What conditions are necessary for an underwater earthquake or volcanic eruption to cause a tsunami?
January 31, 2005
A Guide to Earthquakes What causes tremors? What makes them stop? Can they be predicted? Are our buildings as safe as they can be? » September 15, 2008 
Klaus Jacob, a senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University, explains.
The rapid displacement of a significant volume of ocean water by some external physical process acting either from below at the ocean floor or from above impacting the water surface generates a tsunami. Gravity then provides the restoring force to smooth out the vertical displacement of the ocean surface, causing a wave to propagate away from the source of disturbance.
A variety of events can cause the required vertical displacement of water, including some (but not all) submarine earthquakes; submarine landslides; large calving icebergs; explosive volcanic eruptions in the ocean (or near its coast); slides of land into the ocean; the impact of a meteorite or comet into the ocean (or on land near the coast); even large explosions of ships in harbors can cause local tsunamis.
So why do some submarine earthquakes cause tsunamis but others do not? First, the quakes have to be sufficiently large. Noticeable tsunamis require earthquakes of about magnitude seven or larger and widely-damaging tsunamis usually require earthquake magnitudes of at least eight or greater. But whereas large-magnitude events are necessary, they alone are not sufficient to cause a tsunami. Also essential is that the ocean floor be deformed vertically. This deformation can include faults that intersect the ocean floor and have a vertical component of fault offset. (The fault offset is the distance the rock on one side of the fault slips--or is offset--against the rock on the other side of the fault.) For this reason strike-slip faults (like the San Andreas Fault in California, even where some of its segments run offshore), in which the plates tend to slip horizontally against each other, do not normally generate significant tsunamis. Because water is virtually immune to the horizontal shearing motion of the ocean floor in these regions, little water is displaced. Exceptions occur when a strike-slip earthquake subsequently triggers a large submarine landslide. The latter event can displace water vertically and thus generate a tsunami.
The most notorious tsunamigenic earthquakes occur at subduction zones. During a single large subduction earthquake, one plate (typically an oceanic plate) can slip as much as 20 meters (60 feet) beneath the leading edge of the overriding plate (often a continent or chain of volcanic islands). Preliminary analysis of global seismograms indicate that during the December 26, 2004, great Sumatra-Andaman earthquake the India plate may have indeed slipped past the Burma plate as much as 20 meters at one patch of the fault, but probably somewhat less at other segments. The vertical component of this total inclined slip on the fault that dips to the northeast at an angle of about 10 or 15 degrees is probably on the order of two meters. These very preliminary estimates will surely be refined in future careful studies when all available seismological, tsunamic, geodetic, marine geophysical and geologic, and other data can be jointly analyzed.
A deep oceanic trench usually marks the boundary between the two plates where one plate subducts below the other. In the area near and seaward of the trench, the ocean floor tends to be downthrust during the earthquake, while landward of the trench the leading edge of the overriding plate is raised by several meters. Often the edge of the overriding plate is sliced subvertically into a tilted stack of "imbricated" fault blocks, not unlike playing cards moving against one another inside a tilted deck. All the vertical components of the ocean floor deformation contribute to the tsunami, whether they represent a rise or fall of ocean floor during the quake. Whether the first of a succession of tsunami waves empties out a nearby harbor or coast, or, conversely, floods the coast with a surging wall of water, depends largely on whether the coast faces most closely the down-dropped portion of the ocean floor or, alternatively, the portion that has been uplifted.
Moderate earthquakes can sometimes launch huge tsunamis. An example is the 1946 earthquake that occurred off Unimak Island in the eastern Aleutian Island arc, just west of the tip of the Alaskan Peninsula. The earthquake's magnitude was "only" somewhere between 7.3 and 7.8, but a large submarine block slide triggered by the earthquake set off a Pacific-wide tsunami. A subduction quake acting alone to generate waves of comparable size would have needed to have a magnitude of nearly nine.
When in 1883 the island volcano Krakatau was blasted into pieces in the Straits of Sunda, Indonesia, a gigantic volcanic caldera formed on the seafloor, while the volcanic debris thrown into the air came down onto the surface of the sea surrounding the former island. This double motion from below and above set off a tsunami crashing onto the shores across the entire Indian Ocean, in many places substantially exceeding the tsunami heights from the December 26, 2004, earthquake off northern Sumatra. Volcano- or earthquake-induced slides of portions of the Hawaiian Islands into the Pacific, or from the Canary Islands into the Atlantic are feared to put at risk most of the coasts in and across each of these two ocean basins. While such events have low probability, they could have catastrophic consequences.
