I just read this article and got stuck on the following:
“Immediately after the landslide, a dome of water almost 900 metres (3,000 ft) high and tens of kilometres wide will form, only to collapse and rebound.”
Are these figures accurate? I have a hard time picturing a mushroom cloud of water that huge for a landslide. Wouldn’t this event have to be coupled with an immediate drop in the water level of the Atlantic ocean?
Yes. There was a show on Discovery recently about tsunami. They gave reports about much small waves, only 10-20 meters. In one case, many people were killed because the ocean suddenly withdrew by a bout a hundred meters, and the whole village came out to see what was going on. The wave came in and killed a lot of folks.
As to the OP. I agree with you that I can’t imagine a wave that big, and surely, a bunch of water would get sucked up. Makes me glad I live inland, but with a wave that size, it probably would not matter much.
i don’t know about the size, but this quote makes me think it would be nasty if they were right:
*One of the most famous, and most devastating, tsunamis occurred after the Krakatoa volcano blew itself to pieces in 1883 – a wave six metres high killed 30,000 people. *
that stood out the most to me. if a 20-foot wave could do that, a 3,000 footer could be the most devastating natural incident of all time. but you guys are right, a 3,000 footer just seems unproportional.
If I’m understanding the article, the wave itself wouldn’t be 3,000 feet high; that would only be an initial “swell” that would collapse and not go anywhere. They expect the wave itself will be about 330 feet high when it breaks over Western Sahara, about 160 feet high when it hits Florida, and about 130 feet high at Brazil.
Even so, I think it would still be the worst single disaster in living memory.
I can’t vouch for the accuracy of the figure, but we can take a look at some of the math and see how that might affect the levels of the Atlantic. A couple of assumptions here, but it should be a good picture nonetheless.
The initial dome of water is 3,000 ft high at its peak (~1 km) and tens of kilometers across. Let’s say a radius of 20 km. I don’t know the formula for calculating the volume of an irregular spheroid but the formula for the volume of a cylinder will probably do.
3.14(r²*h) = 3.14(400 * 1) = ~1256 km³ of sea water.
Now we take the surface area of the earth:
(4 * 3.14)13000² = 12.56 * 1.710[sup]8[/sup] = 2.110[sup]9[/sup] km²
Since the ocean floor is only an average of 4 km deep, the volume of sea water, if it covered the earth uniformly at that depth, would be the same figure in km³.
Of course the earth is only covered by water over 75% of its surface, so the actual volume of water is around 1.6 * 10[sup]9[/sup] km³.
In other words, that humongous dome of water isn’t even 1% of the total volume of water of either the world ocean or even the Atlantic. There wouldn’t be a drop in ocean levels when the dome forms.
OK, mathematics Dopers, how’d I do?
Looks good to me. I’d just spin it a little differently. I’ll accepting your estimate of 1256 km[sup]3[/sup] of water in the initial dome. The cylinder assumption overestimates, which is what you want for this kind of scaling.
Compare that to the surface area of the Atlantic Ocean. That’s about 106,450,000 sq km (41,100,000 sq mi) including the connected seas, according to this site:
http://oceanographer.navy.mil/atlantic.html
So, dividing 1256/106^6 gives about 1.2 cm. That means this initial swell would drop the sea level of the Atlantic by a little more than one centimeter. That’s way below the “standard deviation” caused by wind-driven waves.
This approach just skips the assumption about the depth of the ocean and puts the result in more visual terms (centimeters vs. percentage of volume).
What you need to do is read this
http://www.bbc.co.uk/science/horizon/mega_tsunami_transcript.shtml
link.
The BBC broadcast this programme some months ago, it is hardly new.
One point made in the tv show was that the usual tsunami is limited in size by the nature of the thing that caused it, a sudden change in seabed topology and results in waves of a maximum of 8 metres but generally very much smaller, even so a wave of just three metres can be devastating as the residents of the Bay of Bengal can verify through hard and bitter experience.
The next large wave is caused by a collapsing undersea volcano caldera such as at Krakatoa, Santorini and Tambura which are the largest explosions recorded in human history. These have caused much larger waves but they soon dissipate, locally they are incredibly destructive.
The next largest categories are sudden catastrophic landslip and asteroid strike, which appear to have no upper limit for size of wave generation, there is evidence on the Pacific US-Canadian seaboard of an enormous inundation of a more widespread nature than the potential Grand Canarias wave.So far there have been likely candidates put forward for this, but in fact there are so many of them it is very difficult to pin one down.
The BBC link does mention that there are many other potential sites and that these must have caused disasters on a regular basis, but as yet they have either not been recognised (maybe even have disrupted the oceanic currents with all that that implies for local environments) or have ocurred outside of recorded history.
Put Lituya Bay into your search engine and be amazed at a wave that reached over 1200 feet in height, and this is a relatively small rockfall compared to other potential sites.
True, but hardly noticeable, I should think. The seismic event would be easier to detect, no?
What gets me is how tsunamis are completely sneaky bastards. I remember one show (I think it was PBS’ Savage Earth telling how the fishermen of a Japanese village were out to sea and the tsunami that destroyed their village was nothing more than a gentle rise under them.
It is interesting to note that these ‘humongous waves’ are pretty much harmless if you’re out at sea (way out). The reason being is the wave is entirely underwater. Only as the wave approaches land does the seabed form a floor that the wave can lift itself out on. It makes the starting premise of The Poseidon Adventure somewhat dubious unless the ship was close to shore (in which case one would suppose rescuers would have arrived sooner than they did).
