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tsunamis:
tsunamis travel fast but not at infinite speed

 

 

 

a briefing document

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tsunamis: tsunamis travel fast but not at infinite speed vulcania
tectonic plates – floating on the surface of a cauldron
is a sub-document to this document and discusses tectonic plates, including their relationship to tsunamis
futuroscope
introduction
a tsunami strikes
lack of preparation sumatra tsunami present and past
causes of tsunamis end notes

 

 

 

 

 

 


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introduction

In the context of the Sumatra tsunami, this briefing document gives information on what is a tsunami, why do they occur, what are the results (wave size, speed, distance travelled) and the effects on humanity and the environment. As well as an wide-ranging investigation on tsunamis, this briefing document at abelard.org provides illustrations and diagrams, together with numerous links to specialised websites.

[Tsunami means “harbour wave” in Japanese.]click to return

 

a tsunami strikes

This is probably the main source tracking the Sumatra tsunami. An animation of the first few hours only, so far. Bangladesh appears to have escaped lightly, despite having 17 million people living less than one metre above sea level. Another animation of the first three hours. These show weaker waves going North and South, than the waves going East and West. This is probably why much of India and Bangladesh has been little affected.

Seismic diagram of the Sumatran quake of 26 December 2004. Image credit: UNESCO.
Seismic diagram of the Sumatran quake of 26 December 2004.
Image credit: UNESCO

terrestrial consequences of the sumatran undersea quake

  • “The shockwave shortened the period of our planet's rotation by some three microseconds. [...] [NASA] estimate that Earth now tilts by an extra 2.5 centimetres in the wake of the jolt.” [Quoted from nature.com]

  • “The earth is regularly deformed by the daily passage of the sun and moon, raising a tide in the earth of about 20cm, and the vibrations we saw on Boxing Day shortly after the earthquake off the Indonesian coast were of a similar magnitude on our instruments.”

    A fortnight after, “equipment was showing much of the planet was still ringing like a bell.[...] "The main signal we now see is a steady oscillation of a few parts in 10 billion of normal gravity, which corresponds to about a millimetre of vertical motion of the earth," ” [Quoted from Herald Sun]

A tsunami animation of the Pacific region [2.4mb .mov file].

There is a good write-up from the overseers of the US Tsunami Service, NOAA and Tsunamis.

“The waves radiate outward in all directions from the disturbance and can propagate across entire ocean basins. For example, in 1960 an earthquake in Chile caused a tsunami that swept across the Pacific to Japan. Tsunami waves are distinguished from ordinary ocean waves [1] by their great length between peaks, often exceeding 100 miles in the deep ocean, and by the long amount of time between these peaks, ranging from five minutes to an hour. The speed at which tsunamis travel depends on the ocean depth. A tsunami can exceed 500 mph in the deep ocean but slows to 20 or 30 mph in the shallow water near land. In less than 24 hours, a tsunami can cross the entire Pacific Ocean.

“In the deep ocean, a tsunami is barely noticeable and will only cause a small and slow rising and falling of the sea surface as it passes. Only as it approaches land does a tsunami become a hazard. As the tsunami approaches land and shallow water, the waves slow down and become compressed, causing them to grow in height. In the best of cases, the tsunami comes onshore like a quickly rising tide and causes a gentle flooding of low-lying coastal areas.”

(See also radar satellites capture tsunami wave images.)

The behaviour of a tsunami wave when caused by subduction [as happened with Sumatra]. With diagrams.

Mt Fuji seen from the sea, Hokusai, 
        woodcut, 1834
Mt Fuji seen from the sea, Hokusai, woodcut, 1834

When a wave hits the shore, it tends to act like a motorway pile-up to varying degrees. This is called the ‘run up’. A typical beach has a run up of 3; that is, a 5 foot wave will rise to 15 feet on beaching (three times the starting height of the wave).

This will, of course, depend on the beach slope. Naturally, a steep slope causes greater run up than a gentle beach. Even greater run ups would be expected in an inlet like a river or delta. Regions with run ups of approximately 40 are known.

Various types of tsunami give different patterns of waves formation and propagation, this is an area of study that is rapidly developing and involves computer modelling, real-world and experimental data. Study of the Shoemaker-Levy comet, which crashed into Jupiter in 1994, has resulted in increased confidence in computer models.

