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
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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.
[An edited transcript of the interview follows.]
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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13 Comments
Add CommentHmmmm... I have been curious about tsunamis for many years and bits of this explanation do not fit what I have learned. Following comments are also questions looking for more information.
Reply | Report Abuse | Link to thisThey are a form soliton, a solitary wave used to transmit data through fibre unaffected by other waves on the same fibre. They pass through each other, retaining shape and signal without dissipating.
The latter is characteristic of a tsunami, conserving energy after travelling thousands of kilometres through water. The questions that came up for me in my enquiry were 1) specifically what originates the wave, 2) the nature of the energy transmitted, 3) how that energy is transmitted through water, 4) how that energy transmits into physical waves at destinations and 5) why some waves are destructive and others almost unnoticeable.
The transcript suggests that the wave originates from a physical movement of water and that there is physical movement of water at the destination. I question that.
I scoured a wall of texts on solitons in a university library. I found plenty of mathematical formulae, but little text and no descriptions, confirming my suspicion that solitons are natural phenomena for which there is no real explanation, further confirmed when I read in one of the texts a comment about "the miracle of the soliton".
I understand that as the height of the wave increases in deep water the length increases correspondingly, so the same number of cycles covers much greater distance in the same time.
Scientists I contacted told me it was unknown whether, or how energy is transmitted from one water molecule to another, or whether there is physical movement of molecules. I suggested to another who had published a number of papers on tsunamis that when a wave comes into shallow water and is pushed upwards, it looked to me like the energy in the wave is trying to preserve its shape as a wave above the water surface, into which it pulls water in front (sucking back before a tsunami hits) and from behind (why water at the rear moves faster than in front). The greater the energy, the higher the wave, but the weight of water flattens the wave, explaining the long, flat body of water, which is sucked back, then pushed back in with successive waves. The waves are not water moving from great distance, but energy attempting to retain its shape in the physical form of water.
The scientist told me he could not come up with a better explanation. Can anyone else? Any thoughts on the source and nature of the energy?
re "hoamingin"
Reply | Report Abuse | Link to thisnot sure what the problem is .... if you have a body of water and a mass displaces it then it has to move somewhere . If there is uplift from the sea floor then the water is pushed up .... and then because of its own weight it has to move sideways. Take the example of Lituya Bay where the mountain wall at the head of the fjord fell into the water .... which was then pushed sideways ..... big time !
Understanding the transfer of energy in wind generated waves is more difficult.... but in tsunamis it is surely a simple mechanical effect ? .... or am I being too simplistic ?
Amikescott,
Reply | Report Abuse | Link to thisI think that is being too simplistic. Think of a radio wave that moves through the air. There is no physical movement of air, just a wave passing through it. Visual representations of a tsunami wave are like that, with some differences. The height of the wave expands as water gets deeper, so the top is the surface of the water and the bottom is, well, the bottom. Greater height, greater length of wave and greater speed. As mentioned in the article, at 500m depth, speed is 250kph, at 5000m, 800kph. In deep water there is no visible wave on the surface. Ships might not notice it passing by, or just as a tremor. That changes when the wave is squeezed as water gets shallower.
I live in Sydney and we have traditional ferries that put out bow waves that often disappear before they hit shore. We also have catamaran ferries designed to leave no wake along rivers. However, the props on those ferries leave waves that are not visible on the surface, that can be felt as a tremor passing by a swimmer in the water, and some time after the ferry has gone a small wave looks like it is trying to climb up the bank.
Maybe all underwater disturbances create some kind of wave, but the energy transmitted is dependent on the size, speed and direction of the movement.
famulla,
Reply | Report Abuse | Link to thisThis website has an international audience, so if I wrote using my name I do not think anyone is likely to know me better than they would through the content of my writing. If you want to know my name, check my website:
www.ideasintuitionandthinking.com
Which brings me to the content of your comments which, to be honest, left me a bit confused. Are you suggesting that Allah had something to do with the earthquake and tsunami (as far as I know there was no typhoon)?
I am sorry you lost the love you had back in the 60s and 70s, but I guess most of those who can remember back that far are not getting as much as they did then.
I think you should contact some other scientists, perhaps someone who has graduated from high school - energy is transmitted between water molecules through electromagnetic forces while gravitational forces keeps it all on the ground. Nothing mysterious about it. And there certainly is movement of water molecules.
Reply | Report Abuse | Link to thisFor the rest, amikescott gave a pretty good explanation of how it gets started.
Now the specific details in real world situations can get overwhelmingly complicated for calculations, but the general principles are simple and well-known.
