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Environment & Nature News - Asian quake even more
powerful - 31/03/2005
[This is the print version of story
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Asian quake even more powerful
Richard Ingham
Agençe France-Presse
Thursday, 31 March 2005
The 26 December earthquake that unleashed the
Indian Ocean killer tsunami measured 9.3 on the
Richter scale, scientists say, more than twice as
powerful as originally estimated and the second
biggest quake ever recorded.
The quake split the ocean floor northwards from
Sumatra along 1200 kilometres, twice as long as
previously thought, according to their research,
which appears in today’s issue of the journal Nature.
The event released so much strain along this
particular part of the fault that in theory there should
be no quake of similar magnitude, or a similar
tsunami, there for another 400 years, say US
geologists Professor Seth Stein and Professor Emile
Okal of Northwestern University, Illinois.
But further south, it is a different picture.
The scientists, who wrote before last Monday’s
quake that also struck western Sumatra, warn with
uncanny prescience that "a great earthquake" with
the potential to generate a large tsunami remained a
threat south of the 26 December site.
The 26 December earthquake
that caused the killer tsunami
was the second most powerful
ever, with a magnitude of 9.3
not 9.0 (Image: NOAA)
The December quake occurred off northwestern Sumatra, at the nexus where the
Indian plate of the Earth’s crust is sliding under a tongue-shaped sliver called the
Burma microplate.
It was initially thought to be 9.0 on the Richter scale.
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But an evaluation of very low frequency data from seismograms shows that the
quake was in fact 9.3 magnitude.
As the Richter scale is logarithmic, the difference between 9.3 and 9.0 is 2.5 times,
the study says.
"Conventional methods used to assess earthquake size dramatically underestimated
it," the study says.
"This makes the Indonesian earthquake the second largest ever to be instrumentally
recorded."
Only one measured quake has been bigger: a 9.5 event that struck Chile in 1960. But
that quake caused far less damage when compared with 26 December, largely
because the grinding plates in east Asia meet at an unusually oblique angle, causing
the energy to propagate northwards along a weakened fault.
What happened on the ocean floor?
Research led by Chinese seismologist Ni Sidao of the University of Science and
Technology at Hefei, Anhui province, sheds dramatic light on what happened on the
floor of the eastern Indian Ocean.
Their computer model, also published in Nature, suggests that the quake delivered a
high-frequency shock that lasted a stunning 500 seconds, compared with 340 seconds
for the Chile event in 1960.
From Indonesia to just south of Myanmar, the ocean ruptured at 2.5 kilometres per
second in an arc measuring 1200 kilometres.
The rip occurred along the so-called Sunda megathrust, the great tectonic frontier
along which the Australian and Indian plates begin their descent beneath Southeast
Asia.
Stein and Okal also estimate a rupture of 1200 kilometres, and say that this figure
would explain why Sri Lanka and southern India were so badly hit by the tsunami.
The reason: the biggest waves that struck their shores came not from the quake site
off Sumatra, to the southeast, but from the thrust of the ocean floor to the east.
"Tsunami amplitudes are largest when perpendicular to the fault," Stein and Okal
note.
The pair estimate the rupture to be some 11 metres deep and 200 kilometres wide.
Although the bust occurred at vicious speed, its slip was insidiously slow, a
phenomenon that proved to be ideal conditions for a tsunami.
The March quake also occurred west of Sumatra, to the south of the December
disaster, on the same plate boundary.
Seismologists had said that the energy transmitted by the December quake added to
tension in this region and fuelled the likelihood of an imminent, very big shock.
There was no major tsunami in the March quake, probably because the shock was
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much smaller and occurred beneath the ocean floor.
Related Stories
Deep quake saves Asia from second tsunami, News in Science 30 Mar 2005
Repeat Indian Ocean quake likely – study, News in Science 17 Mar 2005
Old records show tsunamis were common, News in Science 1 Feb 2005
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The New York Times > AP > Science > Scientists Reexamine De...
http://www.nytimes.com/aponline/science/AP-Tsunami-Earthquak...