Humanity has not experienced a major global tsunami from a meteorite impact into an ocean. But probably one of the most extreme tsunamis in the geological history of the earth was caused by the impact of a meteorite near what is now known as the Chicxulub structure, on the Yucatan Peninsula in Mexico. This event occurred some 65 million years ago and caused the extinction of many species, including the dinosaurs, most likely from a global drop of temperatures due to volcanic debris and dust in the atmosphere shielding the earth's surface from the sun's energy. But the meteor impact and associated ballistic debris fallout also caused a global tsunami estimated to be on the order of 1,000 feet high. Evidence of this great wave continues to be found at the coasts of many continents, including the U.S. states facing the Gulf of Mexico.
A very local, but still deadly tsunami occurred on Dec 12, 1917, in the harbor of Halifax, Nova Scotia. The explosion of a World War I-ammunition-laden freighter in the harbor set off a local tsunami, causing a 60-foot high wall of water to rush into town.
When enclosed water bodies such as lakes, reservoirs or fjords are subjected to sudden displacements of their waters by rock or landslides, they can also cause deadly and often spectacular upward surges of water on the opposing shorelines. The declared record in historic time is held by a rockslide in Lituya Bay in southeast Alaska, triggered by the 1958 Fairweather Fault strike-slip earthquake with an approximate magnitude of eight. A huge rockslide had gained high speed as it descended into the narrow fjordlike Lituya Bay, which made the displaced water run up on the opposing rock wall of the fjord to heights of 1,700 feet (520 m). The surging water stripped millions of forest trees and all soil from the entire rock wall up to this height.
How Does an Earthquake Trigger Tsunamis Thousands of Kilometers Away?
The Japan Earthquake and Tsunami On March 11, a powerful, 8.9-magnitude quake hit northeast Japan, triggering a tsunami with 10-meter-high waves that reached the U.S. West Coast. Here's the science behind the disaster » March 11, 2011
TSUNAMI PROPAGATION: This graphic illustrates, in dark blue, the propagation of waves from Japan's March 11 tsunami across the Pacific Ocean, toward Australia, North America and South America. Image: COURTESY OF NOAA CENTER FOR TSUNAMI RESEARCH
The massive magnitude 8.9 earthquake that struck near the east coast of Honshu, Japan's main island, at 2:46 P.M. local time and unleashed a fierce tsunami claiming hundreds of lives is already being felt as far away as the west coast of North America, about 8,000 kilometers away. Much of this has to do with the depth of the ocean that the tsunamis waves traversed as well as the sheer size of the quake, which was the strongest recorded in Japan's history.
The tsunami hit Hawaii about seven hours after it washed away entire towns along Japan's northern coast. Whereas the waves that struck Japan have been reported as high as seven meters, Hawaii was spared serious damage. Still, the threat of the tsunami closed ports in Honolulu and Guam and led to warnings, watches and coastal evacuations in 20 countries, including the U.S., Indonesia and Chile.
To find how a tsunami could pose a serious threat from such a great distance, Scientific American spoke with Greg Valentine, a geology professor and director of the University at Buffalo, The State University of New York Center for GeoHazards Studies. As it turns out, Valentine had been scheduled to fly to Japan Friday morning for a meeting to discuss a collaborative program on earthquake and volcano hazards. The meeting was cancelled as Japanese officials deal with the aftermath of this disaster and the possibility that more tsunamis will follow.
How can an earthquake create a tsunami?
This tsunami was probably generated by an earthquake where the crust from beneath the Pacific Ocean is diving down beneath Japan. What's happening is the uppermost 50 to 100 kilometers of the solid earth in the Pacific Ocean is generally moving to the northwest at a speed that's very slow by our standards, maybe several centimeters per year. But the crust around Japan is less dense and lighter than the crust in the Pacific. When the two come together, the ocean crust goes down, and those two plates really grind against each other. Along this area where the two are grinding, forces build up over time. And then it will suddenly snap, where the Pacific plate will go down very suddenly and the Asian plate that Japan is part of will sort of bounce up a little bit. The sudden motion of those two plates displaces a huge volume of water, and that's what causes the tsunami.