A common convenient, if grim, measure of the energy is by megatons of TNT, or numbers of Hiroshima-type bombs. Hiroshima was rated at about 15,000 tons of TNT.

click to return

sumatra tsunami present and past

  • “Many who escaped death in what was possibly the deadliest tsunami in more than 200 years now face hunger and disease. The United Nations mobilized what it called the biggest relief operation in its history [...] ”

Before:

Image credits: digitalglobe.com

Click images for full-size version.

Before the tsunami hit a part of Sri Lanka. Image credit: digitalglobe.com

After:

Note that previously gray roads and light-coloured compounds are now brown from floodwater.

After the tsunami hit part of Sri Lanka. Image credit: digitalglobe.com

  • The epicentre of the 9.0 Richter undersea quake that formed the tsunami of 26 December 2004 was about 155 miles from the north-western coast of Sumatra.

  • “A tsunami in 1883 at Krakatoa, off southern Sumatra, killed 36,000 and one in the South China Sea in 1782 killed 40,000, according to the U.S. National Geophysical Data Center.” [Quoted from Reuters]

  • “The deadly Asian earthquake may have permanently accelerated the Earth's rotation -- shortening days by a fraction of a second -- and caused the planet to wobble on its axis [...] ” [Quoted from tvnz.co.nz]

  • Present estimates suggest that this earthquake had the energy of more than 10,000 Hiroshima bombs, that is between 150 to 200 million tons of TNT

  • The wave went up to 5 kilometres inland.

  • The last large tectonic event also on this fault, that resulted in an earthquake, was in 1843. Thus the pressures and forces have had 162 years to build up. This is why the sea-quake was so powerful (9.0 on the Richter scale). The tectonic rupture is reported as being 1,200 kilometres long, and as occurring as three events, within seconds of each other. The first was to the west of North Sumatra, initiating two more slips further north.

  • The largest tsunami recorded measured 210 feet, about 18 stories above sea level, when it reached Siberia's Kamchatka Peninsula in 1737 - probably!
    [33 feet has been reported so far for the Sumatran tsunami.]

  • 2004 Sumatran quake disaster - toll so far exceeds over 250,000 [February, 2005]
    2003 earthquake in Bam, Iran - 6.6 Richter scale shake kills 30,000
    1976 Earthquake in Tangshan, China, kills 242,000
    1970 Cyclone in Bangladesh kills 500,000
    1923 Tokyo, Japan kills 140,000
    1887 China's Yellow River breaks its banks in Huayan Kou killing 900,000
    1883 Krakatoa kills 36,000
    1826 Tsunami kills 27,000 in Japan
    1815 Volcanic eruption of Mount Tambora on Indonesia's Sumbawa Island kills 90,000
    1556 Earthquake in China's Shanxi and Henan provinces kills 830,000

  • “The most damaging tsunami on record before 2004 was the one that killed an estimated 40,000 people in 1782 following an earthquake in the South China Sea. [...] In northern Chile more than 25,000 people were killed by a tsunami in 1868.

    “The Pacific is by far the most active tsunami zone, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). But tsunamis have been generated in other bodies of water, including the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans. North Atlantic tsunamis included the tsunami associated with the 1775 Lisbon earthquake that killed as many as 60,000 people in Portugal, Spain, and North Africa. This quake caused a tsunami as high as 23 feet (7 meters) in the Caribbean.”
    [Quoted from National Geographic.com] click to return

 

lack of preparation

“LOS ANGELES: US officials who detected a massive earthquake off Asia's coast tried frantically to warn the deadly wall of water was coming, the head of the Pacific Tsunami Warning Centre has said.

“But there was no official alert system in the region because such catastrophes only happen there about once every 700 years, said Charles McCreery, director of the National Oceanic and Atmospheric Administration's Centre in Honolulu.

“ "We tried to do what we could," McCreery said. "We don't have contacts in our address book for anybody in that part of the world."

“Within moments of detecting the quake, McCreery and his staff were on the phone to Australia, then to US Naval officials, various US embassies and finally the US State Department.”

“ US officials are now trying to help officials in the region set up some sort of informal warning system and feeling badly that more couldn't have been done, McCreery said.

“ "It took an hour and a half for the wave to get from the earthquake to Sri Lanka and an hour for it to get...to the west coast of Thailand and Malaysia," he said. "You can walk inland for 15 minutes to get to a safe area." ” [Quoted from stuff.co.nz]

Reports [30.12.04] are now giving estimates of well over 100,000 dead - approximately 1/60,000 of the world population, or the population of a UK town like Oxford (135,000); and maybe 5 millions washed out (getting on for 1/1000 of the world’s population).

warning signs

Most people think that a tsunami is one giant wave, like the ones they see surfers sliding down. A tsunami is more like a river bore, a “wall of water”, than a surfer’s wave tastefully curling over.