I don’t know if it is apocryphal but I heard the April Fool’s Day tsunami in 1946 that formed off of the Aleutian Islands and nailed Hawaii was first spotted by a ship at sea. The ship itself wasn’t affected in the slightest by the tidal wave even though it passed directly under the ship. What caught the crew’s attention, however, was a surface disturbance moving (IIRC) at something like 150+ mph. The tidal wave, when it hit Hawaii was 25 feet tall…that may not seem like very much (ocean swells can get much larger than this) but add a 150mph velocity to the thing at it becomes very dangerous. A 150 foot tall wave moving that fast (not to mention a 300 foot wave) is beyond wanting to contemplate in its destructive capacity.
Just out of curiosity…wouldn’t something like a barrier island (or even a hefty breakwater) be pretty good protection against this sort of thing (for the people on the mainland that is…obviously it’s not good for anyone actually on the barrier island)? While those types of islands are relatively puny wouldn’t the wave lift itself and crash down on the island thus drastically if not completely robbing the wave of it power to continue on to the mainland? (I know this would only protect the part of the mainland in the ‘shadow’ of one of these islands and the rest of the wave off to the sides would continue merrily on to wreak havoc elsewhere.)
This is because they’re very long wavelength waves. Even though the amplitude may be very high, the long wavelength means the curvature of the water surface fairly flat, not a radical breaking wave.
This also leads to the phenomenon related by WEW because the trough of the wave preceeding the crest is both long and deep, causing the water at the beach to recede.
Can you imagine the chaos on the East Coast of the USA if it were announced that a huge wave was 8 or 9 hours away?
Yikes.
wondering how high the Pallisades are . . .
Right, just before the wave impacts. What I was saying is that the shoreline wouldn’t drastically recede on the formation of the original dome of water out in the middle of the Atlantic.
I think you’re asserting the same thing but I get confused easily.
So can anything be done about this?
This does sound to me like something that could kill several hundred thousand people.
Several hundred thousand??? I’d put in the millions… not counting the effects of economic disruption caused by destroying most of the harbours and half the cities around the North Atlantic.
I’m wondering how high the Appalachians are…
[sub](very glad to be living a long way from oceans, major earthquake zones or volcanoes)[/sub]
There is not a whole lot you can do about a possible tsunami before the fact, and what there is, most people won’t do. Planning an evacuation for the entire eastern tidal plain in not something with which political people want to be associated. It would be incredibly expensive, even if you never actually evacuated. Then when the alarm goes off no one really believes it, and half your precious eight-hour warning is over before folks actually succeed in blocking all the highways. What you can save is the ships of the US Navy. They will at least do what they are told, if they are told to do it. Even four hours out to sea, the effects of a tsunami would be fairly modest.
Then, there’s the bad news. The volcano at the Canary Islands is one source for a tsunami. There are others. Some are as near as New Jersey, or Virginia. When one of those cuts loose, the warning time is minutes. We just don’t have the attention span, as a species to keep this sort of thing in mind for a hundred, or three hundred years, in some fashion that makes a five hour long response possible. If you live at the beach, you die. If you live up on a hill, inland, you don’t die from a tsunami. But of course you could get struck by lightning.
“What you cannot enforce, do not command” ~ Sophocles ~
The reason that Tsunami can pass by a ship at sea and have little effect is that it is shock energy that is moving rather than water.
If you had one of those desktop swinging ball toys you can see the effect.
The balls in the middle recieve a slight shock but the energy passes to the last ball in line which is kicked outward.
If the last ball in line was say half the weight of the others it would be kicked away more rapidly.
As the tsumani hits shallower waters the energy has to be dissipated in a smaller volume which results in seawater being piled up and pushed, the shock enrgy is transferred more and more to mechanical motion.
This water has to come from somewhere and its a common enough phenomenon for the sea to retreat back from the shore a long way as it is dragged into the tsunami wave.
Some of the biggest killers have been as a result of destruction of lines of communication, farmland, drainage systems and spreading of waterborne diseases such as cholera and typhus and all from waves of less than five metres, imagine how much worse 300 metre wave hitting the eastern seaboard would be.
Yeah, but think of the surfing, dudes!
I came in on the middle of this story on the news the other night (“the wave will be 130 feet high when it reaches the coast of Britain”) – talk about your momentary panic attacks…
Someone call?!
If you want to see BIG (non-seismic) waves, check out the Cortes Bank 100 miles off San Diego a couple times a year. When conditions are perfect, the waves reach 60, 70, 80 feet–the world’s biggest. Recently, a verified 66-footer was surfed and the long boarders are now hunting for the “perfect” 100-footer. Instead of being terrified, they think it’s great!
BTW, I doubt the alleged physics behind the 1950s Hawaii 25-foot tsunami racing ashore at 150-mph. Large waves move fast, but a more likely speed is 37-45 mph–still quite devastating.
-tsunamisurfer
Well doubt no more. 37-45 MPH is definitely on the low end for tsunamis.
While I don’t have a cite for the actual speed of the Hawaiian tsunami (I checked but couldn’t find anything that specific) I did find this site which I would say is reliable. It mentions that tsunamis can reach speeds of 450-650 MPH in the open ocean! As the tsunami approaches shore it will be slowed down but can still have speeds ranging from 30-200 MPH. I uncovered several other sites that backed-up this speed…if you’re interested I’ll provide links to them as well.
150 MPH does for the Hawaiian tsunami does not seem unreasonable although I will grant I don’t know it for a fact and it might have been slower.