Edited by Gerald Boerner

    

    
Commentary

JerryPhotoI sat in awe as I watched the Tsunami the hit Sendai, Japan, today. To see that thirty foot high wall of water rushing over the countryside taking houses, trains, cars and any other obstacle out as it progressed. Here in Southern California we live with earthquakes of 6.0 + range, but the quake that generated today’s tsunami was an 8.9, about a thousand times more powerful than the largest one that I remember (an 7.1).

The area that generated this tsunami was about 150 miles by 500 miles in size — larger than anything that I have ever thought of. I remember seeing History Channel programs about the Dam Buster raids in the Ruhr valley during World War II. I also remember seeing the sequence in the movie, “Force 10 from Navarone,” that used the force of water from a “blown” dam to destroy a bridge that could not be taken down by explosives. Such is the power of water!

Tsunami2

The current post presents some basic facts about tsunamis for your understanding. I have included an excerpt from a Washington Post article as well as a short video of the event. Let us remember the victims and survivors of this tragedy in our prayers… GLB

These Introductory Comments are copyrighted:
Copyright©2011 — Gerald Boerner — All Rights Reserved

[ 3050 Words ]
    

      

Quotations Related to TSUNAMI:

 

“Being here, it is just impossible to imagine what that was like, when the tsunami hit.”
— Connie Sellecca

“Britain can be proud of its response to the tsunami appeal.”
— Gordon Brown

“What is perhaps more worthy of note than how many tsunami dead we’ve seen, however, is how many other recent dead we have not seen.”
— Bruce Jackson

“A massive state and federal effort, the likes of which we’ve never seen is going to be needed. We can do it for tsunami victims half a world away. We can do it for our own citizens.”
— Al Roker

“Did you know that the word "tsunami," which is now being used worldwide, is a Japanese word? This is indicative of the extent to which Japan has been subject to frequent tsunami disasters in the past.”
— Junichiro Koizumi

“First, those images help us understand the general and specific magnitude of disaster caused by the tsunami. The huge outpouring of aid would not have happened without those images.”
— Bruce Jackson

“The generosity of the American public toward the victims of Hurricane Katrina and the Tsunami has been reflected in the outpouring of support for the Pakistani earthquake victims.”
— Jon Porter

“We think we have a responsibility. And I think it’s important for all of us in the Western world to realize that we’ve all been blessed a lot and if you go to these parts they don’t have a lot, even before the tsunami.”
— Kevin Rollins

    

Special Exploration: What is a “Tsunami”

    

    
2004-tsunamiA tsunami is a series of water waves (called a tsunami wave train) caused by the displacement of a large volume of a body of water, usually an ocean, but can occur in large lakes. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Owing to the immense volumes of water and the high energy involved, tsunamis can devastate coastal regions.

Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.

The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes, but the understanding of a tsunami’s nature remained slim until the 20th century and is the subject of ongoing research. Many early geological, geographical, and oceanographic texts refer to tsunamis as "seismic sea waves."

Some meteorological conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called a meteotsunami, which can raise tides several meters above normal levels. The displacement comes from low atmospheric pressure within the center of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge inundated Burma in May 2008.

    

History of the Concept

The term tsunami comes from the Japanese, meaning "harbor" (tsu, ) and "wave" (nami, ). Tsunami are sometimes referred to as tidal waves. In recent years, this term has fallen out of favor, especially in the scientific community, because tsunami actually have nothing to do with tides. The once-popular term derives from their most common appearance, which is that of an extraordinarily high tidal bore. Tsunami and tides both produce waves of water that move inland, but in the case of tsunami the inland movement of water is much greater and lasts for a longer period, giving the impression of an incredibly high tide. Although the meanings of "tidal" include "resembling" or "having the form or character of" the tides, and the term tsunami is no more accurate because tsunami are not limited to harbors, use of the term tidal wave is discouraged by geologists and oceanographers.

As early as 426 B.C. the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and was the first to argue that ocean earthquakes must be the cause.

The cause, in my opinion, of this phenomenon must be sought in the earthquake. At the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation. Without an earthquake I do not see how such an accident could happen.

The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15-19) described the typical sequence of a tsunami, including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave, after the 365 A.D. tsunami devastated Alexandria.

    
Generation Mechanisms

The principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to either earthquakes, landslides, volcanic eruptions, or more rarely by meteorites and nuclear tests. The waves formed in this way are then sustained by gravity. It is important to note that tides do not play any part in the generation of tsunamis, hence referring to tsunamis as ‘tidal waves’ is inaccurate.

    
Seismicity Generated Tsunamis

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth’s crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.

Eq-gen1Drawing of tectonic plate boundary before earthquake.
    

Atwater.iddOverriding plate bulges under strain, causing tectonic uplift.
    

Eq-gen3Plate slips, causing subsidence and releasing energy into water.
    

Atwater.iddThe energy released produces tsunami waves.
    

Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimeters (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

On April 1, 1946, a magnitude-7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai’i with a 14 meters (46 ft) high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.

Examples of tsunami at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before traveling transoceanic distances.

The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates (methane etc.)

In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could be caused by giant landslides. These phenomena rapidly displace large water volumes, as energy from falling debris or expansion transfers to the water at a rate faster than the water can absorb. Their existence was confirmed in 1958, when a giant landslide in Lituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 metres (over 1700 feet). The wave didn’t travel far, as it struck land almost immediately. Two people fishing in the bay were killed, but another boat amazingly managed to ride the wave. Scientists named these waves megatsunami.

