Another Large Earthquake Off Coast Of Sumatra Likely

ScienceDaily (Dec. 4, 2008) — The subduction zone that brought us the 2004 Sumatra-Andaman earthquake and tsunami is ripe for yet another large event, despite a sequence of quakes that occurred in the Mentawai Islands area in 2007, according to a group of earthquake researchers led by scientists from the Tectonics Observatory at the California Institute of Technology (Caltech).

"From what we saw," says geologist Jean-Philippe Avouac, director of the Tectonics Observatory and one of the paper's lead authors, "we can say with some confidence that we're probably not done with large earthquakes in Sumatra."

The devastating magnitude 9.2 earthquake that occurred off the western coast of Sumatra on December 26, 2004—the earthquake that spawned a lethal tsunami throughout the Indian Ocean—took place in a subduction zone, an area where one tectonic plate dips under another, forming a quake-prone region.

It is that subduction zone that drew the interest of the Caltech-led team. Seismic activity has continued in the region since the 2004 event, they knew. But have the most recent earthquakes been able to relieve the previous centuries of built-up seismic stress?

Yes . . . and no. Take, for instance, an area just south of the 2004 quake, where a magnitude 8.6 earthquake hit in 2005. (That same area had also been the site of a major earthquake in 1861.) The 2005 quake, says Avouac, did a good job of "unzipping" the stuck area in that patch of the zone, effectively relieving the stresses that had built up since 1861. This means that it should be a few centuries before another large quake in that area would be likely.

The same cannot be said, however, of the area even further south along that same subduction zone, near the Mentawai Islands, a chain of about 70 islands off the western coasts of Sumatra and Indonesia. This area, too, has been hit by giant earthquakes in the past (an 8.8 in 1797 and a 9.0 in 1833). More recently, on September 12, 2007, it experienced two earthquakes just 12 hours apart: first a magnitude 8.4 quake and then a magnitude 7.9.

These earthquakes did not come as a surprise to the Caltech researchers. Caltech geologist and paper coauthor Kerry Sieh, who is now at the Nanyang Technological University in Singapore, had long been using coral growth rings to quantify the pattern of slow uplift and subsidence in the Mentawai Islands area; that pattern, he and his colleagues knew, is the result of stress build-up on the plate interface, which should eventually be released by future large earthquakes.

But was all that accumulated stress released in 2007? In the work described in the Nature letter, the researchers analyzed seismological records, remote sensing (inSAR) data, field measurements, and, most importantly, data gathered by an array of continuously recording GPS stations called SuGAr (for Sumatra Geodetic Array) to find out.

Their answer? The quakes hadn't even come close to doing their stress-reduction job. "In fact," says Ali Ozgun Konca, a Caltech scientist and the paper's first author, who did this work as a graduate student, "we saw release of only a quarter of the moment needed to make up for the accumulated deficit over the past two centuries." (Moment is a measure of earthquake size that takes into account how much the fault slips and over how much area.)

"The 2007 quakes occurred in the right place at the right time," adds Avouac. "They were not a surprise. What was a surprise was that those earthquakes were way smaller than we expected."

"The quake north of this region, in 2005, ruptured completely," says Konca. "But the 2007 sequence of quakes was more complicated. The slippage of the plates was patchy, and it didn't release all the strain that had accumulated."

"It was what we call a partial rupture," adds Avouac. "There's still enough strain to create another major earthquake in that region. We may have to wait a long time, but there's no reason to think it's over."

Their findings were published in a letter in the December 4 issue of the journal Nature.

Other authors on the paper include Anthony Sladen, Aron J. Meltzner, John Galetzka, Jeff Genrich, and Don V. Helmberger from Caltech; Danny H. Natawidjaja from the Indonesian Institute of Science (LIPI); Peng Fang and Yehuda Bock from the Scripps Institution of Oceanography in La Jolla; Zhenhong Li from the University of Glasgow in Scotland; Mohamed Chlieh from the Université de Nice Sophia-Antipolis in France; Eric J. Fielding from the Jet Propulsion Laboratory; and Chen Ji from the University of California, Santa Barbara.

The work was supported by funding from the National Science Foundation and the Gordon and Betty Moore Foundation.

Journal reference:

1. . Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence.Nature, December 4, 2008

Adapted from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.

Source :http://www.sciencedaily.com/releases/2008/12/081203131042.htm

Evidence Of Tsunamis On Indian Ocean Shores Long Before 2004

ScienceDaily (Oct. 31, 2008) — A quarter-million people were killed when a tsunami inundated Indian Ocean coastlines the day after Christmas in 2004. Now scientists have found evidence that the event was not a first-time occurrence.