March 31, 2005
Scientists Reexamine December Earthquake
By THE ASSOCIATED PRESS
Filed at 5:49 a.m. ET
DENVER (AP) -- Researchers at a U.S. university have recrunched data from
December’s devastating earthquake in Sumatra and found it was even more
powerful than previously believed, but other scientists suggest their findings are
premature.
A study by seismologists at Northwestern University increases the intensity of
the Dec. 26 earthquake from a magnitude 9.0 to a whopping magnitude 9.3.
That’s about three times more powerful than initial estimates.
However, the new magnitude has yet to be accepted by the U.S. Geological
Survey -- regarded as one of the world’s leading authorities for such information
-- and the international earthquake research community. Several other studies to
recalibrate the earthquake’s magnitude aren’t completed yet.
If the Northwestern calculations are later confirmed, it would make the undersea
Sumatra-Andaman earthquake the second-most powerful ever recorded. The
quake and the tsunami waves it generated already are considered to be among the
world’s worst natural disasters with an estimated 300,000 people killed or
missing.
‘‘There is currently no firm consensus on the ’correct’ magnitude of this
earthquake,’’ although it is likely to change from 9.0, said geophysicist Stuart
Sipkin of the USGS Earthquake Information Center in Golden, Colo.
Details of the Northwestern recalibration appear in Thursday’s issue of the
journal Nature.
In a second Nature study, Chinese researchers recalculated the rupture of the
Sunda Trench where the earthquake occurred off the west coast of Sumatra.
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By analyzing high-frequency energy radiation released by the earthquake,
geophysicists at the University of Science and Technology at Hefei concluded
the rupture extended northward along the fault for 720 miles (1,200 kilometers),
or nearly twice as long as initial assessments.
Other scientists said the new, bigger numbers -- both the quake size and the
rupture length -- could be good news for Indonesia and the rest of the region. The
more energy an earthquake releases, the more it reduces the danger of another
catastrophic event along the same segment of a fault, perhaps for hundreds of
years.
But it doesn’t mean that other segments won’t slip with devastating
consequences -- as seen in Monday’s magnitude 8.7 earthquake in another
segment of the Sunda Trench to the south off Nias Island.
Seismologists believe the Sunda’s southern stretch was in the most immediate
danger of rupturing within weeks of the Dec. 26 quake, and Monday’s
earthquake was virtually coupled to the larger quake three months ago.
Sumatra is one of the world’s most seismically active regions. Off its west coast
lies a subduction zone where plates of the Earth’s crust grind and dive,
generating great disturbances of the earth and seas.
Scientists say even greater uncertainty persists in adjacent faults extending
northward to Mynamar, Bangladesh and India, where hundreds of millions of
people live in the shadow of the Himalayas.
‘‘Long sections of the enormous thrust fault along which India is diving down
beneath the Himalayas have not failed for centuries,’’ said California Institute of
Technology seismologist Kerry Sieh, ‘‘and they are only one to three fault
lengths away from the 2004 rupture.’’
Researchers commonly revise earthquake assessments as more data becomes
available from instruments around the world. So it’s normal for scientists to
revise their take on the December event.
But Northwestern seismologists Seth Stein and Emile Okal said they were
concerned that conventional surface seismic measurements ‘‘dramatically
underestimated’’ the size of the earthquake.
This earthquake broke slowly as it ruptured northward along the Sunda Trench,
so they recalculated the new magnitude after analyzing the very low-frequency
seismological signal generated by the quake.
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USGS officials said magnitude 9.0 quakes are so rare that no others have been
measured using the same method and scientists don’t know whether other giant
earthquakes behave the same way.
For example, Sipkin said other measurements show the magnitude 9.2 Alaska
earthquake in 1964 was larger than the Sumatra quake. But slow-slip
measurements did not exist then, and comparing the different data from the two
earthquakes now is ‘‘like comparing apples and oranges,’’ he said.
‘‘Changing the magnitude of the Sumatra earthquake to 9.3 would not only be
premature at this time,’’ Sipkin said, ‘‘but would place it in the wrong position
on the list of largest earthquakes.’’
