Posted on 12/28/2004 2:10:18 PM PST by M. Espinola
How a tsunami develops
A tsunami is made up of a series of travelling ocean waves of extremely long wavelength. They are triggered by large disturbances such as earthquakes, undersea volcanic eruptions or deep sea landslides.
The figure below shows the amplitude, trough and crest of a tsunami wave in relation to the sea floor and still water level. Note how the wave shape changes and the height increases as it approaches the coastline.
Effects of tsunami Tsunami are a threat to people and property in coastal and low-lying estuarine areas. The waves travel quickly, rapidly flooding and damaging coastal communities, picking up debris as they go. A fast moving wave over 10 metres high can quickly destroy homes and communities. Tsunami also create seiching in harbours and confined estuaries.
Tsunami waves can travel inland along river beds as continuous single standing waves. This puts smaller inland communities at risk and contaminates rivers with saltwater.
A tsunami can threaten 'lifeline' services such as water, power, telecommunication and transportation networks. Find out more about managing Lifelines to deal with unexpected emergencies or natural hazards.
An immense wave could one day wreak havoc on the eastern seaboard of the US and elsewhere around the Atlantic.
"It's entirely possible you'd see 50-metre waves coming ashore in Florida, New York, Boston, all the way up to Greenland, and in some cases reaching up to 10 km inland" Dr Simon Day
Scientists say a volcanic eruption on the Canary Islands, off West Africa, could trigger a vast undersea landslide.
This would set off a tsunami wave capable of inundating coastal regions thousands of kilometres away.
The warning comes from Dr Steven Ward, of the University of California, US, and Dr Simon Day, of the Benfield Greig Hazard Research Centre at University College London, UK.
Writing in Geophysical Research Letters, they refine an earlier estimate of the likely consequences of the collapse of the western flank of the Cumbre Vieja volcano on the island of La Palma in the Canaries.
They believe a build-up of groundwater could destabilise a block of rock up to 500 cubic km in size, which could break off in a future eruption, rushing into the sea at up to 350 km an hour (220 mph).
The energy released by the collapse would equal the entire US electricity consumption for six months.
The dome of water it caused would be 900 metres (2,950 feet) high, and the resulting tsunami, higher than any in recorded history, would travel outwards, reaching speeds of 800 km an hour (500 mph).
Waves 100 m (330 ft) from crest to trough would strike the African coast, while north-eastwards they would affect Spain, Portugal and France, and could still be approaching 12 m (40 ft) when they hit the UK.
That is almost three times the maximum recorded after the Lisbon earthquake of 1755.
Dr Day said the waves striking the UK coast would not penetrate more than two or three kilometres inland.
But he told BBC News Online: "Weird things happen when tsunamis enter harbours or estuaries.
"If those resonate at a certain frequency, that may substantially increase the damage."
Many centuries
Across the Atlantic the damage would be far worse, with wave heights of more than 40 m (130 ft) expected in northern Brazil.
Dr Day said: "It's entirely possible you'd see 50-m waves coming ashore in Florida, New York, Boston, all the way up to Greenland, and in some cases reaching up to 10 km inland.
"And that would be about nine hours or more after the initial collapse."
The ensuing economic losses would probably be in trillions of dollars, even if there were enough warning to evacuate threatened areas and avoid massive loss of human life.
If the speed of the landslide proved to be 150 metres per second (490 feet per second), not the 100 m/s (330 ft/s) assumed, that could double the height of the waves reaching the US.
But Dr Day had some reassurance on the probable timescale of the catastrophe.
He said a collapse was unlikely this century, and perhaps for many centuries.
It could take several eruptions to destabilise the volcano enough to dislodge the western flank, and collapse would occur only after days or weeks of seismic activity.
He told BBC News Online: "We think you have to see some evidence of subterranean movement before there's a risk of collapse.
"The fact that we aren't seeing any movement gives us a lot of confidence Cumbre Vieja won't collapse spontaneously.
"But we've found that eruptions do tend to come in clusters. And there've been two in the recent past."
I guess the movie Deep Impact got it pretty close.
In Thailiand, they said the sea bottom was exposed for over a thousand feet from the normal shoreline..
Or Ted Kennedy immediately belly flopping off the side of his yacht to get a bottle of bourbon he knocked overboard.
Or if her forgot something important in a submersed '69 Cadillac.
Fascinating video of the tsunami hitting a SE Asian beach resort (Thailand?) is found at this link: http://homepage.mac.com/jlgolson/tsunami.wmv
The wave has so much volume...as it travels through the ocean, it's using water in front of it - as it approaches the shore, it's pulling that water into itself. There's some footage from the Anchorage quake that shows the same phenomena (or however you spell it).
OK..I think I get it..at first glance..it's counter-intuitive..
Waves are basically up and down motion. The water in the wave doesn't move sideways, the up/down motion does. The wave usually starts with a central down motion, then the water around it rises. As the wave crest moves, it is preceded and followed by a depressions. A depression hits first. THe depression is there, because the water to form the crest has to come from somewhere.
So when you see the water go out, the crest is forming- or, the the shot is being loaded.
In this case it looks like an island moved sideways ~100ft. On one side the water went up. On the other, it went down.
I've read how the tsunami are formed at the epicenter of the quake site..the movement of the sea floor rapidly, and violently, displaces water..which is not compressible..thus generating a wave..I got thataprt. The sea floor can be thrust up, or the opposite..it can collapse...will both activities generate a tsunami..and the same type, and with the same direction?..if the bottom collapses...water rushes in, right..does the wave run TOWARDS the epicenter?
Thanks again..
Ah, you ain't seen nuthin' yet!
I just heard a reputable scientist say it is unlikely we would have a tsunami on the east coast because the plates move horizontally not vertically.
Think of it as the 'trough', or low part of the wave, hitting first.
know of any links to see this footage?
thanks :)
I don't think those two men who were trying to reach to porch made it. This was a dramatic piece of footage that shows the immense power of a tsunamis and the extreme duration. This was apparently the second or third wave.
I second your opinion of spunkets reply......
I found this while trying to inderstand it....might interest you or others, too:
The Physics of Tsunamis
The mechanisms of tsunami generation and propagation
sections:
What does "tsunami" mean?
How do tsunamis differ from other water waves?
How do earthquakes generate tsunamis?
How do landslides, volcanic eruptions, and meteorites generate tsunamis?
What happens to a tsunami as it approaches land?
What happens when a tsunami encounters land?
http://www.geophys.washington.edu/tsunami/general/physics/physics.html
Thnks for the link..took a while to load..the server must be swamped..bookmarked for tonite late.
I've never seen it on the web - just did a quick and dirty search and couldn't find it - but I've seen it plenty of times on television. It's 8mm, black and white and grainy, but you can see what's going on in it.
I believe the distinguished drunk from Massachusettes was the captain of an Oldsmobile.
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