The first water wall is followed by other waves that can occur over a period of hours. In all, a tsunami is more like a coastal flood. Unless the observer is close to the originating cause, they will probably receive little warning of the event; that is, there will be no associated change in the weather.

On a beach, the only regularly reported clear warning sign that an dangerous sea event is going to happen soon is

all the sea retreats fast and far, maybe about 10 metres,
giving an unexpected and very sudden low tide.

Another warning sign is to see a relatively high line of frothing, white water on or near your sea horizon. That is the tsunami wave coming in. Run!

Move inland as fast as you can, and uphill if possible, taking others with you. Forget your belongings, save yourself.

Remember, the water that retreated abnormally will not stay retreated, it will return and, even it if it is not of tsunami proportions, will be a considerable mass of water.

If you are lucky enough to hear about a large sea-based earthquake in time, get away from the water’s edge - now, and stay away. Of course, major earthquake-related tsunami events occur rarely, but they are not the sort of phenomenon in which you would wish to participate.

As you will know, water en masse can often be dangerous or, at the very least, a nuisance. Every year, a few people are killed by flash floods, breaking dams, or even a fast-rising tide when they have become trapped on a beach without local knowledge.

In more naïve times, during a business lunch-hour, I sat on a rock, reading and sunning myself, alongside a fairly broad channel. I had the special treat of watching a fairly large naval ship moving down the channel. I then went back to my reading. Five or ten minutes later, I was up to my armpits in water and with my clothes floating away. I didn’t know that a ship’s wash could do that — then....

If you feel an earthquake or see a landslide when on the coast, at a beach, again think “tsunami” and move. Tsunamis are a likely consequence of an earthquake or a landslip (see next section).

It is possible that some animals may also behave abnormally, particularly some large mammals and some birds known to be able to sense very low-frequency sounds. Associated with earthquakes, infrasound (below human hearing, less than ~20 Hertz) travels through the the earth at several times the speed of sound in air, very much faster than tsunamis. Animals try to escape that which frightens them. This might provide a warning indicator, though difficult to verify.

click to return

Causes of tsunamis

  • earthquakes,
  • landslips or landslides,
  • asteroids.

earthquakes

From Earthquake Central, tectonic maps and many links.

Earthquake statistics.

Tectonic plates in Indoian Ocean region. Image credit:
Tectonic plates in Indian Ocean region. Image credit: U.S. Geological Survey

Indian Ocean tectonic plates.

Details for Sumatra area, Richter 9.

Preliminary Earthquake Report, with tectonic plate map (over large region)

“"From the size of the earthquake, it is likely that the average displacement on the fault plane was about fifteen meters. The sea floor overlying the thrust fault would have been uplifted by several meters as a result of the earthquake.”

  • Islands are being reported as moved laterally in the area by several metres have probably not moved sideways, but been upthrusted thus changing their coast lines.

  • large quakes from the past century:

    “[...] the magnitude 9.5 1960 Chile earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake, the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the magnitude 9.0 1952 Kamchatka earthquake.”

    List of significant earthquakes from 1556 to 2003.

Seismic map.

Marker at abelard.org

The Richter scale and the JMA Seismic Intensity Scale are two methods of describing earthquake activity.

  • The Richter scale is a measure, or derivation, of the energy released by the quake.

    A large tsunami requires an undersea quake of at least 7.5 on the Richter scale to form. A regional or local tsunami may be formed by a quake of about 6.5.

    The measurement of earthquakes using a scale was only developed in 1935, by Charles Richter at the California Institute of Technology.

Earthquake Severity - Richter scale
scale effects approx. annual frequency
Less than 3.5 Generally not felt, but recorded. millions
3.5 - 5.4 Often felt, but rarely causes damage.  
5.5 - 6.0 At most slight damage to well-designed buildings.
Can cause major damage to poorly constructed buildings over small regions.
 
6.1 - 6.9 Can be destructive in areas up to about 100 kilometres across where people live. 150
7.0 - 7.9 Major earthquake.
Can cause serious damage over larger areas.
15
8 and over Great earthquake.
Can cause serious damage in areas several hundred kilometres across.
1

The largest earthquake ever recorded by this method occured on 22 May 1960, reading 9.5 on the Richter scale, badly damaging much of Chile. That quake also generated a 10m/30ft tsunami that washed away whole villages in Chile and killed 61 in several hundred miles away in Hawaii.

