Mega-Tsunamis: The Truth Behind the Discovery Channel Special

[JustStuit]

Have you guyz heard about these Mega-Tsunamis? They form from large landslides and can be much taller and more desructive than a tsunami. A show on discovery said that a part of La Palma island off Africa is going to break off in like a century or so. It would create a huge wave headed to America. Whats your opinions on this?

They showed this on a show on discovery, and they may have exaggerated some things because Wikipedia said different stuff.

http://en.wikipedia.org/wiki/Megatsunami

and the discovery show was
http://dsc.discovery.com/schedule/e...d=0&channel=DSC
but the link says little about it.

 

[Andre]

Well, hypes are quite popular nowadays since we don't seem to face the nuclear winter anymore.

For a counter expertise opinion:

http://www.lapalma-tsunami.com/

Having been there myself (awesome island), I have not seen any evidence of those dropping blocks indeed.

 

[JustStuit]

Thanks for the website. I had a feeling they were exaggerated this, and many of the facts made no sense or didn't add up. I figured the media would twist and make up many things.

 

[LURCH]

I tend do imediately discount any source that starts with a statement like:

FACT: There has NEVER been a megatsunami in the Atlantic or Pacific oceans. NEVER

 

[Ivan Seeking]

I tend do imediately discount any source that starts with a statement like:

I agree. It's better to stick with reputable sources.

According to a new model by Simon Day of University College London and Steven Ward of the University of California, this mega-tsunami, which they describe in the September 2001 issue of Geophysical Research Letters, would rouse waves 100 meters high along the West Saharan shore, seas more than 40 meters high on the north coast of Brazil, and water walls towering 50 meters in the air off the coast of Florida and in the Caribbean. Britain, Spain, Portugal and France would also face sizeable waves. [continued]

http://www.sciam.com/article.cfm?articleID=0006DF17-5A60-1C61-B882809EC588ED9F

Mega-tsunamis are defined as waves of over
100-meter height in the deep ocean. The
primary source of mega-tsunamis are
asteroid impacts with the ocean, volcanic
explosions, and landslides. A mega-tsunami
did occur in 1958 in Lituya Bay, Alaska;
however the mechanism of its formation is
uncertain. Various models for the event are
described in [1].

Lituya Bay, Alaska is on the northeast shore
of the Gulf of Alaska. It is an ice-scoured
tidal inlet with a maximum depth of 220
meters and a narrow entrance with a depth
of only 10 meters. It is a T-shaped bay, 7-
miles long and up to 2-miles wide. The two
arms at the head of the bay, Gilbert and
Crillon Inlets, are part of a trench along the
the Fairweather Fault. On July 8, 1958, a
7.5-magnitude earthquake occurred along
the Fairweather fault with an epicenter near
Lituya Bay.

A mega-tsunami wave was generated that
washed out trees to a maximum altitude of
520 meters at the entrance of Gilbert Inlet. [continued]

http://t14web.lanl.gov/Research/TDAC2000/mader.tsunami.00.pdf

 

[Integral]

I also have seen the Discovery channel show on Mega Tsu. I was not real impressed with the experimental setup they used nor was I convenced that it would accuratly reflect what would happen in a deep water large ocean invironment. The apparatus they used may well model the Alaska mega Tsu very well but it is not clear that they considered that in a ocean environment the wave would spread on a circiular front, the wave that strikes the US coast line would be part of a wave with a radius of several thousand miles, this would spread the energy of the souce over a tremoundous area of the ocean.

Qualifier, I have not done any of the computations, perhaps the researchers did a better job then was presented on the TV.

 

[Ivan Seeking]

Hundreds of terrorists wielding crowbars and large levers have been spotted on La Palma island.

 

[Andre]

http://www.lapalma-tsunami.com/mader_lapalma.pdf
http://homepage.eircom.net/~rockyenterprises/La_Palma_grl.pdf

It still occurs to me that the numbers may be slightly exagarated.

