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Sunday 22nd May 2005: BBC 2. 21:00hrs
Horizon programme: The next megaquake.
On Boxing Day 2004 the world was shocked by one of the worst natural disasters of all time. The cause of so much devastation was the most powerful kind of earthquake on the planet - a megathrust. Megathrust earthquake only occur on a particular kind of fault. Scientists have now discovered that just such a fault could cause a huge megathrust earthquake and tsunami right off the coast of North America.
The subduction zones
The surface of the earth is divided into giant plates of rock - and most earthquakes occur at faults where two of the plates meet. Where the plates are colliding one of the plates usually gets pushed down under the other - this is subduction.
Not surprisingly, the process of subduction can be very violent. The two plates can get stuck together and the result is that the area where subduction is occurring (the subduction zone) gets compressed. Eventually the strain on the fault becomes too much. The plates suddenly slip past each other. The result is a megathrust earthquake.
Subduction zones are mainly found in South East Asia (like the subduction zone that caused the Indian Ocean Tsunami) and around the Pacific Rim. It had long been known that a subduction zone runs along the Pacific north-west coast, from northern California all the way to Vancouver Island in Canada. It's called the Cascadia Subduction Zone, and is linked to the Cascade Range of volcanoes that includes Mt St Helens.
Quiet in Cascadia
No one, however, thought that this area was at significant risk from earthquakes, largely because there was no historical record of large earthquakes there. The first suggestion of a problem came when plans were drawn up to build a number of nuclear power stations near the Washington Coast.
Tom Heaton, a geophysicist and engineer from the California Institute of Technology, was brought in to examine the proposal from a geohazards perspective. Heaton pointed out the possibility that the Cascadia Subduction Zone might be capable of producing a megathrust earthquake.
The lack of historical record might be explained if the last such earthquake had occurred before Europeans arrived in the region during the 18th century. Heaton pointed to the existence of Native American legends that might be describing a megathrust earthquake that had happened before written records began. In the end, the nuclear power station project ran out of money and the reactors were never completed. Heaton's concerns were still just theoretical - there was no scientific evidence that such earthquakes had actually occurred.
When Brian Atwater, a specialist in marshes and estuaries, heard about Heaton's theory he decided to take a look himself. He started investigating in the very areas where the Native Legends had been recorded. He found evidence that some time in the past there had been a sudden change in land level. The coast had dropped down, drowning forests under layers of mud. Other geologists soon found similar evidence all along the Pacific Northwest Coast. The simplest explanation was that there had been a huge megathrust earthquake in the past. This evidence, however, still wasn't enough to convince.
The orphan tsunami
The most intriguing piece of evidence came from Japan. As the Indonesian earthquake has shown, megathrust earthquakes can create tsunamis capable of crossing entire oceans. If there had been a giant earthquake in Cascadia it should have sent a tsunami across to Japan. So Japanese geologist Kenji Satake looked in old Japanese texts for any record of such a tsunami.
What he was looking for was an "orphan tsunami", a wave the comes out of nowhere, with no local earthquake recorded. Satake and his colleagues found several such records from January 1700. The final proof came when scientists in America were able to date the land level change there by looking at the tree rings of drowned red cedar trees. They found that entire forests had been killed in the winter of 1699/1700, matching the Japanese records perfectly.
This evidence enabled scientists to put together a picture of the last Cascadia megathrust earthquake. It occurred around 9pm on 26 January 1700, and would have had a devastating effect on over 600 miles of coast. It's little wonder that the Native people of the region passed on legends of that event that survive to this day.
A deadly warning
Before the 1700 event was discovered, the people of the Pacific north-west would have had little awareness of the threat of earthquakes and even less of tsunamis. Now that is starting to change. Tsunami evacuation signs are being installed along the coast. The region's building codes are now some of the strictest in the world. All new buildings are designed to withstand a powerful earthquake.
However, building to survive a megathrust earthquake is a major challenge. These earthquakes tend to last several minutes - much longer than other quakes - and there is little knowledge about how modern buildings will react to such shaking.
As the events of Boxing Day 2004 showed, the tsunami unleashed by the sudden movement of the sea floor can be even more devastating than the earthquake. Unlike the Indian Ocean, the Pacific has a warning system that should give the other nations of the Pacific Rim many hours of warning.
On the Pacific north-west coast, however, a tsunami from Cascadia will arrive in some places in under half an hour. In such regions the key to saving lives is education. People there have to know that if they feel an earthquake they should move inland and to high ground. This knowledge could save many lives.
