May 23, 2005
Seismologists publish detailed analysis of
the great Sumatra-Andaman earthquake
By Tim Stephens
The great Sumatra-Andaman earthquake of December 26, 2004,
was an event of stunning proportions, both in its human dimensions--nearly
300,000 lives lost--and as a geological phenomenon.
The sudden rupture of a huge fault beneath the Indian Ocean
unleashed a devastating tsunami. It was the largest earthquake
in the past 40 years and was followed by the second largest
just three months later (March 28, 2005) on an adjacent fault.
Modern monitoring technology gathered an unprecedented amount
of seismological data on these extraordinary events.
Three papers published in the May 20 issue of the journal Science
by an international group of seismologists provide a comprehensive
scientific analysis of the earthquakes.
The Incorporated Research Institutions for Seismology (IRIS),
a university research consortium, played a central role in bringing
about this coordinated report from three teams of experts. IRIS,
funded by the National Science Foundation, operates a global
network of seismic monitoring stations that provided much of
the data for the analysis.
"We wanted to give as unified and comprehensive a report
as possible, rather than having bits and pieces of it come out
in separate papers," said Thorne Lay, professor of Earth
sciences and director of the Institute of Geophysics and Planetary
Physics at UCSC and chair of the board of directors of IRIS.
Lay organized the overall effort and solicited contributions
for the three papers from leading seismological researchers
at U.S. and international research programs. David Simpson,
president of IRIS, helped Lay arrange for the papers to be published
together in a special section of Science.
"This is really a watershed event," Lay said. "We've
never had such comprehensive data for a great earthquake, because
we didn't have the instrumentation to gather it 40 years ago.
And then the sheer size of the event is so awesome. It is nature
at its most formidable, and it has been humbling to all of us
who have studied it. The willingness of the seismological research
community to work together to give a comprehensive report on
the earthquake reflects our understanding of the importance
of this event."
Lay is lead author of the first Science paper, which
provides an overview of the two earthquakes, and he is a coauthor
on the second paper, which focuses on the processes involved
in the rupture of the fault. The third paper describes how the
earthquakes caused the whole planet to vibrate with "free
oscillations," like the ringing of a bell.
The two earthquakes are the largest that have happened since
the global deployment of highly sensitive digital broadband
seismometers. These instruments, deployed around the world by
IRIS and other organizations, recorded both the huge ground
motions from the mainshocks and the tiny motions from small
aftershocks and free oscillations of the planet.
Record-setting features of the Sumatra-Andaman earthquake of
December 26, 2005, include the longest fault rupture ever observed
(1,200 to 1,300 kilometers, or 720 to 780 miles) and the longest
duration of faulting (at least 10 minutes). The aftershocks
included the most energetic earthquake swarm ever observed.
The ground motions during the prolonged, intense shaking of
the mainshock were greater than in any earthquake previously
recorded by global broadband seismometers. As far away as Sri
Lanka, a thousand miles from the epicenter, the ground moved
up and down by more than 9 centimeters (3.6 inches). Ground
motions greater than 1 centimeter (0.4 inch), but too gradual
to be felt, occurred everywhere on Earth's surface as seismic
waves from the event spread around the globe.
The 10-minute duration of the rupture complicated the seismological
analysis, Lay said. An earthquake generates many different kinds
of seismic waves, including fast-moving P waves and slower-moving
S waves. In an earthquake with a more typical duration of 30
seconds, S waves would start to arrive at seismic monitoring
stations minutes after the P waves had passed. But in the Sumatra-Andaman
earthquake, the P waves were still coming when the S waves started
to arrive, making it hard to sort out the signals.
"Nobody's algorithms were tuned to work with this kind
of earthquake, so we had to take all of the methods we have
applied successfully to smaller earthquakes and expand and adapt
them for this earthquake that just went on and on," Lay
The new analysis gives the Sumatra-Andaman earthquake a seismic
magnitude of at least 9.1, and possibly as high as 9.3. Earlier
estimates had put it at magnitude 9.0. By comparison, the 1960
Chile earthquake was magnitude 9.5, and the 1964 Alaska earthquake
was magnitude 9.2.
