RandomGuy
03-30-2011, 09:55 AM
http://media.economist.com/sites/default/files/imagecache/original-size/20110319_STP507.jpg
HAVING spent half his adult life in seismically active regions of the world—first in Japan and more recently in California—your correspondent takes earthquake precautions seriously. He knows precisely what to do when the initial P-wave tinkles the wine glasses in the kitchen cabinet, providing precious seconds of warning before the more muscular S-wave comes barging across the threshold. Though he admits to having been unnerved more than once, he grabs his shoes and a torch (if it is dark), turns off the gas at the mains and opens the garage door. He goes back about his business if the shaking turns out to be 4 or less on the scale of 0 to 7 the Japanese have used for over a century to measure the local intensity of an earthquake.
Unlike the logarithmic “moment-magnitude” scale, which measures the energy released at an earthquake’s focus, the more practical Japanese shindo scale classifies the actual shaking taking place at any given location. A shaking of 4 on the shindo scale wakes up most people; pots and pans rattle, and dangling light-shades sway back and forth. If you feel unsteady on your feet, if dishes and books start falling off shelves and if the furniture moves around, you are probably in the throes of a lower-5. Pray you never get caught in a full-blooded 7, with the whole building being ripped apart around you. The most your correspondent has ever experienced is a simulated upper-6. Though braced for the expected violence, it literally took his breath away.
The moment-magnitude 9 megaquake that struck 130 kilometres (80 miles) off the Pacific coast of north-east Japan on March 11th caused upper-6 shaking in Sendai, the nearest big city to the offshore epicentre. The physical wrenching alone must have been terrifying. With the structurally weakened buildings swept aside like matchboxes by a surging tsunami up to ten metres (33ft) high in places, the loss of life and property has been horrendous. The final count is still unknown, but fatalities will surely top 14,000. Could something comparable happen in America? The most likely place, if it did, would be on the West Coast.
Ironically, the prime suspect is not the infamous San Andreas fault that stretches 810 miles from off the coast of northern California to the inland Salton Sea close to the Mexican border. The magnitude 7.8 earthquake that killed 3,000 people in San Francisco in 1906 sprang from the northern end of the San Andreas fault. With a magnitude of 6.9, the Loma Prieta earthquake that toppled buildings, bridges and elevated highways around the Bay Area in 1989 had its epicentre further down the fault near Santa Cruz.
Yet, while the San Andreas fault is capable of inflicting untold damage on built-up areas, it is not the monster portrayed in popular culture. If it ruptured from end to end, it would unleash an earthquake with a magnitude of 8.3—less than a tenth the size of the megaquake that wreaked havoc on Japan last week. Besides, it is most unlikely ever to do so. There is a chunk in the middle where the two sides of the fault creep gently passed one another, relieving the stress as they do. In short, the San Andreas is effectively two separate, shorter faults, each capable of giving birth to a quake only a third the size of the whole. That makes them tiddlers compared with the leviathans stalking the depths off Japan.
That is not to say that a diminished San Andreas could not devastate a major conurbation like Los Angeles. Indeed, a forecast prepared by the United States Geological Survey in 2008 reckoned that if a magnitude 7.8 quake ruptured the San Andreas fault in the desert 100 miles east of Los Angeles, it would topple 1,500 buildings in the metropolitan area and leave 2,000 people dead, 50,000 injured and 250,000 homeless. As grim as that may sound, it would be modest compared with the destruction wrought by the smaller magnitude 6.9 shock that erupted a stone’s throw from Kobe in 1995. Like real estate, the damage earthquakes inflict has more to do with location than size.
The most likely megaquake on the West Coast would be much further north—in fact, 50 miles off the coast between Cape Mendocino in northern California and Vancouver Island in southern British Columbia. This 680-mile strip of seabed is home to the Cascadia subduction zone, where oceanic crust known as the Juan de Fuca plate is forced under the ancient North American plate that forms the continent. For much of its length, the two sides of this huge subduction zone are locked together, accumulating stresses that are capable of triggering megaquakes in excess of magnitude 9.0 when they eventually slip. As such, Cascadia is more than a match for anything off the coast of Japan.