How are tsunami waves different from normal waves?
Tsunami waves travel very, very rapidly. The normal waves that we're accustomed to on the ocean are mostly driven by the wind, so they travel at the speed that the wind blows. With a tsunami, the speed depends on the depth of the water. Out in the open ocean, for example, where it's 5,000 meters deep, the speed of a tsunami's waves will be about 220 meters per second. The speed of the waves in 500-meter deep ocean drops to about 70 meters per second. If there's a lot of deep ocean between where a tsunami starts and where it's going these waves will get there extremely fast.
How does a tsunami wave change as it reaches the coast?
Part of what makes the tsunamis so damaging is that, as this mass of water is approaching a shoreline, it's also slowing down, so the water at the front of the wave is moving slower than the water coming in at the back of the wave. As a result, you get this huge piling up effect of the water.
As this approaches the shoreline, the sea level there can increase by several meters or even 10 meters—maybe 20 meters in an extreme case—and stay that way for some long period of time because these waves have a very long wavelength. So it's really different than a wind-driven wave that crashes on the shoreline and rolls back into the sea. In the case of a tsunami, the sea level at that shoreline is increasing for some period of time. Anything below that level on the shore will be flooded for a length of time.
Can the height of tsunami waves be predicted before they reach shore?
We can predict the path and the speed pretty well, but the height at a given location can be pretty hard to predict. It has to do with understanding the details of the fluid dynamics within the waves and the way the seafloor is shaped—how quickly it gets shallow.
Is there a danger of aftershocks from this earthquake creating additional tsunamis in the coming days and weeks?
Yes, there is. Often when an earthquake releases some stress along a subduction zone, such the one where the ocean crust is pushing below Japan, it can throw other parts of that zone out of equilibrium. This subduction zone is a very large-scale feature that is maybe hundreds to thousands of kilometers long, where the Pacific plate is going down beneath the Asian plate. Only a small part of that snapped to generate this earthquake and tsunami, but the fact that part of it snapped now changes the whole balance of forces along that system. So there could be another one, although most aftershocks are not as strong as the original earthquake.
How strong would an aftershock need to be to create a tsunami?
In general a magnitude 6 or higher would give you substantial enough motion to trigger a tsunami, but it also depends on the local situation, so I wouldn't say that's written in stone in any way. The higher the magnitude, the more likely an earthquake is to cause a tsunami because the magnitude reflects the amount of motion of the crust when it snaps.
But it also depends on the type of earthquake. Some earthquakes are caused when two plates are sliding horizontally past each other. The San Andreas Fault in California is a good example of this. If a horizontal slide were to happen on the seafloor, there wouldn't be as much vertical motion of the crust so that might generate a smaller tsunami. The reason subduction zones like the one near Japan are so bad for generating tsunamis is that it's a lot of up-and-down motion that really moves a lot of water.
The rapid displacement of a significant volume of ocean water by some external physical process acting either from below at the ocean floor or from above impacting the water surface generates a tsunami. Gravity then provides the restoring force to smooth out the vertical displacement of the ocean surface, causing a wave to propagate away from the source of disturbance.
A variety of events can cause the required vertical displacement of water, including some (but not all) submarine earthquakes; submarine landslides; large calving icebergs; explosive volcanic eruptions in the ocean (or near its coast); slides of land into the ocean; the impact of a meteorite or comet into the ocean (or on land near the coast); even large explosions of ships in harbors can cause local tsunamis.
So why do some submarine earthquakes cause tsunamis but others do not? First, the quakes have to be sufficiently large. Noticeable tsunamis require earthquakes of about magnitude seven or larger and widely-damaging tsunamis usually require earthquake magnitudes of at least eight or greater. But whereas large-magnitude events are necessary, they alone are not sufficient to cause a tsunami. Also essential is that the ocean floor be deformed vertically. This deformation can include faults that intersect the ocean floor and have a vertical component of fault offset. (The fault offset is the distance the rock on one side of the fault slips--or is offset--against the rock on the other side of the fault.) For this reason strike-slip faults (like the San Andreas Fault in California, even where some of its segments run offshore), in which the plates tend to slip horizontally against each other, do not normally generate significant tsunamis. Because water is virtually immune to the horizontal shearing motion of the ocean floor in these regions, little water is displaced. Exceptions occur when a strike-slip earthquake subsequently triggers a large submarine landslide. The latter event can displace water vertically and thus generate a tsunami.
