, where a giant
earthquake occurred in 1700 and where another may occur before long.

But at the site of the 2004 Sumatra-Andaman earthquake the oceanic crust
is old and dense, and the relative motion between the plates is quite
slow. Yet another factor that should have lessened the likelihood of a
giant earthquake in the Indian Ocean is the fact that the oceanic crust is
being stretched by formation of a so-called back-arc basin off the
continental margin.

"For all these reasons, received wisdom said that the giant 2004
earthquake should not have occurred," says Jean-Philippe Avouac, a Caltech
professor of geology, who is also a contributor to the paper. "But it did,
so received wisdom must be wrong. It may be, for example, that a slow rate
of motion between the plates simply causes the giant earthquakes to occur
less often, so we didn't happen to have seen any in recent times-until
2004."

Many subduction zones that were not considered to be at risk of causing
giant earthquakes may need to be reassessed as a result of the 2004
disaster. "For example, the Ryukyu Islands between Taiwan and Japan are in
an area where a large rupture would probably cause a tsunami that would
kill a lot of people along the Chinese coast," says Sieh.

"And in the Caribbean, it could well be an error to assume that the entire
subduction zone from Trinidad to Barbados and Puerto Rico is aseismic. The
message of the 2004 earthquake to the world is that you shouldn't assume
that your subduction zone, even though it's quiet, is incapable of
generating great earthquakes."

According to Sieh, it's not that all subduction zones should now be
assigned a high risk of giant earthquakes, but that better monitoring
systems-networks of continuously recording GPS stations, for
example-should be put in place to assess their seismic potential.

"For most subduction zones, a $1 million GPS system would be adequate,"
says Sieh. "This is a small price to pay to assess the level of hazard and
to monitor subduction zones with the potential to produce a calamity like
the Sumatra-Andaman earthquake and tsunami. Caltech's Tectonics
Observatory has, for example, begun to monitor the northern coast of
Chile, where a giant earthquake last occurred in 1877."

In addition to Sieh and Avouac, the other authors of the Nature paper are
Cecep Subarya of the National Coordinating Agency for Surveys and Mapping
in Cibinong, Indonesia; Mohamed Chlieh and Aron Meltzner, both of
Caltech's Tectonics Observatory; Linette Prawirodirdjo and Yehuda Bock,
both of the Scripps Institution of Oceanography; Danny Natawidjaja of the
Indonesian Institute of Sciences; and Robert McCaffrey of Rensselaer
Polytechnic Institute.