Copyright 2005 The Associated Press
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04/01/05 10:10
MSNBC - December quake was longest on record
http://msnbc.msn.com/id/7338077/print/1/displaymode/1098/
MSNBC.com
December quake was longest on record
Rupture that triggered tsunami extended over 750 miles
Reuters
Updated: 5:27 p.m. ET March 30, 2005
LONDON - The earthquake that triggered the devastating Asian tsunami was the longest on
record — extending over 750 miles (1,200 km) —but poses little further immediate danger,
scientists said on Wednesday.
However, large earthquakes and possible giant tidal waves are a threat on the segments of
the tectonic plate boundary to the south, they wrote in the science journal Nature in papers
penned well before Monday’s major quake in the danger zone.
The Dec. 26 subsea earthquake off northern Indonesia measuring 9.0 on the open ended
Richter scale triggered a tidal wave that swept across the Indian Ocean spreading death
and devastation from Sri Lanka to Thailand.
Three months after the disaster, nearly 300,000 people are still missing or known to be
dead, and more than one million are homeless.
The two groups of scientists working independently at Northwestern University in the United
States and the University of Science and Technology of China both reached the same
conclusions on the size of the rupture.
They said the size of the quake was far bigger than previously estimated and had the odd
characteristic of having had a long, slow slip in the northern end of the giant fault followed
by a sharp rapid move in the southern section.
"We determine the rupture length to be 1,200 km -- the longest ever recorded," wrote the
team from Anhui in China, noting that the quake lasted more than eight minutes at its peak
power.
The team from Illinois said the odd characteristics of the earthquake and its extent made a
repeat unlikely in the near future.
"Strain accumulated on the northern part of the rupture has been released. There is
therefore no immediate threat of an oceanwide tsunami being generated ... because such
earthquakes should be at least 400 years apart," they wrote.
"However, the danger of a large tsunami resulting from a great earthquake on segments to
the south remains," they added.
The article appeared just two days after the major earthquake measuring 8.7 struck some
200 km south of the epicenter of the December quake.
Although there was no repeat of the giant tsunami, the March 28 earthquake killed an
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MSNBC - December quake was longest on record
http://msnbc.msn.com/id/7338077/print/1/displaymode/1098/
estimated 1,000 people and devastated coastal communities in northern Indonesia still
struggling to rebuild three months after the Boxing Day wave.
© Reuters 2005. All rights reserved. Republication or redistribution of Reuters content,
including by caching, framing or similar means, is expressly prohibited without the prior
written consent of Reuters. Reuters and the Reuters sphere logo are registered trademarks
and trademarks of the Reuters group of companies around the world.
URL: http://msnbc.msn.com/id/7338077/
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ScientificAmerican.com
March 31, 2005
Reanalysis Reveals Tsunami-Spawning Quake to Be Second Largest
Known
The death toll from Monday's 8.7 on the Richter earthquake near Indonesia reached 1,000 on Wednesday
as additional tremors measuring up to 6.4 on the Richter scale continued to shake the region,
demonstrating the uncertainty and unpredictability of geologic activity. Indeed, scientists are still
uncovering details about the devastating earthquake that triggered the deadly tsunami in the vicinity in
December. A new analysis published today in the journal Nature indicates that it released more than twice
as much energy as previously believed. If the results are confirmed, it would make that earthquake the
second largest on record, behind a 1960 earthquake in Chile.
Seth Stein and Emile A. Okal of Northwestern University analyzed the low-frequency seismological signal
that the December 26, 2004, quake generated. They determined that it was 2.5 times stronger than initial
estimates and measured 9.3 on the Richter scale. Scientists based the initial 9.0 magnitude estimate on
surface waves. The authors posit that slow slip between the plates that was not detectable in the surface
waves accounts for their larger measurement. A second analysis published in the same issue of Nature
indicates that the December earthquake also involved a longer section of the fault than previously
believed. Sidao Ni of the University of Science and Technology of China and colleagues determined that
the Sumatra-Andaman quake resulted in a rupture length of 1,200 kilometers--about twice as long as had
been inferred from analyses performed soon after the event.