A team working on Phra Thong, a barrier island along the hard-hit west coast of Thailand, unearthed evidence of at least three previous major tsunamis in the preceding 2,800 years, the most recent from about 550 to 700 years ago. That team, led by Kruawun Jankaew of Chulalongkorn University in Thailand, included Brian Atwater, a University of Washington affiliate professor of Earth and space sciences and a U.S. Geological Survey geologist.

A second team found similar evidence of previous tsunamis during the last 1,200 years in Aceh, a province at the northern tip of the Indonesian island of Sumatra where more than half the deaths from the 2004 tsunami occurred.

Sparse knowledge of the region's tsunami history contributed to the loss of life in 2004, the scientists believe. Few people living along the coasts knew to heed the natural tsunami warnings, such as the strong shaking felt in Aceh and the rapid retreat of ocean water from the shoreline that was observed in Thailand.

But on an island just off the coast of Aceh most people safely fled to higher ground in 2004 because the island's oral history includes information about a devastating tsunami in 1907.

"A region's tsunami history can serve as a long-term warning system," Atwater said.

The research will reinforce the importance of tsunami education as an essential part of early warning, said Jankaew, the lead author.

"Many people in Southeast Asia, especially in Thailand, believe, or would like to believe, that it will never happen again," Jankaew said. "This will be a big step towards mitigating the losses from future tsunami events."

The team found evidence for previous tsunamis by digging pits and auguring holes at more than 150 sites on an island about 75 miles north of Phuket, a Thai tourist resort area ravaged by the 2004 tsunami. That tsunami was generated 300 miles to the west when the seafloor was warped during a magnitude 9.2 earthquake.

At 20 sites in marshes, the researchers found layers of white sand about 4 inches thick alternating with layers of black peaty soil. Witnesses confirmed that the top sand layer, just below the surface, was laid down by the 2004 tsunami, which ran 20 to 30 feet deep across much of the island.

Radiocarbon dating of bark fragments in soil below the second sand layer led the scientists to estimate that the most recent predecessor to the 2004 tsunami probably occurred between A.D. 1300 and 1450. They also noted signs of two earlier tsunamis during the last 2,500 to 2,800 years.

There are no known written records describing an Indian Ocean tsunami between A.D. 1300 and 1450, including the accounts of noted Islamic traveler Ibn Battuta and records of the great Ming Dynasty armadas of China, both of which visited the area at different times during that period. Atwater hopes the new geologic evidence might prompt historians to check other Asian documents from that era.

"This research demonstrates that tsunami geology, both recent and past tsunamis, can help extend the tsunami catalogues far beyond historical records," Jankaew said.

The new findings also carry lessons for the northwest coast of North America, where scientists estimate that many centuries typically elapse between catastrophic tsunamis generated by the Cascadia subduction zone.

"Like Aceh, Cascadia has a history of tsunamis that are both infrequent and catastrophic, and that originate during earthquakes that provide a natural tsunami warning," Atwater said. "This history calls for sustained efforts in tsunami education."

Findings from both teams are published in the Oct. 30 edition of Nature.

Other co-authors of the Thai paper are Yuki Sawai of the Geological Survey of Japan, Montri Choowong and Thasinee Charoentitirat of Chulalongkorn University, Maria Martin of the UW and Amy Prendergast of Geoscience Australia.

The research was funded by the U.S. Agency for International Development, Thailand's Ministry of Natural Resources and Environment, the U.S. National Science Foundation, the Japan Society for the Promotion of Science and the Thailand Research Fund.