The data from those earlier earthquakes are relatively limited,
however, and small differences in magnitude may not be significant,
For those who experienced California's 1989 Loma Prieta earthquake--a
magnitude 6.9 event that caused major destruction from Santa
Cruz to the San Francisco Bay Area--Lay noted that the ground
shook more than 100 times harder during the Sumatra-Andaman
earthquake. Even some of the aftershocks were more powerful
than the Loma Prieta quake.
"Even among seismologists, we call this a monster earthquake,"
The earthquake took place along the curving boundary between
major plates of the Earth's crust, where the Indo-Australian
plate plunges beneath the southeastern Eurasian plate. Before
the fault ruptured, the edge of the Eurasian plate was being
dragged downward by the descending Indo-Australian plate. Released
by the rupture of the fault, the edge of the plate sprang back
up, uplifting the ocean floor and setting off the tsunami that
inundated coastal areas throughout the Indian Ocean. The fault
slipped by as much as 15 meters (50 feet) in places, averaging
about 10 meters (33 feet) of displacement along the segment
off the northwestern tip of Sumatra where the quake was centered.
From the epicenter, the rupture expanded along the fault at
a speed of about 2.5 kilometers per second (1.5 miles per second)
toward the north-northwest. But the initial movement of the
fault was much less along the northern segment than in the south.
This was fortunate, because it spared much of the coastline
in the north from the massive tsunami waves that caused so much
destruction further south. Eventually, the northern part of
the fault slipped about as much as the southern part, uplifting
and tilting the Andaman Islands. The tilting of the islands
shows that the northern part must have slipped about 10 meters,
but much of that slip occurred gradually, without generating
"We think that slip was occurring in the northern part
for about an hour, well after the 10 minutes of rapid motions
were over," Lay said.
UCSC geophysicist Steven Ward generated models of the tsunami
waves that document this long slip process. Ward used a unique
recording of the tsunami spreading across the Indian Ocean obtained
by a radar altimetry satellite (Jason) that happened
to be passing overhead. The satellite data showed a trough in
the central Bay of Bengal two hours after the earthquake, which
is best explained by late slip beneath the Andaman Islands,
according to Ward's tsunami models.
"The satellite image of the tsunami is quite exciting
because such data open a new window through which earthquake
rupture processes can be observed, and it also suggests that
radar satellites might some day be able to provide direct real-time
warning of an approaching tsunami wave," Ward said.
After the earthquake and the tsunami came the aftershocks,
including the most energetic earthquake swarm ever observed.
More than 150 earthquakes of magnitude 5 and greater occurred
over a four-day period in late January on faults beneath the
Andaman Sea that were activated by the rupture of the main fault
along the plate boundary to the west. There were also numerous
aftershocks of magnitude 6 and greater throughout the fault
"It's an incredible aftershock series," Lay said.
"It is hard to get a feeling for the scale of it. If you
take the aftershock zone and superimpose it on California, it
completely covers the state."
Then the March 28 earthquake struck with a magnitude of 8.6
on an adjacent portion of the plate boundary to the southeast.
This was not an aftershock, but a new rupture of an adjacent
segment of the fault. Now, concern about additional earthquakes
is focused on the next area to the southeast, which last failed
in a great earthquake in 1833. Major earthquakes could occur
not only on the thrust fault along the plate boundary, but also
on a related fault system beneath the island of Sumatra. Faulting
on that system involves horizontal shearing, similar to the
San Andreas Fault.
"The Sumatra Fault runs right down the length of the island.
Because it is close to major population centers, the seismic
hazard is significant even for a smaller event," Lay said.
Major faults elsewhere in the world--in northern Turkey, for
example--have experienced sequences of earthquakes moving progressively
along a fault line.
"When one part of the fault slides, that loads up the
adjacent region and transfers stress. So you have a heightened
potential for earthquakes on the adjacent section. The concern
is that something like that could happen in Sumatra," Lay
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