Scientists discovered that extremely large landslides from volcanic island collapses can generate megatsunami, that can travel trans-oceanic distances.

    
Characteristics

Propagation_du_tsunami_en_profondeur_variableWhen the wave enters shallow water, it slows down and its
amplitude (height) increases.
    

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 m (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 km (120 mi). Such a wave travels at well over 800 km per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 m (3.3 ft). This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously, producing a distinctly visible wave. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami’s wave peak reaches the shore, the resulting temporary rise in sea level is termed ‘run up’. Run up is measured in metres above a reference sea level. A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.

About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, and bolides.

    

Warnings and Predictions

 

Drawbacks can serve as a brief warning. People who observe drawback (many survivors report an accompanying sucking sound), can survive only if they immediately run for high ground or seek the upper floors of nearby buildings. In 2004, ten-year old Tilly Smith of Surrey, England, was on Maikhao beach in Phuket, Thailand with her parents and sister, and having learned about tsunamis recently in school, told her family that a tsunami might be imminent. Her parents warned others minutes before the wave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney.

Dart_tsunamicoverOne of the deep water buoys used
in the DART tsunami warning system
    

In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other eastern coasts it reached. This was because the wave moved downwards on the eastern side of the fault line and upwards on the western side. The western pulse hit coastal Africa and other western areas.

A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, and seismologists analyse each earthquake and based on many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and automated systems can provide warnings immediately after an earthquake in time to save lives. One of the most successful systems uses bottom pressure sensors that are attached to buoys.

As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat for all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is being installed in the Indian Ocean.

    
Computer Models

Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors relay information in real time. Based on these pressure readings and other seismic information and the seafloor’s shape (bathymetry) and coastal topography, the models estimate the amplitude and surge height of the approaching tsunami. All Pacific Rim countries collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures. In Japan, such preparation is mandatory for government, local authorities, emergency services and the population.

Some zoologists hypothesize that some animal species have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. If correct, monitoring their behavior could provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and is not widely accepted. There are unsubstantiated claims about the Lisbon quake that some animals escaped to higher ground, while many other animals in the same areas drowned. The phenomenon was also noted by media sources in Sri Lanka in the 2004 Indian Ocean earthquake. It is possible that certain animals (e.g., elephants) may have heard the sounds of the tsunami as it approached the coast. The elephants’ reaction was to move away from the approaching noise. By contrast, some humans went to the shore to investigate and many drowned as a result.

It is not possible to prevent a tsunami. However, in some tsunami-prone countries some earthquake engineering measures have been taken to reduce the damage caused on shore. Japan, where tsunami science and response measures first began following a disaster in 1896, has produced ever-more elaborate countermeasures and response plans. That country has built many tsunami walls of up to 4.5 m (15 ft) to protect populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunami. However, their effectiveness has been questioned, as tsunami often overtop the barriers. For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake on July 12, 1993 created waves as much as 30 m (100 ft) tall—as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it did not prevent major destruction and loss of life.

Natural factors such as shoreline tree cover can mitigate tsunami effects. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed because trees such as coconut palms and mangroves absorbed the tsunami’s energy. In one striking example, the village of Naluvedapathy in India’s Tamil Nadu region suffered only minimal damage and few deaths because the wave broke against a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records. Environmentalists have suggested tree planting along tsunami-prone seacoasts. Trees require years to grow to a useful size, but such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than artificial barriers.

    

Washington Post Coverage of Today’s Tsunami in Sendai, Japan

    

    
Japan tsunami spares major economic zones

By Howard Schneider

Washington Post Staff Writer
Friday, March 11, 2011; 6:12 PM

The earthquake and tsunami in Japan on Friday struck an area that accounts for only a small fraction of the country’s economic activity, but damage could still run into the tens of billions of dollars, according to analysts trying to assess the impact of the disaster.

Hard-hit Miyagi Prefecture is the source of only 1.7 percent of Japan’s gross domestic product, and damage to industrial and commercial facilities in the area appeared to be limited.

Still, the earthquake and subsequent flood struck at a time when Japan is struggling to pull itself out of recession and facing pressure to curb the heaviest public debt load in the world. Any downturn in economic activity resulting from the disaster, at least in the short term, could undercut Japan’s tentative recovery, analysts said, and could force the country to delay efforts to reduce its debt and annual deficits while it rebuilds.

"The timing of the disaster could not have been much worse," according to an analysis written by Japanese and other international economists at London-based Capital Economics. The Japanese government is already divided over how to tame Japan’s debt. So, the economists warned, "the greater the social and economic damage, the larger the threat to the government’s ability and willingness to ward off a fiscal crisis."

The Bank of Japan announced Friday that it would accelerate a meeting scheduled for next week and ensure that banks and the financial system have the funds needed to conduct business.

The human toll and the amount of property destruction are so far unknown, and damage at a nuclear power plant remained a concern. Millions of homes were without power, and public transportation systems in major cities including Tokyo were shut down. […]
    

    

     

References

    

Background information is from Wikipedia articles on:

Wikipedia: Tsunami…
http://en.wikipedia.org/wiki/Tsunami

Washington Post: Japan Tsunami Spares Major Economic Zones…
http://www.washingtonpost.com/wp-dyn/content/article/2011/03/11/AR2011031105407.html?hpid=topnews

Brainy Quote: TSUNAMI Quotes…
http://www.brainyquote.com/quotes/keywords/tsunami.html

    

Other Posts on related Topics:

Prof. Boerner’s Exploration: Hurricane Katrina: Impact on New Orleans…
http://www.boerner.net/jboerner/?p=14131