 

[matthyaouw]

It is an ice-scoured
tidal inlet with a maximum depth of 220
meters and a narrow entrance with a depth
of only 10 meters. It is a T-shaped bay, 7-
miles long and up to 2-miles wide.

It does strike me as a bit of a poor example really. The dimensions are hardly representative of a large ocean.

My computer seems to refuse to view PDFs, so forgive me if the point has been addressed in any of the articles linked to here.

 

[LURCH]

Yeah, I agree that the model and the one observed example leave a huge gap to leap when we try to model ;arger waves in the deep ocean. However, haven't geologists found evidence of wave impacts at hundreds of meters in altitude on both the Hawian and Coastal mountain ranges?

 

[matthyaouw]

However, haven't geologists found evidence of wave impacts at hundreds of meters in altitude on both the Hawian and Coastal mountain ranges?

I can't remember where I heard this, but some believe that other factors like local uplift and subsidence can account for the sediment found on the Hawaiian Islands.

 

[Amp1]

MEGA TSUNAMI DISASTER that faces the US

I also saw the program on discovery and posted about it under my first member ID Amp (since then I haven't been able to use it-forgot password). But getting back to the topic, the mega tsunami would be caused by a large hunk of the island plunging into the ocean. This break off of a chunk of the island is caused in part by the active volcanism in the island, there are vertical shafts through out a fairly large area of the island - centrally located which are filled with or partially filled with water (from the last eruptive cycle of the islands volcano), what has been posited to happen is in the event of a new upwelling of magma, this water would be super heated, turning into steam under tremendous pressure because the previously mentioned vertical shafts are capped. The pressure of this steam it is thought will be great enough to cause huge areas of the island to slough off falling into the ocean on either side of the island this is what would generate the tsunamis. These tsunamis would indeed spread like a ripple similar to throwing a stone in a pond but once they reached the continental shelf traveling to the east coast of the US, they would behave like the waves that come ashore at Hawaiis' famous shores, towering as they build up to a crest.

 

[LURCH]

I can't remember where I heard this, but some believe that other factors like local uplift and subsidence can account for the sediment found on the Hawaiian Islands.

Just saw http://www.soest.hawaii.edu/SOEST_News/News/AGU2003/gfryer.htm which talks about evidence of marine fossils at over 1,000 ft altitude in an area where the rate of uplift is considered to be pretty well undeerstood, on the island of Kohala.

 

[matthyaouw]

Interesting, thanks. A megatsunami does seem to be the most probable cause.

 

[Ophiolite]

It's technically not a megtsunami, but it's still pretty big.
Around 8100 years ago, one of the largest landslides in the world occurred at Storegga, 100 kilometres north west of the Møre coast. An area the size of Iceland slid into the Norwegian Sea.
The slide, which ended up at a depth of 300-2500 metres, created a 10-20 metre high tidal wave that reached the Norwegian coast.
The mass slid around 800 kilometres into the deep sea, and its back edge is around 300 kilometres long. The Ormen Lange field is in the middle of the depression left behind by the Storegga slide and is close to the steep slide edge which rises 200-300 metres up towards the continental shelf.
From http://www.ormenlange.com/en/about_ormen/key_features/storegga_slide/

 

[Andre]

However, it was the largest recent continental slope landslide on record with 3500+ cubic kilometers and may have caused the indications that we interpret as the Holocene 8100bp cold event. Furthermore the preceeding clathrate decomposition killed the mammoths:

https://www.physicsforums.com/showthread.php?t=80137

 

[Cybersteve]

I think I saw the documentary in question and it left me with a couple of questions.
Surely what would be as important as the volume of rock is the speed at which it moved? The model they were using to demonstrate this event looked to me as if it could just slide gracefully into the sea. It could only reach a decent speed by accelerating couldn't it? Wouldn't this cause it to break up?

The other point is that what is so damaging with a tsunami is not so much the height of the wave but its length from front to back. More accurately I suppose its the volume of water it contains. Could a relatively small mountain/hill really move anywhere near as much water as the raising of the seabed did in last year's tsunami?