The terrible events of 26 December are a warning to the world that we should be better prepared for these huge geological catastrophes. This message has particular significance for the people of the Pacific north-west. One day that region will be hit by a megathrust that will probably be very similar to the Indonesian earthquake. At least, thanks to the work of many scientists, they have been forewarned.
Programme transcript: www.bbc.co.uk/sn/tvradio/..._trans.shtml
Horizon programme: The next megaquake.
On Boxing Day 2004 the world was shocked by one of the worst natural disasters of all time. The cause of so much devastation was the most powerful kind of earthquake on the planet - a megathrust. Megathrust earthquake only occur on a particular kind of fault. Scientists have now discovered that just such a fault could cause a huge megathrust earthquake and tsunami right off the coast of North America.
The subduction zones
The surface of the earth is divided into giant plates of rock - and most earthquakes occur at faults where two of the plates meet. Where the plates are colliding one of the plates usually gets pushed down under the other - this is subduction.
Not surprisingly, the process of subduction can be very violent. The two plates can get stuck together and the result is that the area where subduction is occurring (the subduction zone) gets compressed. Eventually the strain on the fault becomes too much. The plates suddenly slip past each other. The result is a megathrust earthquake.
Subduction zones are mainly found in South East Asia (like the subduction zone that caused the Indian Ocean Tsunami) and around the Pacific Rim. It had long been known that a subduction zone runs along the Pacific north-west coast, from northern California all the way to Vancouver Island in Canada. It's called the Cascadia Subduction Zone, and is linked to the Cascade Range of volcanoes that includes Mt St Helens.
Quiet in Cascadia
No one, however, thought that this area was at significant risk from earthquakes, largely because there was no historical record of large earthquakes there. The first suggestion of a problem came when plans were drawn up to build a number of nuclear power stations near the Washington Coast.
Tom Heaton, a geophysicist and engineer from the California Institute of Technology, was brought in to examine the proposal from a geohazards perspective. Heaton pointed out the possibility that the Cascadia Subduction Zone might be capable of producing a megathrust earthquake.
The lack of historical record might be explained if the last such earthquake had occurred before Europeans arrived in the region during the 18th century. Heaton pointed to the existence of Native American legends that might be describing a megathrust earthquake that had happened before written records began. In the end, the nuclear power station project ran out of money and the reactors were never completed. Heaton's concerns were still just theoretical - there was no scientific evidence that such earthquakes had actually occurred.
When Brian Atwater, a specialist in marshes and estuaries, heard about Heaton's theory he decided to take a look himself. He started investigating in the very areas where the Native Legends had been recorded. He found evidence that some time in the past there had been a sudden change in land level. The coast had dropped down, drowning forests under layers of mud. Other geologists soon found similar evidence all along the Pacific Northwest Coast. The simplest explanation was that there had been a huge megathrust earthquake in the past. This evidence, however, still wasn't enough to convince.
The orphan tsunami
The most intriguing piece of evidence came from Japan. As the Indonesian earthquake has shown, megathrust earthquakes can create tsunamis capable of crossing entire oceans. If there had been a giant earthquake in Cascadia it should have sent a tsunami across to Japan. So Japanese geologist Kenji Satake looked in old Japanese texts for any record of such a tsunami.
What he was looking for was an "orphan tsunami", a wave the comes out of nowhere, with no local earthquake recorded. Satake and his colleagues found several such records from January 1700. The final proof came when scientists in America were able to date the land level change there by looking at the tree rings of drowned red cedar trees. They found that entire forests had been killed in the winter of 1699/1700, matching the Japanese records perfectly.
This evidence enabled scientists to put together a picture of the last Cascadia megathrust earthquake. It occurred around 9pm on 26 January 1700, and would have had a devastating effect on over 600 miles of coast. It's little wonder that the Native people of the region passed on legends of that event that survive to this day.
A deadly warning
Before the 1700 event was discovered, the people of the Pacific north-west would have had little awareness of the threat of earthquakes and even less of tsunamis. Now that is starting to change. Tsunami evacuation signs are being installed along the coast. The region's building codes are now some of the strictest in the world. All new buildings are designed to withstand a powerful earthquake.
However, building to survive a megathrust earthquake is a major challenge. These earthquakes tend to last several minutes - much longer than other quakes - and there is little knowledge about how modern buildings will react to such shaking.
