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TOPIC: Sumatra-Andaman earthquake


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RE: Sumatra-Andaman earthquake
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For the first time, scientists have been able to use satellite data to detect the changes in the earth's surface caused by a massive earthquake.

The discovery, reported in the latest issue of the journal Science, signifies a new use for the data from NASA's two GRACE satellites and offers a possible new approach to understanding how earthquakes work.
The research paints a clearer picture of how the earth changed after the December, 2004 Sumatra-Andaman earthquake, the 9.1-magnitude temblor in the Indian Ocean which caused a deadly tsunami killing nearly 230,000 and displacing more than 1 million people.

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Tsunami Warning System
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The UN organisation, UNESCO, has said that a tsunami warning system covering the Indian Ocean region is now "up and running".

26 national tsunami information centres receiving information from 25 new seismographic stations, and three deep-ocean sensors have been set up.
The whole region can now receive and distribute warnings of possible tsunamis.
The system is being overseen by Unesco's Intergovernmental Oceanographic Commission.

-- Edited by Blobrana at 19:48, 2006-06-28

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RE: Sumatra-Andaman earthquake
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Residents along Thailand's southern coast have been warned to be on alert for a repeat of the 2004 tsunami after a series of 31 earthquakes in the Andaman Sea in recent days.
Although there was no immediate risk of a tsunami, having 31 earthquakes in such a short space of time was rare.

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The Indonesian island of Sumatra, is now at risk from two potentially major quakes, one of which could generate waves 10 metres high, seismologists warn on Thursday.

A research team is headed by the same expert who predicted the quake that struck Sumatra on March 28, three months after the December 26 earthquake.
He predicts the next quakes may be as high as 7.5 and 9 respectively on the Richter scale - and in the latter case, cities along much of Sumatra's west coast would be exposed to a tsunami.

"An earthquake under the Mentawai Islands which produced a tsunami would be felt strongly along the cities along the west coast of Sumatra. The tsunami will be generated 200 kms offshore, moving at 750 kms an hour, which gives you 15-20 minutes for people to get on to higher ground."

"I think it would be irresponsible for those in charge of preparing people in this area to ignore the possibility that the earthquake could happen in a year" - John McCloskey, a professor of environmental sciences, University of Ulster.

The study takes a fresh look at Sumatra's seismic mosaic in the light of the last two great quakes, focussing on the two biggest faultlines.
One faultline runs on the land down the western side of Sumatra, and has lateral friction, with one side trying to head northwest and the other trying to move southeast.
Stress on this so-called Sumatra fault, especially in the northwest, in the region of Banda Aceh, remains high

"The threat of an earthquake of magnitude 7.0-7.5 on the Sumatra fault north of four degrees north has not receded"
An even greater threat lies in the Sunda Trench, the second faultline, a notorious seabed crack that runs about 200 kilometres to the west of Sumatra.
This second faultline is known for its vertical movement -- the kind capable of creating big waves by thrusting up sections of the sea bed.
It lies in a so-called "megathrust" area, in which the Australian plate is trying to push its way under the Eurasian plate to the northeast.
The Sunda Trench has been seismically activity for centuries.

Part of its northern section, at the conjunction with the tongue-shaped Burma microplate, was the epicentre for the December 26, 2004 quake, at 9.3 the second highest ever recorded.
The ocean floor ruptured along 1,200 kms, creating a wave in which 217,000 people around the Indian Ocean's rim were killed or left unaccounted for.
That massive event had a domino effect, placing further stress to the south.
On March 28, just 160 kilometres to the south, an 8.7-magnitude earthquake killed more than 900 people.
In turn, this quake has created a new spot of high vulnerability about 500-600 kms farther south underneath the Mentawai Islands, between 0.7 and 5.5 degrees south of the equator.

On average, the Mentawai Islands produce a very big quake every 230 years.
The last big one in the southern part of this section was in 1833, with an 8.5 quake that unleashed a damaging tsunami up to 10 metres high.
By comparison, the wave that scoured the coastline of the northern Indian Ocean on December 26 varied in height from 10 - 15 metres.
In the northern part of this section, there has not been any big quake since 1797, when there was a small slip of only a few metres under the main island of Siberut.

"Slip on the northern part of this section could be greater than 10 metres depending on the timing of the last rupture, slip on the southern portion could be as great as in 1833".
"If this earthquake were to happen within a year, it would not surprise me. Even if the earthquake doesn't happen for 10 years, it's still better to start moving towards a culture of preparedness for these things, because they will happen. ".

