Facts and Details

TSUNAMIS

Damage from 1896 tsunami in Japan A tsunami is a series of waves produced by an event in the ocean such as an underwater landslide or an earthquake that cases large amounts of seawater to be displaced . Sometimes they can be very large. Catastrophic ones occur a few times a century, Other times they are so small they virtually undetectable.

For a long time tsunamis were erroneously called tidal waves. A tidal wave according to Oxford English Dictionary is a “high water wave caused by the movement of the tide.” It is different from a storm surge a flood of water that occur when water pushed inland by a storm such as a typhoon coincides with a high tide. The word tsunami is derived from the Japanese tsu (“harbor”) and nami (“waves”).

Most tsunamis are comprised of a series of crests and troughs called a “wave train” and have a leading wave followed by crests that push it from behind. Damages is not caused so much by huge wall of water crashing down like a large beach wave but rather by a surge of water than pushes far inland. The waves comes in series, sometimes with many minutes passing from one to the next.

Waves a meter high can severely damage houses. A two-meter-high tsunami can destroy wooden buildings. Often the damage caused by receding waves being sucked to the sea is more severe than that caused by advancing waves. Receding waves can also drag people far out to sea.

tsunami refuge in Japan The height of a wave can differ significantly depending on the contour of the land areas that are stricken. A narrow creek can funnel such waves, causing them to rise to a height of 10 meters. If the earthquake that generates the waves is nearby the waves come in rapid succession. If the earthquake is farther away the waves can arrive over a period of several hours.

Good Websites and Sources: Wikipedia article on Tsunamis Wikipedia ; Surviving a Tsunami, Lessons from Chile, Hawaii and Japan pubs.usgs.gov ; Tsunami Warning System in Japan jma.go.jp/jma ; Tsunami Warnings from Japan Meteorological Agency jma.go.jp/en/tsunami ; Book: Tsunami: The Underrated Hazard by Edward Bryant. Tsunamis That Struck Japan Major Tsunamis in Japan in the 20th Century tsunami.civil.tohoku.ac.jp ; Major Earthquakes and Tsunamis in Japan in the 20th Century drgeorgepc.com ; 1933 Earthquake and Tsunami pdf file cidbimena.desastres.hn ; 1983 Tsunami drgeorgepc.com ; Report on the 1993 Tsunami nctr.pmel.noaa.gov ; Small Tsunami in 2010 reuters.com ;

Links in this Website: VOLCANOS AND JAPAN Factsanddetails.com/Japan ; MAJOR VOLCANOS AND ERUPTIONS IN JAPAN Factsanddetails.com/Japan ; EARTHQUAKES AND JAPAN Factsanddetails.com/Japan ; EARTHQUAKES AND LIFE IN JAPAN Factsanddetails.com/Japan ; LARGE EARTHQUAKES IN JAPAN Factsanddetails.com/Japan ; KOBE EARTHQUAKE OF 1995 Factsanddetails.com/Japan ; LARGE EARTHQUAKES IN JAPAN IN THE 2000s Factsanddetails.com/Japan ; TSUNAMIS IN JAPAN Factsanddetails.com/Japan

physics of tsunami wave
as it approaches a shore Good Websites and Sources on Earthquakes: U.S. Geological Survey (USGS) National Earthquake Information Center earthquake.usgs.gov ; Wikipedia article on Earthquakes Wikipedia ; Earthquake severity pubs.usgs.gov ; USGS Earthquake Frequently Asked Questions earthquake.usgs.gov/learn/faq ; Collection of Images from Historic Earthquakes Pacific Earthquake Engineering Research Center, Jan Kozak Collection ; World Earthquake Map iris.edu/seismon ; Most Recent Earthquakes earthquake.usgs.gov ; Interactive Earthquake Guide guardian.co.uk ; USGS Earthquakes for Kids earthquake.usgs.gov/learn/kids ; Earthquake Preparedness and Safety Surviving an Earthquake edu4hazards.org ; Earthquake Pamphlet pubs.usgs.gov ; Earthquake Preparedness Guide earthquakepreparednessguide.com ; Earthquake Safety Site earthquakecountry.info ;

Physics of Tsunamis

Tsunamis are caused when there is a movement of land on the ocean floor and this displaces water causing one area of water to be higher than an other. The water at the surface starts to shift downhill and that is what triggers the tsunami. Tsunamis tend to maintain their force as they travel through the deep seas at speeds of a jetliner. While normal waves are movements of water along the surface of the sea tsunamis are forces that move through the sea sort of like a series of dominoes knocking over the dominoes in front of it. When a tsunami strikes a shallow ocean bottom it loses some energy to friction. In the open sea there is nothing to slow it down.

Tsunami waves usually strike in a train of a dozen or more waves rather than a single wave with each wave slightly weaker that the one in front of it. The distance between the swells can be a be a kilometer or more.

Tsunamis waves generally radiate out in directions opposite from the seismic disturbance. They are distinguished from coastal waves in that can be very long and move at very high speeds. A single wave from a tsunami can be 160 kilometers long and move across the ocean at 965 kilometers per hour.

Tsunamis move like sound waves through the water and get slower and more powerful at the surface as they move through shallow water. When they approach the coast the slow dramatically as their wavelength shortens, causing them to rise up dramatically because of the enormous volume of water that piles up in the shallow water. The top travels at fast speed which increases its height.