As the events of Boxing Day 2004 showed, the tsunami unleashed by the sudden movement of the sea floor can be even more devastating than the earthquake. Unlike the Indian Ocean, the Pacific has a warning system that should give the other nations of the Pacific Rim many hours of warning.
On the Pacific north-west coast, however, a tsunami from Cascadia will arrive in some places in under half an hour. In such regions the key to saving lives is education. People there have to know that if they feel an earthquake they should move inland and to high ground. This knowledge could save many lives.
The terrible events of 26 December are a warning to the world that we should be better prepared for these huge geological catastrophes. This message has particular significance for the people of the Pacific north-west. One day that region will be hit by a megathrust that will probably be very similar to the Indonesian earthquake. At least, thanks to the work of many scientists, they have been forewarned.
Programme transcript: www.bbc.co.uk/sn/tvradio/..._trans.shtml
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Re: The Next Megaquake?
Sat, May 21, 2005 - 5:10 PMI watched a program a few weeks back on "discovery channel" or some such, about this very thing. They focused quite a bit on Seattle and that area....how unprepared for an earthquake (let alone this type) that region is.
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Re: The Next Megaquake?
Mon, March 10, 2008 - 2:09 AMMega-quakes rare but can occur anywhere
Monday, 10 March 2008 Larry O'Hanlon
Discovery News
We can expect an average three catastrophic, magnitude 9 or greater earthquakes around the world each century, according to a new study.
The estimate may sound strange as there have been five such quakes in the past half century, a cluster that includes the deadly 2004 rupture offshore of northern Sumatra.
The good news is that the new estimate is lower than previous ones.
But there is a dark side to the new study, which is published in the March issue of the journal Geology.
It removes certain limits on where such quakes can occur.
Now it looks like any place where one tectonic plate is being actively shoved under another, at a subduction zone, can experience a catastrophic magnitude 9 earthquake.
Previously, different subduction zones were thought to pose greater or lesser risks of magnitude 9 quakes, depending on the speeds at which the plates were colliding, and their stiffness and age.
That popular hypothesis evaporated on 26 December 2004.
"The Sumatra-Andaman earthquake happened in one of the least likely places," says seismologist Dr Robert McCaffrey of New Zealand's government research agency GNS Science.
The hypothesis failed. "It was quite humbling," he says.
What's the chance?McCaffrey decided to re-examine the risks of the largest earthquakes without biasing his simulation with the known recent history of giant earthquakes or with the failed theory that was supposed to predict the danger of any given subduction zone.
When he ran his simulation, it popped out the three-per-century figure, as well as the new possibility that any subduction zone posed a danger for these mega-uakes, albeit not a very frequent danger.
As for why McCaffrey's model came up with fewer magnitude 9 quakes than have been seen in the past 50 years, that's just a matter of the sample size, he says.
"One of the analogies is that you go into a friend's house and their baby is crying the entire 10 minutes you are there," McCaffrey says.
You leave with the impression that the baby cries all the time. However, it might also be that you just happened to see the baby's only fit all day. The same goes for watching really big, rare earthquakes.
"We've been just watching this process that has thousands of years in a cycle," says McCaffrey.
Yet the seismograph was only invented about 100 years ago, and historical records noting magnitude 9 earthquakes only extend back about 300 years, at best.
So whatever patterns we see are not likely to represent a long-term average.
The next tsunami"His argument is, anywhere you have a subduction zone you can't exclude the possibility," says seismologist Professor Seth Stein of Northwestern University.
"What that means is when you think of giant ocean tsunamis, you can't count on where they will come from."
In other words, says Stein, McCaffrey's paper is bad news for tsunami emergency planners because it increases the number of disaster possibilities.
www.abc.net.au/science/ar...2184897.htm
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Re: The Next Megaquake?
Sun, August 16, 2009 - 2:10 PMNew data: Mega-quake could strike near Seattle
August 16th, 2009 By Les Blumenthal Using sophisticated seismometers and GPS devices, scientists have been able to track minute movements along two massive tectonic plates colliding 25 miles or so underneath Washington state's Puget Sound basin. Their early findings suggest that a mega-earthquake could strike closer to the Seattle-Tacoma area, home to some 3.6 million people, than was thought earlier.
The deep tremors, which humans can't feel, occur routinely every 15 months or so and can continue for more than two weeks before they die back to undetectable levels.