 

[Ophiolite]

. Could a relatively small mountain/hill really move anywhere near as much water as the raising of the seabed did in last year's tsunami?

I wouldn't call 3500 cubic kilometres small. Let's do a back of the envelope calculation for the tsunami.
Greatest movement was 30ft or 30mtr? I don't remember. We'll take worst case. That's the movement at the epicentre. As we move back it diminishes. But we'll take worst case and assume it doesn't. Epicentre to coastline. Say 100kms. (I'm sure it's less, but worst case again.) Over what length? Let's say 1000kms. So the volume of rock moved = 1000 x 100 x 0.03 = 3000.
So, at a maximum, with unrealistic worst case scenarios for each number the tsunami just manages to come in at the same order of magnitude as the Norwegian slide.

 

[Cybersteve]

I think I didn't make my post clear. I was referring to the island of La Palma that the original quetion was about. I think I'm right in saying that the whole island is only about 15 km x 8km, I can't remeber how high it is but I'm sure it could be high enough to give a volume of 3000 cu Km!

I also seem to remember an article in the press saying that the sea around the island wasn't deep enough to allow the mountain to slide into it, which would also limit the effect!

 

[Andre]

La Palma is about 3-4 times bigger than that and a beautiful place. Try google Earth and fly into the huge Caldera de Taburiente

Here is the story:

http://www.es.ucsc.edu/~ward/papers/La_Palma_grl.pdf

But I think that's a little exagarated

 

[Ophiolite]

Sorry Cybersteve. I misunderstood what you were relying to. The total moving mass is indeed smaller for La Palma than for the crust in the Asian tsunami. However, the velocity and distance traveled are very much greater for the former, if the assumptions made and model used by the researchers are accurate. I have not read the original paper(s), but given they passed peer review I accept that the thesis is at least plausible.

 

[Andre]

Here is are latest Storega slide papers and tsunami's.

http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V9Y-4FD13HG-5-3&_cdi=5911&_user=10&_orig=browse&_coverDate=01%2F01%2F2005&_sk=999779998&view=c&wchp=dGLbVlz-zSkzS&md5=7374ce82608a562009478ff893a498c9&ie=/sdarticle.pdf

and

http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V9Y-4FDNBWD-3-T&_cdi=5911&_user=10&_orig=browse&_coverDate=01%2F01%2F2005&_sk=999779998&view=c&wchp=dGLbVlz-zSkzS&md5=a1eb369c6d33d86597abb2f954b3934a&ie=/sdarticle.pdf

Actually I think I may have found out that the key to the solution of the ice age mechanism is in the continental slope slides. This is the key article:

http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V9Y-4FMK0M1-1-1&_cdi=5911&_user=10&_orig=browse&_coverDate=01%2F01%2F2005&_sk=999779998&view=c&wchp=dGLbVlz-zSkzS&md5=4bc59d44cc81c317d90a02b59ee064ff&ie=/sdarticle.pdf

 

[Mk]

I'm surprised nobody has brought up rogue waves?
http://en.wikipedia.org/wiki/Monster_wave

 

[Ophiolite]

Well, their origin, behaviour, context, location and effects are all quite different from tsunamis. Interesting as they are, they are a quite different beast.

 

[jhe1984]

I watched that documentary as well in Geo class. Along with the concern in the Azores (or maybe it was Canary (?)) Islands, there was also a segment about the effects of a mega-tsunami in Alaska - they interviewed a fisherman who survived it! He said the wave was super tall and the impact showed it: the treeline ended up something like 150 ft before the shore.

Mega-tsunamis would be much different than rogue waves as they would hardly be noticeable on the ocean water, while rogue waves can (and have!) plowed into ships at sea.

A mega-tsunami would also move at a VERY high velocity.

However, the land breaks they are talking about are only relevant to places with pyroclastic (explosive) lava, as opposed to milder, "pillow" lava that is slow moving and not violently explosive. The documentary was very much concerned with the Azores because the resulting tsunami would submerge theoretically, the entire Eastern seaboard.

May geophysicists excuse any butchering of fact in above post