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Le but de ce blog est d'eduquer et de discuter a propos des desastres naturels avec un focus sur l'activite et la vulnerabilite sismique, de reporter des informations generales relatives au tremblement de terre d'Haiti du 12 janvier 2010 et aux tremblements de terre du monde. Il met l'accent sur les efforts de reconstruction d'Haiti et la necessite d'utiliser des techniques de conception des structures de batiments et construction parasismique dans la construction des infrastructures physiques.

Haitilibre.com / Les dossiers

Friday, March 11, 2011

WORLD EARTHQUAKES

What conditions are necessary for an underwater earthquake or volcanic eruption to cause a tsunami?


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?
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

Japan, tsunami, earthquake 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.

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