11 March 2011: An earthquake, estimated at 8.9 on the Richter scale, hits mid and north-eastern Japan. An associated tsunami has reached across the Pacific Ocean to the western coasts of Canada and the United Staes of Amerca.

Marker at abelard.org

  • The JMA [Japanese Meteorological Agency] Seismic Intensity Scale lists the local effects of different levels of seismic intensity on people, buildings, services and ground.
JMA Seismic Intensity Scale
scale effects
0 Imperceptible to people.
1 Felt by only some people in the building.
2 Felt by most people in the building. Some people awaken.
3 Felt by most people in the building. Some people are frightened.
4 Many people are frightened. Some people try to escape from danger. Most sleeping people awaken.
5 lower Most people try to escape from a danger.Some people find it difficult to move.
5 upper Many people are considerably frightened and find it difficult to move.
6 lower Difficult to keep standing.
6 upper Impossible to keep standing and to move without crawling.
7 Thrown by the shaking and impossible to move at will.

related material

 

tectonic subduction

Subduction of one tectonic plate under another. Image credit: abelard.org, 2005

“What happened in Asia may give a vivid demonstration of the geologic future of the Pacific Northwest. For hundreds of years, these subduction zone plates remain locked, releasing little of their tension. The plate which is being subducted is forced down, while the plate above bulges upwards. Then, in a few minutes of violence every few centuries, the forces are released.[1] The upper plate moves seaward, and a massive tsunami can be produced along with catastrophic destruction from earthquake shaking.

“In the case of the Cascadia Subduction Zone, you could have an area of ocean sea floor that's 50 miles wide and 500-600 miles long suddenly snap back up, causing a huge tsunami," Goldfinger said. "At the same time, we could expect some parts of the upper, or North American plate to sink one to two meters. These are massive tectonic events. Subduction zones produce the most powerful earthquakes and tsunamis in the world.” [Quoted from oregonstate.edu]

Subduction diagram. Image credit: NASA
A NASA subduction diagram

“Description: The majority of earthquakes and volcanoes around the world occur at the intersection of plate boundaries. This diagram shows the subduction of an ocean plate underneath a continental plate. Earthquakes are caused by the two plates moving relative to each other, and volcanoes are formed when ocean crust, forced under the lighter continental crust, melts and then rises to the Earth's surface. The Andes have been forming over the past 170 million years as the Nazca Plate lying under the Pacific Ocean has forced its way under the South American Plate and pushed up its western edge. The subduction of one plate under the other has given rise to a number of volcanoes that dot the western edge of the mountain range.”

Diagram showing overthrust and subduction areas in Sumatra region.

click to return

landslips

A very long and useful item on the risk of landslide/landslip-generated tsunamis (illustrated). The Canary Islands and Hawaii are discussed in detail.

Tambora [ from Enc. Brit.]

  • The largest eruption since 1700 occurred at Tambora Volcano on Sumbawa Island, Indonesia, on April 10-11, 1815.
  • Fifty cubic kilometres of magma were expelled in Plinian ash clouds and pyroclastic flows.
  • Ash greater than one centimetre thick fell on more than 500,000 square kilometres of Indonesia and the Java Sea.
  • Before the eruption Tambora was a 4,300-metre-high stratovolcano; following the eruption 1,400 metres of the summit cone were missing and in its place was a collapsed caldera measuring six by seven kilometres wide and one kilometre deep.
  • About 10,000 people were killed by the explosive eruption and the tsunamis caused by massive pyroclastic flows entering the sea.
  • Agricultural losses from the thick ash deposits resulted in famine and disease, leading to an additional 82,000 deaths.

Fifty cubic kilometres is approximately 12 cubic miles [1 cubic km ~ 4.168 cubic miles], the amount a 500-yard diameter asteroid could be expected to throw into the air, assuming a 50,000 mph impact.

Krakatoa (Also in Indonesia)

The second largest eruption of the 19th century also occurred in Indonesia. Krakatoa (or Krakatau), a compound volcano on a small uninhabited island between Sumatra and Java, erupted explosively on 26-27 August, 1883. The eruption was similar to the Tambora outburst but smaller, involving only about 18 cubic kilometres of magma erupted in Plinian ash clouds and pyroclastic flows. Krakatoa was a much smaller volcano than Tambora, and when the eruption had emptied its magma chamber, it collapsed to form a caldera that was partly below sea level.

