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Sumatra
quake reveals new seismic information
The undersea earthquake off the Indonesian
island of Sumatra on Wednesday was the largest earthquake of its type ever
recorded and has increased the risk of more powerful quakes in the region, a
leading expert has said.
Director
of the Earth Observatory of Singapore, Kerry Sieh, told CNN that the 8.6
Richter scale undersea earthquake was the largest "strike-slip" quake
- steep structures where the two sides of the fault slip horizontally past each
other - ever recorded.
"Before
that we thought that 8.1 was as big as they get, but this 8.6 quake was
phenomenal," Sieh said. "It has been absolutely jaw-dropping and has caused a lot of foment among seismologists."
6.3-quake hits off Indonesia's Sumatra: USGS
AFP | Sep 14, 2012,
11.50AM IST
JAKARTA: A 6.3-magnitude quake struck off Indonesia's Sumatra island
today, the US Geological Survey said, but no tsunami warning was issued and
there were no immediate reports of damage or injuries.
The epicentre of the quake, which struck at 11:51 am (0451 GMT) at a depth of 25 kilometres, was 190 kilometres northwest of the town of Bengkulu.
Indonesia's Meteorology, Climatology and Geophysics Agency measured the magnitude at 6.1.
"There are no reports of damage so far. The quake happened in the sea close to Mentawai island, but no tsunami warning has been issued," the agency's duty officer, Koko Widyatmoko, told AFP.
Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.
The epicentre of the quake, which struck at 11:51 am (0451 GMT) at a depth of 25 kilometres, was 190 kilometres northwest of the town of Bengkulu.
Indonesia's Meteorology, Climatology and Geophysics Agency measured the magnitude at 6.1.
"There are no reports of damage so far. The quake happened in the sea close to Mentawai island, but no tsunami warning has been issued," the agency's duty officer, Koko Widyatmoko, told AFP.
Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.
SUMATRA IN PERSPECTIVE
As the tsunami
warnings are being issued by the Indonesian government on Wednesday following a
massive 8.7 magnitude earthquake, the coastal regions across the Indian Ocean
are evacuated. As the authorities certainly believe the earthquake has the
potential to cause a huge devastation, here is a good reason why.
Take a look
at some of the deadliest tsunamis which were triggered by similar earthquakes
in the past.
27 August
1883 - The tsunami
was caused by the eruptions from the Krakatoa volcano. The volcanic eruption in
turn triggered a deadly tsunami which killed 36,000 people in the
Indonesian islands of western java and southern Sumatra. The killer waves
washed ashore coral blocks as large as 600 tons.
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More than a third (37%) see the
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and water scarcity as a real and present danger, up from just 10% two years
ago, says the latest Carbon Disclosure Project (CDP) survey of top global
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Four-fifths (81%) identify climate
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Of the 379 of the 500 companies
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Historical
Tsunami Generation from the 2004 M=9.2 Sumatra-Andaman
Earthquake
Tectonics of Sumatra-Andaman Islands
Indonesia's Sumatra alive to quake and tsunami risks
There is no doubting
the people of Sumatra are alive to the risks of earthquakes and the possibility
of a tsunami.
News video of panic
and shock emerging from Banda Aceh, the provincial capital and largest city in
the island's northern province, spoke volumes.
Wednesday's major tremor (M8.6) occurred in a similar location, although much
further offshore (some 400km), at 15:38 local time (08:38 GMT). It was followed
by an aftershock of M8.2 just a couple of hours later.
Tsunami alerts were
issued for the entire Indian Ocean basin but, mercifully, the scale of the wave
action appears to have been small - on the order of a few tens of cm in height.
"Our tide gauges
and buoys recorded small tsunamis," Said Kristiawan, of Indonesia's
Meteorology and Geophysics Agency, said.
Underwater
landslide
Geologists reported
quickly that the quake was "predominantly strike-slip" - that is to
say, the movement of rock at the site of fracture was horizontal in nature.
"Tsunami can be
caused in a number of ways, but typically it is where the seafloor can be moved
vertically, and that displaces a large amount of water that travels outward
from that source toward land," observed Bruce Pressgrave, from the US
Geological Survey (USGS).
"One of the ways
that strike-slip earthquakes can cause tsunami is if the shaking itself causes
some kind of underwater landslide that then produces the movement in the water
column," he told BBC News.
Nonetheless, the
alerts stayed in place for several hours as the authorities attempted to get on
top of the latest information, and there is now a lot more of it.
This is one of the big
changes since 2004. There is now a tsunami monitoring system dedicated to the
Indian Ocean, put in place through the leadership of [UN scientific agency]
Unesco in 2006.