The instruments are detecting an inch or two of movement -- known as "episodic tremor and slip" -- as the Juan de Fuca plate grinds and sinks beneath the North American plate. Closer to the surface, the two plates are locked together. When they snap, scientists say, it could produce a massive 9.0 or greater earthquake and a tsunami.
By comparison, the largest earthquake ever recorded was 9.5 on the Richter scale, in Chile in 1960. The largest in North America was the 9.2 Great Alaska Earthquake in 1964, which killed nine people and spawned a tsunami that struck the Northwest coast. The 1906 San Francisco earthquake, which killed 750 to 2,500 people, was estimated to be an 8.2.
Whereas the scientists once predicted that a mega-earthquake would be centered just off the Northwest coast, now -- using data from the tremors research -- they say that it could be 30 miles or more inland, under the Olympic Peninsula, which lies to the west of Seattle and Tacoma across Puget Sound.
"The closer you are to the source, the stronger the shaking," said Steve Malone, a research professor emeritus at the University of Washington.
Exactly how much stronger, and how much more damage such a quake would cause in the Puget Sound area, hasn't been calculated, Malone said.
While there's still plenty of debate about the findings within the scientific community, and while they may not be consistent with the models that geologists have developed, state officials are aware of the latest studies.
"People are aware of the possibility," said John Vidale, a professor of geophysics at the University of Washington and the state's seismologist. "We haven't exactly calculated the impact, but bringing the fault closer (to metropolitan areas) could increase the shaking."
Scientists have spent years studying what's known as the Cascadia subduction zone, an area where the two tectonic plants collide that stretches roughly 600 miles off the coast of Northern California to southern British Columbia, Canada.
As the Juan de Fuca plate slides under the North American plate, the two can become locked. When plates become locked, pressure builds. The pressure is released in what scientists call a mega-thrust earthquake, which easily can be magnitude 9.0. The Sumatra-Andaman Islands earthquake the day after Christmas in 2004 was a 9.2 mega-thrust quake that produced a devastating Indian Ocean tsunami that killed an estimated 233,000 people in 11 countries.
The last mega-thrust earthquake along the Cascadia subduction zone, estimated at 9.2, was in January 1700. It produced a tsunami that reached Japan. Cascadia subduction zone mega-thrust earthquakes happen on average every 400 to 500 years, but they can happen as little as 300 years apart or as much as 800.
A mega-thrust earthquake would be different from those that shake the Northwest occasionally. A mega-thrust quake occurs right on the boundary of two tectonic plates, while other earthquakes occur along cracks in the plate. Vidale likened what's going on beneath the Earth's crust to a bunch of blocks jostling around. Where the smaller blocks collide, you can have more standard-type quakes. Where the biggest blocks, the tectonic plates, collide, you have a mega-thrust earthquake.
Since the deep tremors were first detected 15 years ago, scientists have been trying to determine what was causing them along the Cascadia subduction zone. Eventually, they concluded that the tremors reflected the slippage of the Juan de Fuca plate under the North American plate.
"It's a burst of noise that can go on for up to 24 hours over a period of several weeks," said Herb Dragert, a geophysicist with the Geological Survey of Canada in Victoria, British Columbia, who was among those who first developed the theory.
What's unique about the deep tremors, which occur in an area stretching roughly from Olympia, Wash., to Canada's Vancouver Island, is that they reappear about every 15 months. While tremors have been detected elsewhere along the Cascadia subduction zone, none is as regular or as prolonged as those in the Puget Sound basin, Dragert said.
"Every 15 months it's like tightening the guitar string a little more," Dragert said. "You don't know whether it will take it beyond the break zone."
According to the timetable, episodic tremor and slip should be going on just about now. Instead, it came last spring, catching scientists by surprise. Malone said that some tremors were detected southwest of Olympia last week, but that it was too soon to determine whether they were part of a new episode or just isolated ones.
If all the energy associated with tremors over two weeks were released in 10 seconds, Vidale said, it would equal a 7.0 earthquake.
Similar deep tremors are being tracked at other subduction zones around the globe, including in Alaska, Japan, Mexico and Chile.
ON THE WEB
Pacific Northwest Seismic Network: www.ess.washington.edu/SEIS/PNSN
U.S. Geological Survey: www.usgs.gov
Geological Survey of Canada: tinyurl.com/5n97nf
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Re: The Next Megaquake?
Sat, October 31, 2009 - 7:13 PMAre they sure that the slow slips are not happening in place of big quakes? They could be lessening the pressure too, couldn't they?
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