(And it's better for you not to think of radio waves "moving through air," that concept was abandoned a century and a half ago. They are largely unimpeded by air!)
iWind,
Reply | Report Abuse | Link to thisI agree that I was loose with terminology. Nevertheless, sound is transmitted through air, or other medium, similar to the energy wave that produces a tsunami being transferred through water. One difference is that sound is a linear wave, a tsunami is the result of a non-linear wave, a soliton, that does not dissipate.
However, I will still use sound as comparison. When a guitar string is plucked, the physical movement of the string displaces air, but it is not the movement of air that is transmitted, it is a wave, the frequency of which varies with the extent and speed of movement of the string.
Tsunamis start from what is sometimes described as a large mass of a plate that has been compressed "snapping" with a large physical movement. Obviously that displaces water, which might explain why some waves are localised. I am sure that the physical movement of water is not what starts the energy wave that shows up thousands of kilometres away hours later, with no loss of energy. That is a soliton, the characteristics of which, I believe, can be attributed to the speed and distance of movement of the edge of the plate, perhaps affected by the direction in which the plate moves.
My enquiries have not been with people who graduated high school, but with people who have studies of solitions out in the public domain. They have told me that they do not know whether anything, or what, happens among molecules of water through which the wave passes. The wave passes through deep water of consistent depth with no noticeable physical presence, suggesting that this is a wave passing through a medium, without physically affecting the medium.
That seems to change when the wave comes to a bottom that slopes upwards. The energy in the wave seems to attempt to preserve its shape by sucking in water from the troughs in front of and behind the energy wave. The water that is moved by the energy is also affected by gravity, explaining the flat body of water that hits a coastline.
Attached is a reference that purports to explain solitons:
http://www.fas.org/sgp/othergov/doe/lanl/pubs/00326980.pdf
I am still asking if anyone out there can explain a soliton or a tsunami wave in words, not numbers?
When the earth quake started then possibly the center of earth and center of moon connecting straight line passes to the epicenter of the earth quake and due to moon being very near to earth the pull became stronger due to which the molten iron rotating core of earth hit the crust causing earth quake and as the sea level has been rising due to global warming, the extra mass of water contributed to higher waves causing a tsunami in addition to earth quake. We know that crust of earth in Japan is weak compared to many other places due to hidden volcano.
Reply | Report Abuse | Link to thisEveryone says "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."
Reply | Report Abuse | Link to thisMy question is how fast does this process occur? everything just says sudden. Five seconds or 5 minutes? We are dealing with very large masses, so I am very curious as to how fast this shift actually occurs.
@ ScottL
Reply | Report Abuse | Link to thisconsidering the length of faultline that can "unzip" during a large event like this it happens very fast.
400-500 km of faultline rupturing in less than a minute
this can provide 2-4 minutes of shaking depending on your proximity to the fault. The further away the longer the shaking (within the felt effects region). ie. the people in Tokyo had a longer shaking period than those in the Sendai region.
There are 2 reasons for a variation in the length of time it will take the rupture to occur.... Bi-lateral rupture ... if the fault starts rupturing and spreads both directions then that will be a rupture time of only 1/2 that, than if the fault started rupturing from one end to the other, a uni-lateral rupture.
cheers
Dave
@hoamingin
Reply | Report Abuse | Link to thisjust a clarification for you :) Note that the sea doesnt always recede before the tsunami wave arrives.
That is dependant on factors like distance from the wave generation point.
Over the many years there have been a number of examples where the wave just arrived at the peak first followed by a trough and the next peak.
Dave
@hoamingin
Reply | Report Abuse | Link to this[quote]I am sure that the physical movement of water is not what starts the energy wave that shows up thousands of kilometres away hours later, with no loss of energy.
[/quote]
of course it is... the energy pulse had to come from somewhere ... it the earthquake..movement of the seafloor :) whether its your soliton that keeps the wave propagating I have no idea, what keeps a sound wave propagating ??
D
Well if you are looking for a solution in words, maybe this website can help you with a picture.
Reply | Report Abuse | Link to thishttp://science.howstuffworks.com/nature/natural-disasters/tsunami2.htm
yoshyusmc,
Reply | Report Abuse | Link to thisThanks for that. I quote the main point. "When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes the water upward above normal sea level. This is the birth of a tsunami."
I agree with the snapping and the force being transferred to the water, but then it, and the accompanying diagram imply that it is the physical movement of water that is the tsunami. If a physical wave is produced by an earthquake, I think that it is separate to the shockwave transmitted through the water.