Pressure sensors on
the ocean floor detect anomalous behaviour in the water column and signal that
information to surface buoys, which then relay the data, via satellite, to
onshore control centres.
The system is much
needed, particularly in Indonesia. Its Sumatra island lies close to an active
subduction zone, where the Indian-Australian tectonic plate presses into and
under the Eurasian plate.
This monumental
collision is evident on the ocean floor by a huge depression known as the Sunda
Trench.
The slab of cold,
dense rock that descends into the Earth at this point gets stuck, and strain
builds up that has to be released at some stage in the form of an earthquake.
More
aftershocks
But what was key to
Wednesday's outcome was that the main event occurred some distance to the west
of the Sunda Trench, and so did not produce the very big megathrust action
capable of deforming the seafloor in a way likely to generate large tsunamis.
"This is not one
of those; it was the seafloor moving horizontally - one part moving relative to
the other," explained Dr Richard Luckett, a seismologist with the British
Geological Survey.
"What we think
this is, is some kind of correction to do with all the massive earthquakes that
have happened in the Sunda trench in the last 10 years.
"And so this kind
of earthquake, although very big and widely felt, is much less likely to cause
a serious tsunami," he told the BBC.
"We're unlikely
to get another aftershock as big as the M8.2, although it can happen. But there
will be aftershocks - fives, sixes, maybe even sevens, going on for several
months."
To put these
magnitudes in context: one would expect about two or three quakes a year greater
than M8.0 to occur somewhere on Earth. On Wednesday, they had a pair of big
ones off the coast of Sumatra.
Already a curiosity for its sheer
size, the 8.6-magnitude earthquake that shook the seafloor west of the
Indonesian island of Sumatra on April 11 appears to have been even weirder than
scientists thought.
A new study reveals the quake zigzagged along four faults, three
of which are set perpendicular to each other. From above, the layout looks like
a city street grid. "We call it an earthquake in a maze," said
Lingsen Meng, lead author of the study and a graduate student in seismology at
Caltech.
"We were very excited to see this because an earthquake
this large, involving this complicated a fault system, does not happen very
often," Meng told OurAmazingPlanet. "This may be the only one I will
see in my lifetime."
The April Sumatra shaker was a strike-slip earthquake,
in which two parts of the Earth's crust slide past each other horizontally. The
quake was not only the 11th largest quakerecorded by seismometers, it's also
the largest strike-slip quake on the books. It's also one of the rare big intraplate earthquakes ; that is, it happened away from a plate
boundary, where two plates of the Earth's crust meet and where most of the world's largest earthquakes occur.
The results of the new study, detailed in tomorrow's (July 20)
issue of the journal Science, provide the first in-depth picture of the
rupture. The study also offers intriguing clues about the physics of extremely large earthquakes, and the
properties of oceanic plates. The observations could shed new light on
intraplate quakes and how tectonic plate boundaries evolve.
This earthquake "provides an exceptional opportunity to
probe the mechanical properties of Earth’s materials deep beneath the
oceans," Meng said. [ April 2012 Sumatra Quake
(Infographic) ]
Seismic back-tracking
To better understand how the Earth unzipped during this quake, Meng and his Caltech colleagues applied a technique called back-projection imaging to seismic waves recorded from the Sumatra quake. The imaging process is similar to how our brain uses sound to determine the position of moving objects.
To better understand how the Earth unzipped during this quake, Meng and his Caltech colleagues applied a technique called back-projection imaging to seismic waves recorded from the Sumatra quake. The imaging process is similar to how our brain uses sound to determine the position of moving objects.
Indonesia quake a record, risks for Aceh grow
(Reuters) - The powerful undersea earthquake off the
Indonesian island of Sumatra this week was a once in 2,000 years event, and
although it resulted in only a few deaths, it increases the risks of a killer
quake in the region, a leading seismologist said.
Wednesday's 8.6 magnitude quake and a powerful aftershock were
"strike-slip" quakes and the largest of that type recorded, Kerry
Sieh, director of the Earth Observatory of Singapore, told Reuters.
"It's a really an exceptionally large and rare event,"
he said.
"Besides it being the biggest strike-slip earthquake ever
recorded, the aftershock is the second biggest as far as we can tell,"
said Sieh, who has studied the seismically active, and deadly, fault zones
around Sumatra for years.
Strike-slip quakes involve the horizontal movement of colliding
earth plates, and are typically less powerful than those where there is
vertical movement. They are also less likely to trigger big tsunamis, or tidal
waves.
A magnitude 9.1 quake in roughly the same region on Boxing Day in
2004 decimated Aceh province on Sumatra and killed over 230,000 people in 13
countries around the Indian Ocean.