What makes Cascadia such a monster is not just its length, but also the shallowness of the angle with which the encroaching tectonic plate dives under the continental mass. The descending plate has to travel 40 miles down the incline before it softens enough from the Earth’s internal heat to slide without accumulating further frictional stresses. Could the fault unzip from end to end and trigger a megaquake—along with the mother of all tsunamis? You bet. By one account, it has done so at least seven times over the past 3,500 years. Another study suggests there have been around 20 such events over the past 10,000 years. Whatever, the “return time” would seem to be within 200 to 600 years.
And the last time Cascadia let go? Just 311 years ago. We know the time and date precisely, thanks to records kept by diligent Japanese officials on the far side of the Pacific. On January 26th 1700, between nine and ten o’clock at night, the Cascadia fault ruptured along its length, unleashing a megaquake of magnitude 9.0 or more. While the shaking was not felt across the Pacific, the Japanese were alarmed enough by the “orphan tsunami” that inundated their coastal villages to record the details. Knowing the distance and the speed such long waves travel at (around 750 kilometres per hour), the Japanese records allow the time the megaquake struck to be known to within an hour.
Numerical models suggest that a seismic event that size would have made the seafloor bounce six metres or more, causing tsunami waves to rise as high as 30 metres along the nearby coasts of Oregon and Washington. This past week Sendai looked devastated enough. Imagine how much worse Portland and Seattle would fare if hit by a tidal surge three times bigger.
The common view is that there is a one-in-seven chance of an earthquake of magnitude 9.0 or higher occurring along the Cascadia subduction zone within the next 50 years. More recent studies suggest the probability of that happening is more like one-in-three. Those odds suggest greater attention needs to be paid to preparations for the inevitable inundation. This past week’s pictures from Japan show just how dire, even in a wealthy country, the circumstances following a megaquake and tsunami can be.
http://www.economist.com/blogs/babbage/2011/03/megaquakes
-------------------------------------------------------------
The Oregon Dept. of Geology has a section of their website devoted to such things:
http://www.oregongeology.com/sub/earthquakes/coastal/HistoricTsunamis.htm
HAVING spent half his adult life in seismically active regions of the world—first in Japan and more recently in California—your correspondent takes earthquake precautions seriously. He knows precisely what to do when the initial P-wave tinkles the wine glasses in the kitchen cabinet, providing precious seconds of warning before the more muscular S-wave comes barging across the threshold. Though he admits to having been unnerved more than once, he grabs his shoes and a torch (if it is dark), turns off the gas at the mains and opens the garage door. He goes back about his business if the shaking turns out to be 4 or less on the scale of 0 to 7 the Japanese have used for over a century to measure the local intensity of an earthquake.
Unlike the logarithmic “moment-magnitude” scale, which measures the energy released at an earthquake’s focus, the more practical Japanese shindo scale classifies the actual shaking taking place at any given location. A shaking of 4 on the shindo scale wakes up most people; pots and pans rattle, and dangling light-shades sway back and forth. If you feel unsteady on your feet, if dishes and books start falling off shelves and if the furniture moves around, you are probably in the throes of a lower-5. Pray you never get caught in a full-blooded 7, with the whole building being ripped apart around you. The most your correspondent has ever experienced is a simulated upper-6. Though braced for the expected violence, it literally took his breath away.
The moment-magnitude 9 megaquake that struck 130 kilometres (80 miles) off the Pacific coast of north-east Japan on March 11th caused upper-6 shaking in Sendai, the nearest big city to the offshore epicentre. The physical wrenching alone must have been terrifying. With the structurally weakened buildings swept aside like matchboxes by a surging tsunami up to ten metres (33ft) high in places, the loss of life and property has been horrendous. The final count is still unknown, but fatalities will surely top 14,000. Could something comparable happen in America? The most likely place, if it did, would be on the West Coast.
Ironically, the prime suspect is not the infamous San Andreas fault that stretches 810 miles from off the coast of northern California to the inland Salton Sea close to the Mexican border. The magnitude 7.8 earthquake that killed 3,000 people in San Francisco in 1906 sprang from the northern end of the San Andreas fault. With a magnitude of 6.9, the Loma Prieta earthquake that toppled buildings, bridges and elevated highways around the Bay Area in 1989 had its epicentre further down the fault near Santa Cruz.