Twenty-three square kilometres of the island of Krakatoa disappeared, and where a volcanic peak 450 metres high once stood was water as deep as 275 metres. The largest explosion on the morning of August 27 produced an ash cloud that was reported to have reached 80 kilometres high, and the detonation was heard in Australia, 4,800 kilometres away. A tsunami, over 30 metres high, followed the explosion and apparent caldera collapse, killing about 36,000 people on the adjacent shores of Java and Sumatra.

 

asteroids and meteors

About one third of the world is land, so an asteroid or meteor impact is more likely to occur at sea.

On a big messy site about tsunamis with vast numbers of links, there is a section on asteroids.

Large asteroids also form a ‘chimney’ in the atmosphere, by which large quantities of material may be sucked into the atmosphere. Thus the effect of an asteroid can be greater than a major volcanic eruption. The Tambora event in 1815 (also in Indonesia) is associated with “the year with no summer”, causing major crop failures around the world.

The effects of large amounts of water being vapourised and ejected into the upper atmosphere is yet unknown. Remember, water vapour is a major greenhouse gas.

There are approximate frequencies of asteroid/meteor impact by size, as calculated from past data. The force of an asteroid varies with size and speed of impact. It has effects similar to a pebble dropped in a pond, but can be a damned big, fast pebble.

approximate probabilities of various levels of strike impact
impact diameter  kinetic energy area devastated average interval (years) between asteroid impacts
(m)  Mt  TNT sq km anywhere on Earth on a ‘city’ on inhabited region /
expected death toll
50  10 1900 100   30,000,000 900 yr
1,000,000
100 75 7200  1000 70,000,000 8000 yr
3,000,000
200 600 29,000 5000 90,000,000 30,000 yr
14,000,000
500 10,000 70,000 40,000 290,000,000 180,000 yr
30,000,000
1 km  75,000 200,000 100,000 260,000,000 290,000 yr
60,000,000
2 km  1,000,000 -  1,000,000 - 1 ,000,000 yr
1.5 billion
based on the impact of a stony asteroid,
assuming velocity=20km/s [approx. 72,000 kph/45,000 mph], density=3 g/cc

Table derived from section on asteroids. You want to study asteroids further? Go to the linked site!
 

Acknowledgement

With additional research from the auroran sunset, (detail reporting).

 

click to return

end notes

  1. Ordinary ocean and lake waves are formed by the friction of wind on the water. Thus, they are kicked up according to the intensity of the wind. Waves are not a movement of water forward, the movement only is transmitted through the water, molecule to molecule. Hence, the speed of a tsunami can be much greater, because of the much greater generating forces. As with the waves and also with a tsunami, it is the wave that travels and not the water.

    A tectonic plate slip can be a very fast, explosive event. Think in terms of bending a stick, where the tension gradually increases and then suddenly, the stick snaps back. If you want to see this happen, push a fairly thin green branch at an angle, up against a brick or concrete wall and the stick will bend, and then at some point the branch will slip and spring back. (If you are youthful, don’t do this without supervision; and if you’re more experienced, take precautions, because the energy when the branch snaps back can be dangerous.) The speed of the event triggering the tsunami will determine the speed of the propagating wave, and the size/volume of the event will determine the size of the wave.


    Windows Media Player

    Of course, all earthquakes are not equal. They happen at various depths and, to be sure, some happen on land. The depth of water where the event occurs can also differ greatly. In fact, it often takes considerable time before all the data is gathered and analysed, and a clear idea of the position and power of the event is assessed.

    The present estimate for the Chilean earthquake [February 2010] is it occured at a depth of 21 miles, whereas the Haitian earthquake [January 2010] is estimated to have been at about 8 miles depth.

    With some earthquakes, much of the energy will dispersed sideways. In others, the snapback - that lifts and drops vast masses of water - may be dominant.

    how fast does a tsunami travel - the chilean event

    The 27th February, 2010 Chilean main event, at 06:34:17, was originally assessed at 8.8 on the Richter scale.

    This much milder tsunami event, relative to the Sumatrian ’quake-generated tsunami, took about 12 hours to reach Hawaii and about 13 to 14 hours to reach New Zealand. It was detected in Japan after a little over 24 hours, a distance of about 17,000 km or over 10,000 miles.

    So the tsunami appears to have been propagated at approximately 700 kph or 400 mph.

    Error: Thread 1474 does not exist.


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