Source: http://www.sciencedaily.com/releases/2008/10/081029141037.htm

Study helps predict big Mediterranean quake

LONDON, March 9 (Reuters) - Scientists have found evidence that an overlooked fault in the eastern Mediterranean is likely to produce an earthquake and tsunami every 800 years as powerful as the one that destroyed Alexandria in AD 365.
Using radiocarbon dating techniques, simulations and computer models, the researchers recreated the ancient disaster in order to identify the responsible fault, they said in a study published in the journal Nature Geoscience on Sunday.
"We are saying there is probably a repeat time of 800 years for this kind of earthquake," said Beth Shaw, a seismologist at the University of Cambridge, who led the study.
Scientists study past earthquakes in order to determine the future likelihood of similar large shocks. Identifying the fault for the AD 365 earthquake and tsunami is important for the tens of millions of people in the region, Shaw said.
The fault close to the southwest coast of Crete last produced a big enough quake to generate a tsunami about 1300, which means the next powerful one could come in the next 100 years, she added in a telephone interview.
Shaw and her colleagues calculate the likely intervals by measuring the motion of either side of the fault to gauge how often such large earthquakes would have to occur to account for that level of motion, she said.
Their computer model suggested an 8 magnitude quake on the fault would produce a tsunami that inundates the coastal regions of Alexandria and North Africa, the southern coast of Greece and Sicily all the way up the Adriatic to Dubrovnik, Shaw said.
This would be similar to the ancient quake in AD 365 that caused widespread destruction in much of Greece and unleashed a tsunami that flooded Alexandria and the Nile Delta, likely killing tens of thousands of people, she said.
"This is consistent with the historical record of the tsunami," she said. (Reporting by Michael Kahn; Editing by Janet Lawrence)

Source: http://www.alertnet.org/thenews/newsdesk/L07804229.htm

'Ultrasound' Of Earth's Crust Reveals Inner Workings Of A Tsunami Factory

ScienceDaily (Nov. 15, 2007) — Research just announced by a team of U.S. and Japanese geoscientists may help explain why part of the seafloor near the southwest coast of Japan is particularly good at generating devastating tsunamis, such as the 1944 Tonankai event, which killed at least 1,200 people. The findings will help scientists assess the risk of giant tsunamis in other regions of the world.

Geoscientists from The University of Texas at Austin and colleagues used a commercial ship to collect three-dimensional seismic data that reveals the structure of Earth's crust below a region of the Pacific seafloor known as the Nankai Trough. The resulting images are akin to ultrasounds of the human body. The results, published in the journal Science, address a long standing mystery as to why earthquakes below some parts of the seafloor trigger large tsunamis while earthquakes in other regions do not. The 3D seismic images allowed the researchers to reconstruct how layers of rock and sediment have cracked and shifted over time. They found two things that contribute to big tsunamis.

First, they confirmed the existence of a major fault that runs from a region known to unleash earthquakes about 10 kilometers (6 miles) deep right up to the seafloor. When an earthquake happens, the fault allows it to reach up and move the seafloor up or down, carrying a column of water with it and setting up a series of tsunami waves that spread outward.

Second, and most surprising, the team discovered that the recent fault activity, probably including the slip that caused the 1944 event, has shifted to landward branches of the fault, becoming shallower and steeper than it was in the past. "That leads to more direct displacement of the seafloor and a larger vertical component of seafloor displacement that is more effective in generating tsunamis," said Nathan Bangs, senior research scientist at the Institute for Geophysics at The University of Texas at Austin who was co-principal investigator on the research project and co-author on the Science article.

The Nankai Trough is in a subduction zone, an area where two tectonic plates are colliding, pushing one plate down below the other. The grinding of one plate over the other in subduction zones leads to some of the world's largest earthquakes.

In 2002, a team of researchers led by Jin-Oh Park at Japan Marine Science and Technology Center (JAMSTEC) had identified the fault, known as a megathrust or megasplay fault, using less detailed two-dimensional geophysical methods. Based on its location, they suggested a possible link to the 1944 event, but they were unable to determine where faulting has been recently active. "What we can now say is that slip has very recently propagated up to or near to the seafloor, and slip along these thrusts most likely caused the large tsunami during the 1944 Tonankai 8.1 magnitude event," said Bangs. The images produced in this project will be used by scientists in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an international effort designed to, for the first time, "drill, sample and instrument the earthquake-causing, or seismogenic portion of Earth's crust, where violent, large-scale earthquakes have occurred repeatedly throughout history." "The ultimate goal is to understand what's happening at different margins," said Bangs. "The 2004 Indonesian tsunami was a big surprise. It's still not clear why that earthquake created such a large tsunami. By understanding places like Nankai, we'll have more information and a better approach to looking at other places to determine whether they have potential. And we'll be less surprised in the future."
Bangs' co-principal investigator was Gregory Moore at JAMSTEC in Yokohama and the University of Hawaii, Honolulu. The other co-authors are Emily Pangborn at the Institute for Geophysics at The University of Texas at Austin, Asahiko Taira and Shin'ichi Kuramoto at JAMSTEC and Harold Tobin at the University of Wisconsin, Madison. Funding for the project was provided by the National Science Foundation, Ocean Drilling Program and Japanese Ministry of Education, Culture, Sports and Technology.

Source: http://www.sciencedaily.com/releases/2007/11/071115164101.htm