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Australia's populous east coast could be struck by a devastating tsunami during the next decade, researchers said on Friday as the government prepares to create a national tsunami warning system.
Australia is surrounded by 8,000 km of active tectonic plate boundaries capable of generating tsunamis that could reach the country's coastline within two to four hours.
One third of the world's earthquakes take place along these boundaries.
On Dec. 23, just three days before the Indian Ocean tsunami, a magnitude 8.1 quake occurred several hundred km south of New Zealand and was felt in parts of Australia and New Zealand.
Professor Peter Mora, director of the Earth Systems Science Computational Centre at the University of Queensland, said there was a misconception Australia was safe from earthquakes and tsunamis, when the country was instead very prone.
"Our international research collaboration partners in the USA forecast that within the next 10 years, a great earthquake with a magnitude of at least 7 on the Richter scale is likely to strike to the north of New Zealand. This could mean a potential tsunami hazard for the east coast of Australia." - Professor Peter Mora.
The U.S. researchers who have made the prediction are from the NASA QuakeSim project and the University of California.
New Zealand would also be hit hard and possibly low-lying Pacific island states to the north.
New Zealand, about 2,000 km east of Australia, straddles two of the tectonic plates that make up the Pacific "ring of fire."
New Zealand's Institute of Geological & Nuclear Sciences records about 14,000 earthquakes in and around the country each year. Most are small, but between 100 and 150 are big enough to be felt.
But while Geoscience Australia seismologists agree a large earthquake occurring north of New Zealand is possible, they believe it would have to be a bigger earthquake than predicted to create a tsunami capable of devastating Australia's east coast.
"It would take something much bigger than magnitude 7 out there to cause any significant damage on the east coast of Australia." - John Schneider, head of Geoscience Australia's Geohazards Division.
A magnitude 9 earthquake in the Indian Ocean off the coast of the Indonesian island of Sumatra on Dec. 26 created a tsunami that killed or left missing about 300,000 people in 13 nations.
"You would be looking at magnitude 8 and larger earthquakes before you would have earthquakes that would be likely to cause significant damage to Australia." - John Schneider.
Geoscience Australia said that for every unit increase in magnitude on the Richter scale, there was about a thirty-fold increase in the energy released by an earthquake. A magnitude 8.6 earthquake releases energy equivalent to about 10,000 atom bombs.
Geoscience Australia is setting up the Australia's A$70 million tsunami warning system.



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Two primitive tribes in India's Andaman and Nicobar islands are believed to be direct descendants of the first humans who migrated from Africa at least 50 000 years ago, according to a study by Indian biologists.
A team of biologists at the Centre for Cellular and Molecular Biology in the southern Indian city of Hyderabad studied the DNA of 10 Onge and Great Andamanese people in the Indian Ocean archipelago who lived for tens of thousands of years in "genetic isolation" from other human contact.
The findings suggest the tribes are descended from the "oldest population of the world and were among the first batch of modern humans to migrate from Africa", said professor Lalji Singh, director of the centre.
The tribals have similar physical features to Africans and their DNA suggests that they have close links with Africa.
Mitochondrial DNA, which is passed maternally and found in every human cell, can be traced to a single female ancestor who lived about 150 000 to 200 000 years ago.
It is believed the descendants of this "Eve" that all humans claim as their ancestor began migrating out of Africa in batches about 70 000 years ago.
The tribals' DNA is extremely close to the so-called African root gene of the single female ancestor, Singh said this week.
This made it likely that they migrated from Africa via a sea route 50 000 to 70 000 years ago and have lived "in genetic isolation" since in the Andaman Islands, said Singh.
"These islanders could hold the key to the mystery of our origins. They are windows to look into the past and hence need to be preserved." - Professor Lalji Singh.
The two other "Stone Age" tribes living in the islands - the Jarawas and the Sentinelese - were not studied because they are resistant to contact with outsiders.
The Indian government has discouraged outside contact with the tribals in an effort to preserve their traditions and protect them from falling victim to illnesses to which they might have no resistance.
The Nicabarese, another tribal group in the islands who were tested, showed they had a closer DNA affinity with people from Asia, suggesting a more recent arrival from the east in the past 18 000 years..
There were fears some of the tribes living in the chain of islands might have been wiped out in the devastating tsunamis of December 26 that were triggered by a powerful underground earthquake nearby off the coast of Indonesia.
But the four Stone Age tribes - the 99-member Onge, 250 Sentinelese, the 39 almost extinct Andamanese and 350 Jarawas - have been accounted for in a post-tsunami headcount.
There were many casualties, however, among the far more numerous Nicobarese tribals who are farmers and live along the coast.
About 1 900 people died in the tsunamis that hit the islands and 5 554 people are listed as missing.


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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." - 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. 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." - Thorne Lay.
Lay is lead author of the first Science paper, which provides an overview of the two earthquakes, and he is a co-author 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 main shocks 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 kilometres) 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 main shock were greater than in any earthquake previously recorded by global broadband seismometers. As far away as Sri Lanka, a thousand miles from the epicentre, the ground moved up and down by more than 9 centimetres. Ground motions greater than 1 centimetre, 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." - Thorne 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, 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." - Thorne Lay.
The earthquake took place along the curving boundary between major plates of the Earth's crust, where the Indo-Australian plate plunges beneath the south-eastern 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 metres in places, averaging about 10 metres of displacement along the segment off the north-western tip of Sumatra where the quake was centred.
From the epicentre, the rupture expanded along the fault at a speed of about 2.5 kilometres 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 metres, 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."- Thorne Lay.
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." - Steven Ward.
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. 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." - Thorne Lay
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 centres, the seismic hazard is significant even for a smaller event."- Thorne Lay.
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." - Thorne Lay.


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