Sumatra, the westernmost island in the sprawling Indonesian
archipelago, has a history of powerful quakes and tsunamis, most triggered by
an offshore zone along its entire length, where the Indian-Australian tectonic
plate is forced under the Eurasian plate.
This creates a deep ocean trench as one plate slides under the
other at a rate of several centimeters a year. In this zone, called the Sunda
megathrust, stress builds up when the subducting Indian-Australian plate bends
the Eurasian plate like a spring board as it moves down into the Earth's crust.
Eventually enough stress builds up that the edge of Eurasian plate
suddenly jolts upward, triggering an earthquake. The sudden uplift of the
seafloor and huge pulse of seawater triggers a tsunami.
Over the centuries, repeated magnitude 8 and 9 quakes have struck
along portions of the megathrust zone off the coast of Sumatra, flattening
towns and killing thousands of people.
STING IN THE TALE
Wednesday's event was different, Sieh said, because it occurred
further west from the megathrust zone in a fault that runs north-south. This
strike-slip fault involved a sudden horizontal movement of the Indian and
Australian plates along hundreds of kilometers, preliminary data suggest.
Sieh said the Indian plate and Australian plate are moving
relative to each other horizontally at about 1 cm a year.
"If all of that ... is taken up on this one fault and if you
make some crude calculations about how much slip occurred during this
earthquake, say 20 meters. It means that this earthquake shouldn't happen more
than once every 2,000 years."
Wednesday's quake caused few casualties and triggered very small
waves, despite its magnitude. But the sting in the tale is that it likely to
have increased stress on the plate boundaries near Aceh, increasing the risks
of another major earthquake in the same area as the 2004 disaster.
In addition, research by Sieh and colleagues published in 2010
showed that the 2004 Aceh quake only relieved about half the stress that has
built up over the centuries along a 400 km portion of the megathrust faultline.
That makes another major quake in the area a matter of time.
Adding to concerns, further south along another 700 km portion of
the megathrust fault under the Mentawai islands, Sieh and colleagues in a
separate 2008 research said so much stress was building up on this section that
one or more major quakes were likely within years.
The Mentawai islands, a popular surfing destination, are a chain
of about 70 islands off the western coast of Sumatra. They face the city of
Padang on Sumatra, home to about one million people and likely to be in the
path of any tsunami that is triggered.
"I am very confident that we are very likely to have within
the next few decades to have this great Mentawai earthquake that will have a
magnitude at least as big as yesterday's," said Sieh.
And when it does, history shows there will be more than one quake
within a few years.
He said a magnitude 8.4 quake in 2007 that struck this part of the
megathrust relieved only a small portion of the pent-up pressure. The last time
it ruptured was a magnitude 9 quake in 1833 and an 8.4 quake in 1797.
"We've had so many big earthquakes around in Sumatra in the
past few years that it seems like an awful lot of the faults around there seem
ready to go."
Panic in
Indonesia as 8.6 quake hits off Sumatra (PHOTOS, VIDEO)
A massive quake off the
west coast of Indonesia sent fears across the country, prompting residents in
coastal cities to rush to higher ground. A tsunami that followed a similar
quake in 2004 killed nearly 230,000 people, most of them in Indonesia.
The Aceh region, which
lost over 170,000 people in the 2004 disaster, was paralyzed with traffic jams
soon after the first tremor on Wednesday, measured 8.6 magnitude.
Chaos reigned in streets
as many towns along the coast line were full of shoppers stocking up before the
traditional New Year on Friday. Locals poured from their homes screaming “God is great” as they
searched frantically for family members.
"Sirens and Quran recitals from mosques
are everywhere," said a spokesman for the Indonesia's
disaster management agency.
Patients ran out of
hospitals, some with drips still attached to their arms. In schools, children
were terrified as teachers started an evacuation. Timbang Pangaribuan told El
Shinta radio a guest of his hotel was injured as he jumped from the window of
his room.
A strong 8.2 aftershock
nearly three hours later sparked a new wave of panic. Electricity and telephone
lines were down in several areas. Even more people fled the coast after the
government issued a fresh tsunami warning.
"What did we do to deserve this? What sins have we committed? I'm so scared, I don't want
to lose my family again," cried Aisyah Husaini, clinging to
her two children in a mosque in Banda Aceh, where hundreds of people sheltered.
The woman, accoding to the Associated Press, lost both her parents and a son in
the 2004 tsunami.
Eventually, the tsunami
warning was lifted. The highest wave which so far has rolled onto the country's
coast is reported to be less than a meter high. But hours afterwards people
were still standing outside their homes and offices, scared to go back inside.
The big hits were
followed by over 20 minor shakes, up to five per hour, says the US Geological
Service. Later quakes were registered at 5.0 magnitude.
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