The most notorious tsunamigenic earthquakes occur at subduction zones. During a single large subduction earthquake, one plate (typically an oceanic plate) can slip as much as 20 meters (60 feet) beneath the leading edge of the overriding plate (often a continent or chain of volcanic islands). Preliminary analysis of global seismograms indicate that during the December 26, 2004, great Sumatra-Andaman earthquake the India plate may have indeed slipped past the Burma plate as much as 20 meters at one patch of the fault, but probably somewhat less at other segments. The vertical component of this total inclined slip on the fault that dips to the northeast at an angle of about 10 or 15 degrees is probably on the order of two meters. These very preliminary estimates will surely be refined in future careful studies when all available seismological, tsunamic, geodetic, marine geophysical and geologic, and other data can be jointly analyzed.
A deep oceanic trench usually marks the boundary between the two plates where one plate subducts below the other. In the area near and seaward of the trench, the ocean floor tends to be downthrust during the earthquake, while landward of the trench the leading edge of the overriding plate is raised by several meters. Often the edge of the overriding plate is sliced subvertically into a tilted stack of "imbricated" fault blocks, not unlike playing cards moving against one another inside a tilted deck. All the vertical components of the ocean floor deformation contribute to the tsunami, whether they represent a rise or fall of ocean floor during the quake. Whether the first of a succession of tsunami waves empties out a nearby harbor or coast, or, conversely, floods the coast with a surging wall of water, depends largely on whether the coast faces most closely the down-dropped portion of the ocean floor or, alternatively, the portion that has been uplifted.
Moderate earthquakes can sometimes launch huge tsunamis. An example is the 1946 earthquake that occurred off Unimak Island in the eastern Aleutian Island arc, just west of the tip of the Alaskan Peninsula. The earthquake's magnitude was "only" somewhere between 7.3 and 7.8, but a large submarine block slide triggered by the earthquake set off a Pacific-wide tsunami. A subduction quake acting alone to generate waves of comparable size would have needed to have a magnitude of nearly nine.
When in 1883 the island volcano Krakatau was blasted into pieces in the Straits of Sunda, Indonesia, a gigantic volcanic caldera formed on the seafloor, while the volcanic debris thrown into the air came down onto the surface of the sea surrounding the former island. This double motion from below and above set off a tsunami crashing onto the shores across the entire Indian Ocean, in many places substantially exceeding the tsunami heights from the December 26, 2004, earthquake off northern Sumatra. Volcano- or earthquake-induced slides of portions of the Hawaiian Islands into the Pacific, or from the Canary Islands into the Atlantic are feared to put at risk most of the coasts in and across each of these two ocean basins. While such events have low probability, they could have catastrophic consequences.
Humanity has not experienced a major global tsunami from a meteorite impact into an ocean. But probably one of the most extreme tsunamis in the geological history of the earth was caused by the impact of a meteorite near what is now known as the Chicxulub structure, on the Yucatan Peninsula in Mexico. This event occurred some 65 million years ago and caused the extinction of many species, including the dinosaurs, most likely from a global drop of temperatures due to volcanic debris and dust in the atmosphere shielding the earth's surface from the sun's energy. But the meteor impact and associated ballistic debris fallout also caused a global tsunami estimated to be on the order of 1,000 feet high. Evidence of this great wave continues to be found at the coasts of many continents, including the U.S. states facing the Gulf of Mexico.
A very local, but still deadly tsunami occurred on Dec 12, 1917, in the harbor of Halifax, Nova Scotia. The explosion of a World War I-ammunition-laden freighter in the harbor set off a local tsunami, causing a 60-foot high wall of water to rush into town.
When enclosed water bodies such as lakes, reservoirs or fjords are subjected to sudden displacements of their waters by rock or landslides, they can also cause deadly and often spectacular upward surges of water on the opposing shorelines. The declared record in historic time is held by a rockslide in Lituya Bay in southeast Alaska, triggered by the 1958 Fairweather Fault strike-slip earthquake with an approximate magnitude of eight. A huge rockslide had gained high speed as it descended into the narrow fjordlike Lituya Bay, which made the displaced water run up on the opposing rock wall of the fjord to heights of 1,700 feet (520 m). The surging water stripped millions of forest trees and all soil from the entire rock wall up to this height.
