Seismologists publish detailed analysis of the great Sumatra-Andaman earthquake

05/13/05

SANTA CRUZ, CA--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 this week 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 the University of California, Santa Cruz, 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. A complete list of the authors of each paper can be found at the end of this press release.

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 said.

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, Lay said.

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," Lay said.

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 seismic waves.

"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 zone.

"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 said.

Source: Eurekalert & others

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