Seabed at Tsunami's Center Shows Ruptures

The first images of the seabed that was rocked by the earthquake that triggered Asia's catastrophic tsunami revealed huge ruptures spanning several miles along the Indian Ocean's floor.

The United States, meanwhile, said it was preparing to more than double its pledge for tsunami relief to $950 million.

The images of the seabed were from a British naval ship collecting data off the coast of Indonesia's Sumatra Island that could be used to help governments develop a tsunami early warning system for the Indian Ocean region.

"There are features which we would think are something like the Grand Canyon would look," Tim Henstock, a scientist aboard the HMS Scott, told BBC News. "You can see huge piles of mud maybe a few hundred meters (yards) thick."

The images show "slide scars" more than six miles wide resulting from the 9.0 magnitude quake on Dec. 26, the world's biggest in 40 years.

President Bush said he would ask Congress for $950 million - up from $350 committed for tsunami relief so far - a pledge that would put the United States atop of the list of donors to the disaster.

Australia has promised $810 million, followed by Germany's $660 million, the European Commission's $624 million and Japan's $540 million.

"We will use these resources to provide assistance and to work with the affected nations on rebuilding vital infrastructure that re-energizes economies and strengthens societies," Bush said in a statement.

The gruesome task of retrieving the dead in Indonesia is unlikely to be finished by June as earlier expected as corpses continue to be found in the rubble, said Yrsa Grune from the International Federation of Red Cross and Red Crescent Societies.

Volunteers have been helping a government-led effort to collect and bury victims in Aceh.

Indonesian officials have said they expect the death toll to rise for weeks by an average of 500 a day, but Grune said the search could stretch on for months.

"The plan was to continue until June. Now, it might be that plan will have to be revised," she said. "It's inevitable. Every time you lift a stone you might find something under it because there's still lots of rubble."

Meanwhile, hundreds of residents of Banda Aceh gathered at the city's historic mosque Thursday for a memorial service on the Islamic New Year.

"This is a trial from God, an opportunity for the Islamic community in Aceh to reflect on their lives," Aceh's vice governor, Azwar Abubakar, said in an address broadcast over loudspeakers at the 17th century Baiturrahman Mosque, where many sought shelter from the disaster.

"Let us draw closer in brotherhood," Abubakar said. "Let us gather together to do good deeds and reject sin."

Aftershocks have rattled the region since the Dec. 26 quake that spawned the tsunami, killing more than 160,000 people in 11 Indian Ocean nations.

A 5.7-magnitude temblor struck Aceh on Wednesday, according to the U.S. Geological Survey. It shook buildings and sent terrified residents fleeing for higher ground while police shouted "Tsunami! Tsunami!" There were no reports of injuries or damage.

Tens of thousands of people are still missing following the December disaster, although officials say it's too early to add them to the toll.

After weeks of keeping the names private, police in Sweden released a list of its 565 missing citizens. It made for harrowing reading: A 9-month-old boy, entire families and nearly 100 children younger than 13.

New Zealand's Foreign Ministry slashed the number of nationals it listed as missing by about 200 to just four. Two people were confirmed to have died when the tsunami struck in Thailand, and the four missing are presumed dead, ministry spokeswoman Emma Riley said.

Analysis of seismograms from the December 26, 2004 Sumatra earthquake that generated the devastating tsunami shows that it was much bigger than previously thought and explains in part why the tsunami was so destructive.

Measurements of seismic energy at vibration periods much longer than previously studied show that the earthquake was approximately three times larger than previously reported. Its revised moment magnitude, Mw= 9.3 instead of the previously reported 9.0, makes it the second largest ever recorded since the invention of the seismometer about 100 years ago. The additional energy release occurred by slow slip along the 1200-km long fault delineated by aftershocks, making the rupture zone much larger than previously thought from analysis of shorter period waves. [Image]

This long rupture played a key role in generating the devastating tsunami. In particular, the large tsunami amplitudes in Sri Lanka and India result from rupture on the northern, north-trending, segment because tsunami amplitudes are largest perpendicular to the fault.

Because the entire rupture zone slipped, strain accumulated from subduction of the Indian plate beneath the Burma microplate has been released, leaving no immediate danger of a comparable tsunami being generated by water moved by an earthquake on this segment of the plate boundary. However, the danger of a local tsunami generated by a large aftershock or a comparable ocean-wide tsunami resulting from a great earthquake on segments to the south remains.

[Image]

These results come from analyzing the earth's normal modes - ultra long vibrations by which the earth rings like a bell (or more precisely rattles like a garbage can) for days and even weeks after such a gigantic earthquake. Analysis of long seismograms shows distinct energy peaks whose height reflects the earthquake's seismic moment, which gives its magnitude. Because the magnitude scale is logarithmic, the threefold increase in seismic moment raises the magnitude by 0.3 units, making it second only to the 1960 Chile earthquake.

Curiously, the analysis technique relied on results we developed with Robert Geller (now at the University of Tokyo) as graduate students almost 30 years ago. However, because such gigantic earthquakes are rare, these methods had been essentially unused until records of the Sumatra earthquake on modern seismometers became available. These data were provided by the Global Seismographic Network of the Incorporated Research Institutions for Seismology .

Examples of seismic data from around the world that we analyzed. [Image]

The size of the earthquake is given by its moment magnitude Mw, defined as
Mw = (log Mo / 1.5) - 10.73
from the seismic moment Mo in dyn-cm. Thus the Harvard CMT solution at 300 s period measured a moment of 4e29 dyn-cm giving Mw 9.0, and the longest period normal mode 0S2 gives Mo 1.3e30 dyne-cm or Mw 9.3. The moment and magnitude rise smoothly with period, as shown in the technical report linked below. This makes the earthquake gigantic, as illustrated by comparison with some other earthquakes below.

[Image]

Seth Stein and Emile A. Okal
Department of Geological Sciences
1850 Campus Drive
Northwestern University, Evanston Illinois 60208 USA

seth@earth.northwestern.edu
emile@earth.northwestern.edu (847) 491-5265 FAX: (847) 491-8060 http://www.earth.northwestern.edu/people/seth

For a technical report of these results click here .

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