Scientists will continue to try to piece together just how and why the earth moves below the Bay of Bengal
region. Kerry Sieh of the California Institute of Technology comments in an accompanying commentary
that "over the next year or two, figuring out what happened will be a showcase both of what modern
observations and analysis can do and of the multidisciplinary nature of modern earthquake science." -Sarah Graham
© 1996-2005 Scientific American, Inc. All rights reserved.
Reproduction in whole or in part without permission is prohibited.
Dec. 26 quake miscalculated, scientists say
Source of giant tsunamis may have been 2½ times more powerful than reported
BY ESTANISLAO OZIEWICZ
THURSDAY, MARCH 31, 2005 PAGE A3
The Dec. 26 earthquake that caused the Indian Ocean killer tsunamis was at least twice as powerful as
originally thought, making it the second-biggest quake ever recorded, according to two U.S. researchers.
The quake ruptured the ocean floor northward from Sumatra along a line extending 1,200 kilometres -twice as long as previously believed, according to a study by seismologists at Chicago's Northwestern
University. And they have assigned the temblor an intensity of magnitude-9.3 -- 2.5 times greater than
initial reports, which estimated it at approximately 9.0.
Their research, along with another study by Chinese scientists, appears in today's edition of the British
science journal Nature. They are among the first published analyses of the Sumatra quake, which
unleashed tsunamis killing an estimated 300,000 people.
Northwestern's Seth Stein and Emile Okal say their non-standard evaluations of extremely long-period, or
"slow-slip," seismic waves shows the devastating quake had a magnitude of 9.3. (Because the scale is
logarithmic, the increase represents a 2.5-times size difference.) Only one measured quake has been
bigger, a 9.5-magnitude whopper that hit Chile in 1960.
However, the U.S. Geological Survey, a world-leading authority on earthquakes, is waiting for an
agreement among scientists to emerge before making its own pronouncement on the magnitude of the
Boxing Day quake.
"The Stein and Okal results are using a non-standard technique. It doesn't mean they're wrong, but it is
non-standard," USGS geophysicist Stuart Sipkin cautioned in an interview. "It doesn't seem yet that
there's a consensus in the scientific community about what the result should be."
Dr. Sipkin also said it is difficult to compare today's earthquakes to "historical" ones, such as the one
measured in Chile, because the data were collected differently.
"We need to make sure that when we do that, we are comparing apples to apples," he said.
In the other Nature study, geophysicists at the University of Science and Technology at Hefei, China, also
concluded the rupture length was 1,200 kilometres, running from the coast of the Indonesian province of
Aceh to the Andaman Islands.
They also say the shaking lasted a remarkable 500 seconds, substantially longer than the 340-second
Chilean quake.
Dr. Stein and Dr. Okal estimate the rupture to be 11 metres deep and 200 kilometres wide and may
explain why Sri Lanka and India were so badly hit by the "excited" tsunamis. The biggest waves that hit
them came not from the quake site itself, to the southeast, but from the thrust of the ocean floor to the
east.
"Tsunami amplitudes are largest when perpendicular to the fault," they wrote.
The scientists determined that the enormous strain accumulated on the northern part of the rupture has
been released, saying there appears to be no immediate threat of an oceanwide tsunami on this segment
because such great earthquakes are typically at least 400 years apart.
But it's another story on the segments to the south, where the danger of a large tsunami remains.
Dr. Stein said in a telephone interview yesterday that this week's 8.7-magnitude earthquake off Sumatra is
consistent with his findings, although it did not result in a giant tsunami.
Other scientists say dangers lie in adjacent faults extending north to Myanmar, Bangladesh and India,
where hundreds of millions of people have little access to emergency communication and services.
Kerry Sieh, of the Tectonic Observatory at the California Institute of Technology, warns in the same issue
of Nature that since big earthquakes often occur in clusters, the time may be ripe for a rupture beneath the
Himalayas.
"Because many of the giant faults in the Aceh-Andaman neighbourhood have been dormant for a very
long time, it is quite plausible that the recent giant earthquake and tsunami may not be the only disastrous
21st-century manifestation of the Indian plate's unsteady tectonic journey northward," he said.
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