How Does an Earthquake Trigger Tsunamis Thousands of Kilometers Away?
As Japan suffered the worst earthquake in the country's recorded history, tsunami waves fanned out across the Pacific Ocean at the speed of a jetliner
March 11, 2011
The Japan Earthquake and Tsunami On March 11, a powerful, 8.9-magnitude quake hit northeast Japan, triggering a tsunami with 10-meter-high waves that reached the U.S. West Coast. Here's the science behind the disaster » March 11, 2011
TSUNAMI PROPAGATION: This graphic illustrates, in dark blue, the propagation of waves from Japan's March 11 tsunami across the Pacific Ocean, toward Australia, North America and South America. Image: COURTESY OF NOAA CENTER FOR TSUNAMI RESEARCH The tsunami hit Hawaii about seven hours after it washed away entire towns along Japan's northern coast. Whereas the waves that struck Japan have been reported as high as seven meters, Hawaii was spared serious damage. Still, the threat of the tsunami closed ports in Honolulu and Guam and led to warnings, watches and coastal evacuations in 20 countries, including the U.S., Indonesia and Chile.
To find how a tsunami could pose a serious threat from such a great distance, Scientific American spoke with Greg Valentine, a geology professor and director of the University at Buffalo, The State University of New York Center for GeoHazards Studies. As it turns out, Valentine had been scheduled to fly to Japan Friday morning for a meeting to discuss a collaborative program on earthquake and volcano hazards. The meeting was cancelled as Japanese officials deal with the aftermath of this disaster and the possibility that more tsunamis will follow.
How can an earthquake create a tsunami?
This tsunami was probably generated by an earthquake where the crust from beneath the Pacific Ocean is diving down beneath Japan. What's happening is the uppermost 50 to 100 kilometers of the solid earth in the Pacific Ocean is generally moving to the northwest at a speed that's very slow by our standards, maybe several centimeters per year. But the crust around Japan is less dense and lighter than the crust in the Pacific. When the two come together, the ocean crust goes down, and those two plates really grind against each other. Along this area where the two are grinding, forces build up over time. And then it will suddenly snap, where the Pacific plate will go down very suddenly and the Asian plate that Japan is part of will sort of bounce up a little bit. The sudden motion of those two plates displaces a huge volume of water, and that's what causes the tsunami.
How are tsunami waves different from normal waves?
Tsunami waves travel very, very rapidly. The normal waves that we're accustomed to on the ocean are mostly driven by the wind, so they travel at the speed that the wind blows. With a tsunami, the speed depends on the depth of the water. Out in the open ocean, for example, where it's 5,000 meters deep, the speed of a tsunami's waves will be about 220 meters per second. The speed of the waves in 500-meter deep ocean drops to about 70 meters per second. If there's a lot of deep ocean between where a tsunami starts and where it's going these waves will get there extremely fast.
How does a tsunami wave change as it reaches the coast?
Part of what makes the tsunamis so damaging is that, as this mass of water is approaching a shoreline, it's also slowing down, so the water at the front of the wave is moving slower than the water coming in at the back of the wave. As a result, you get this huge piling up effect of the water.
As this approaches the shoreline, the sea level there can increase by several meters or even 10 meters—maybe 20 meters in an extreme case—and stay that way for some long period of time because these waves have a very long wavelength. So it's really different than a wind-driven wave that crashes on the shoreline and rolls back into the sea. In the case of a tsunami, the sea level at that shoreline is increasing for some period of time. Anything below that level on the shore will be flooded for a length of time.
Can the height of tsunami waves be predicted before they reach shore?
We can predict the path and the speed pretty well, but the height at a given location can be pretty hard to predict. It has to do with understanding the details of the fluid dynamics within the waves and the way the seafloor is shaped—how quickly it gets shallow.
Is there a danger of aftershocks from this earthquake creating additional tsunamis in the coming days and weeks?