Yet, while the San Andreas fault is capable of inflicting untold damage on built-up areas, it is not the monster portrayed in popular culture. If it ruptured from end to end, it would unleash an earthquake with a magnitude of 8.3—less than a tenth the size of the megaquake that wreaked havoc on Japan last week. Besides, it is most unlikely ever to do so. There is a chunk in the middle where the two sides of the fault creep gently passed one another, relieving the stress as they do. In short, the San Andreas is effectively two separate, shorter faults, each capable of giving birth to a quake only a third the size of the whole. That makes them tiddlers compared with the leviathans stalking the depths off Japan.
That is not to say that a diminished San Andreas could not devastate a major conurbation like Los Angeles. Indeed, a forecast prepared by the United States Geological Survey in 2008 reckoned that if a magnitude 7.8 quake ruptured the San Andreas fault in the desert 100 miles east of Los Angeles, it would topple 1,500 buildings in the metropolitan area and leave 2,000 people dead, 50,000 injured and 250,000 homeless. As grim as that may sound, it would be modest compared with the destruction wrought by the smaller magnitude 6.9 shock that erupted a stone’s throw from Kobe in 1995. Like real estate, the damage earthquakes inflict has more to do with location than size.
The most likely megaquake on the West Coast would be much further north—in fact, 50 miles off the coast between Cape Mendocino in northern California and Vancouver Island in southern British Columbia. This 680-mile strip of seabed is home to the Cascadia subduction zone, where oceanic crust known as the Juan de Fuca plate is forced under the ancient North American plate that forms the continent. For much of its length, the two sides of this huge subduction zone are locked together, accumulating stresses that are capable of triggering megaquakes in excess of magnitude 9.0 when they eventually slip. As such, Cascadia is more than a match for anything off the coast of Japan.
What makes Cascadia such a monster is not just its length, but also the shallowness of the angle with which the encroaching tectonic plate dives under the continental mass. The descending plate has to travel 40 miles down the incline before it softens enough from the Earth’s internal heat to slide without accumulating further frictional stresses. Could the fault unzip from end to end and trigger a megaquake—along with the mother of all tsunamis? You bet. By one account, it has done so at least seven times over the past 3,500 years. Another study suggests there have been around 20 such events over the past 10,000 years. Whatever, the “return time” would seem to be within 200 to 600 years.
And the last time Cascadia let go? Just 311 years ago. We know the time and date precisely, thanks to records kept by diligent Japanese officials on the far side of the Pacific. On January 26th 1700, between nine and ten o’clock at night, the Cascadia fault ruptured along its length, unleashing a megaquake of magnitude 9.0 or more. While the shaking was not felt across the Pacific, the Japanese were alarmed enough by the “orphan tsunami” that inundated their coastal villages to record the details. Knowing the distance and the speed such long waves travel at (around 750 kilometres per hour), the Japanese records allow the time the megaquake struck to be known to within an hour.
Numerical models suggest that a seismic event that size would have made the seafloor bounce six metres or more, causing tsunami waves to rise as high as 30 metres along the nearby coasts of Oregon and Washington. This past week Sendai looked devastated enough. Imagine how much worse Portland and Seattle would fare if hit by a tidal surge three times bigger.
The common view is that there is a one-in-seven chance of an earthquake of magnitude 9.0 or higher occurring along the Cascadia subduction zone within the next 50 years. More recent studies suggest the probability of that happening is more like one-in-three. Those odds suggest greater attention needs to be paid to preparations for the inevitable inundation. This past week’s pictures from Japan show just how dire, even in a wealthy country, the circumstances following a megaquake and tsunami can be.
http://www.economist.com/blogs/babbage/2011/03/megaquakes
-------------------------------------------------------------
The Oregon Dept. of Geology has a section of their website devoted to such things:
http://www.oregongeology.com/sub/earthquakes/coastal/HistoricTsunamis.htm