Yes, there is. Often when an earthquake releases some stress along a subduction zone, such the one where the ocean crust is pushing below Japan, it can throw other parts of that zone out of equilibrium. This subduction zone is a very large-scale feature that is maybe hundreds to thousands of kilometers long, where the Pacific plate is going down beneath the Asian plate. Only a small part of that snapped to generate this earthquake and tsunami, but the fact that part of it snapped now changes the whole balance of forces along that system. So there could be another one, although most aftershocks are not as strong as the original earthquake.
How strong would an aftershock need to be to create a tsunami?
In general a magnitude 6 or higher would give you substantial enough motion to trigger a tsunami, but it also depends on the local situation, so I wouldn't say that's written in stone in any way. The higher the magnitude, the more likely an earthquake is to cause a tsunami because the magnitude reflects the amount of motion of the crust when it snaps.
But it also depends on the type of earthquake. Some earthquakes are caused when two plates are sliding horizontally past each other. The San Andreas Fault in California is a good example of this. If a horizontal slide were to happen on the seafloor, there wouldn't be as much vertical motion of the crust so that might generate a smaller tsunami. The reason subduction zones like the one near Japan are so bad for generating tsunamis is that it's a lot of up-and-down motion that really moves a lot of water.
How to Cool a Nuclear Reactor
Japan's devastating earthquake caused cooling problems at one of the nation's nuclear reactors, and authorities scrambled to prevent a meltdown
March 11, 2011
The Japan Earthquake and Tsunami On March 11, a powerful, 8.9-magnitude quake hit northeast Japan, triggering a tsunami with 10-meter-high waves that reached the U.S. West Coast. Here's the science behind the disaster » March 11, 2011 
COOLING POWER: Japan's 8.9 magnitude earthquake cut power to reactor No. 1 at Fukushima Daiichi nuclear power plant--pictured here in 1975--making it difficult to cool the core. Image: National Land Image Information (Color Aerial Photographs), Ministry of Land, Infrastructure, Transport and Tourism As a precautionary measure, the Japanese government has declared a nuclear emergency and asked people living within three kilometers of the facility to evacuate and people living within 10 kilometers to remain indoors. Tokyo Electric Power, for its part, planned to vent some of the radioactive steam from inside the containment building.
Scientific American spoke with Scott Burnell, public affairs officer at the U.S. Nuclear Regulatory Commission (NRC), the government agency charged with monitoring the safety of the 104 nuclear reactors in the U.S., about what it takes to cool down a reactor.
How do you typically cool a reactor?
The approach to cooling is very simple: push water past the nuclear core and carry the heat somewhere else. The chain reaction that actually runs the reactor can be shut off in a matter of seconds. What's left over in the core, the radioactive material, will continue to give off heat for a long time. Unless you have a mechanism to remove that, the heat can build up and can eventually damage the radioactive fuel or the reactor.
Pushing water past the core means pumps that are generally run by electricity. What happens when a reactor gets disconnected from the grid?
There are emergency diesel generators. You also have a battery system to keep instruments running, but that can also provide power to safety systems [which prevent a meltdown by cooling the reactor core]. It's all meant to provide defense in depth. First you rely on the grid. If the grid is no longer available, you use diesel generators. If there is an issue with the diesels, you have a battery backup. And the batteries usually last long enough for you to get the diesels going.
How much time is there before a meltdown?
It depends on the plant. It depends on whether it's a boiling-water reactor or a pressurized-water reactor. Basically, [in both] you have the benefit of natural forces such as convection. There is a coolant loop no matter what, so you end up to some degree cooling the core because the heated water rises and colder water gets pulled in. But that's not as effective as a pump bringing in cool water. Just to speak very broadly, you have many hours to restore power to the system to get normal cooling going. It's really not possible to get more specific than "many hours."
But generally, less than 24 hours.
That's fair to say.
What's the worst-case scenario?
The event we are looking to avoid is damaging the core. Once you start damaging the core, you are then releasing radioactive material into the coolant and thereby increasing the chances that something travels outside the reactor.
The reactor that was not cooling properly in Japan, the Fukushima Daiichi No. 1 reactor, was a boiling-water type. How are these different from pressurized-water reactors in terms of cooling?
Particularly useful to boiling-water reactors is a system that is steam driven. It does not require an outside power source. Steam generated by the heat of a cooling down reactor has enough force to run a turbine, which then runs a pump that provides coolant to the core. That sort of system is supposed to withstand an earthquake, and that can run for an extended period. It's a self-limiting condition. That system does use batteries for the controls, but it can also be operated manually. So even in the face of a complete station blackout—you don't have any power at all—there are methods for using the steam-driven pump to continue to keep cooling going.
Japan earthquake demonstrates the limits and power of science
Mar 11, 2011 01:10 PM
Will seismologists ever be able to reliably predict the exact location, time and magnitude of earthquakes like the one that just devastated Japan and sent tsunamis racing across the Pacific Ocean? If so, they might be able to save many lives. Consider how many people have been killed by large earthquakes just in the last decade: more than 20,000 people in India in 2001, 30,000 in Iran in 2003, 227,000 in Sumatra in 2004, 86,000 in Pakistan in 2005, 87,000 in China in 2008, and 222,000 in Haiti last year, according to the U.S. Geological Survey. Early reports from Japan suggest that the death toll could be in the tens of thousands. The pioneers of earthquake studies were the Chinese, who began keeping records of where and when earthquakes occurred as early as 780 B.C. In the second century A.D. the Chinese invented a kind of weathervane for detecting and locating the center of earthquakes. The device consisted of a weight delicately suspended in a large bronze urn, ringed by dragons with hinged jaws. Jostling of the urn tipped the weight toward one side of the urn, causing the jaws of the dragon on that side to swing open and release a ball. The ball would supposedly fall on side of the urn from which the earthquake originated.
Modern seismometers are exquisitely sensitive, capable of calculating the exact location and strength of earthquakes on the other side of the planet. Moreover, the theory of plate tectonics—first proposed by Alfred Wegener in 1915 and finally accepted by other scientists in the 1960s--provides a firm foundation for understanding why earthquakes happen. Temblors tend to occur at the boundaries between the vast, shifting plates that comprise the earth’s crust.
A few decades ago, seismological technology and theory had advanced so far that many researchers became confident they could predict the exact date and location of earthquakes, providing time for evacuation and other life-saving measures. In 1985 scientists working with the U.S.G.S. funded an experiment intended to serve as a test bed for earthquake prediction. The experiment was based in Parkfield, Calif., a small town that sits astride the notorious San Andreas fault. Since the mid-1800s, Parker had been struck by earthquakes of magnitude 6 or greater every 22 years, on average.
Scientists outfitted the Parkfield fault with seismometers, strain gauges and other sensors that ideally would provide warning of an impending quake. The leaders of the experiment claimed there was a 95 percent probability that an earthquake of magnitude 6 or greater would happen by 1993. This claim—and the possibility of a precise prediction--received much attention from the media, including Scientific American. Parkfield was indeed struck by an earthquake—in 2004, 11 years after the initial prediction period had expired.
Other nations, notably China and Japan, have funded earthquake-prediction programs, but they have not been successful, either. One forecasting method focuses on the minor foreshocks that often portend a large quake. Unfortunately, this method is prone to false alarms, because the vast majority of minor tremors are not followed by major ones. Moreover, not all big quakes are preceded by foreshocks. China claims that foreshock-detection allowed it to successfully predict and evacuate people in the vicinity of a 7.3-magnitude quake in 1975. But over the past 20 years the Chinese program has issued more than 30 false alarms, and it failed to predict the 2008 quake that devastated eastern Sichuan.
Many other prediction methods have been proposed and in some cases tested. These involve detection of such alleged quake precursors as surges in ground water; emissions of the radioactive gas radon; fractoluminescence, or flashes of light emitted by compressed rock; unusual tidal activity; low-frequency electromagnetic waves; and unusual animal behavior. One long-running experiment in Japan involves monitoring catfish, which are supposedly sensitive to electromagnetic activity that precedes quakes. None of these approaches has proven reliable.
On the other hand, science and engineering have helped us reduce the damage of quakes. Whatever the final death toll from Japan's quake turns out to be, it would have been orders of magnitude greater if Japan had not designed its buildings, roads, nuclear power plants and other structures to withstand a vigorous shaking. Fewer than 1,000 people were killed by an enormous, 8.8-magnitude quake that struck Chile last year, because Chile has hardened its infrastructure against quakes. Tsunami-warning networks have also saved lives by quickly disseminating alerts to coastal regions.
So even if seismologists never achieve precise, short-term predictions of earthquakes, there is much that science can do, and has done, to protect us from this ancient scourge.
Source: Scientific American
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