A super powerful tsunami can move at 900kph in deep water, producing only a ripple at the surface, not even noticed by ships in the open sea. It slows to 300kph at the continental shelf and then suddenly rises to 20 meters when it hits the shore at 35 kph, potentially killing thousands. When a wave slows it does not lose energy it simply rearranges it. In many case the full energy of a tsunami is not released until it hits something.

When the water off the coast is very deep, tsunamis come a shore like a high tide that just keeps coming and coming. When the water is shallow large violent, breaking waves are more likely. When trains of waves hit shallow water the leading waves slows down first and the waves behind them slow down less rapidly and pile on from behind, shortened the distance between the waves and adding to the height of the leading waves. Hours after an earthquake tsunamis loose energy because of friction over the rough sea bottom and/or the dissipation of energy over long distances.

Causes of Tsunamis

Hawaii 1946 Tsunamis can be caused by earthquakes, earthquake-induced landslides, non-earthquake landslides caused by water from aquifers, volcanic eruptions, and collapsing sea cliffs and explosions on volcanic islands, volcanic eruptions or meteor or asteroid impacts. The most common and destructive tsunamis are caused by earthquakes.

Large tsunamis are typically the result of quakes known as subduction earthquakes. They're the most powerful earthquakes on the planet, and they occur at plate boundaries, where one tectonic plate is grinding inexorably beneath another. When the bottom plate suddenly lurches deeper, a colossal amount of energy is released, unleashing the sorts of massive earthquakes and calamitous tsunamis that hit the Indian Ocean in 2004 and the coast of Japan in March 2011. Generally an earthquake of 7.5 of larger is needed to deform the sea bottom enough to displace enough water to produce tsunamis wave pulses that can travel long distances across the ocean. These earthquakes typically occur along thrust faults, where an ocean plate is thrust under a continental plate, dragging it down until the fault ruptures, causing an earthquake that cause the sea floor to rise.

Landslides caused by earthquakes, rock falls or ice falls can produce enormous local tsunamis. Places that are vulnerable to these events include the unstable flanks of volcanos and continental shelves with huge deposit of sediments. Some of the largest tsunamis have been generated by earthquake-induced landslides. These can occur after any large earthquake, say 6.5 on the Richter scale and above. An earthquake in Lutuya Bay, Alaska in 1958 caused a massive rock fall that generated a tsunami that was 1,720 feet high, 267 feet higher than the Empire State Building. Fortunately it struck a wilderness area and did not travel across the sea to Japan or Hawaii.

deaths from the 2004 Indonesian tsunami Volcanic eruptions, the collapse of volcanic features and huge profusions of ash are all capable of producing a tsunami. The volcanic explosion of Krakatau in Indonesia and the collapse of its caldera produced 130-foot-high waves and killed 36,0000 people. There are concerns about specific volcanos triggering huge landslides that could generate tsunamis over 1000 feet high.

An asteroid three of four miles in diameter crashing into the Atlantic Ocean could produce a tsunami large enough to wash over New York City and climb up the Appalachian mountains. A 53-mile-wide asteroid that landed in Chesapeake 35 million years is believed to have produced waves that were thousands of feet high. One that landed on land 65 million years is believed to have kicked up enough debris to cause the climate to change and the dinosaurs to become extinct.

Dangerous Tsunamis

1976 Lebak Tsunami in the Philippines
killed 8000 people It is difficult to predict the likelihood of a dangerous tsunami simply on the magnitude of earthquake. A lot of factors come into play: the kind of earth quake, where on the ocean floor it occurs, the depth of water, the distance from coastal areas, the configurations of harbors and bays and the topography of the land where a tsunami comes a shore.

A dangerous tsunami is more likely to develop in deep water than in shallow water. It is also more likely to develop if the movement of earth is large but concentrated in an small area rather than spread out over a large area. Destructive tsunami often occur where the water is channeled either in the sea or by land and focused on one area.

Generally an earthquake of a magnitude of more than 8.5 on the Richter scale is necessary to produce tsunamis that affect areas long distances away from the epicenter of an earthquake. Smaller earthquakes can generate powerful local tsunamis. But even large earthquakes do not necessarily produce large tsunamis. The worst tsunamis often occur when there is a huge vertical movement on ocean floor, causing sudden, large displacement of water. This contrasts to dangerous land earthquake which often have violent horizontal movements that cause violent shacking over a large area.

Obviously tsunamis are most dangerous when they hit places with a lot of people. Hotels and houses are built along beaches and harbors are particularly vulnerable. Coral reefs, marshes, forests and mangroves act as natural buffers. When they are removed or disturbed they remove obstacles that can slow a tsunami down.

Tsunami Detection

2004 tsunami in Thailand Tsunami detection is very difficult. Scientists use seismometers, sensitive devices used to detect earth movements and measuring earthquakes; tide gauges, that measure changes in the water surface level; and wave-sensing buoys. Water level gauges cost between $5,000 and $20,000 depending on the sensitivity the instruments and the quickness of their communication ability.

The most effective technology are deep ocean instruments called tsunameters than detect passing waves and transmit the information to satellites via buoys at the surface. They cost $250,000 a piece and require $50,000 a year for maintenance. As of 2005 there were only seven of them in operation.

In an efforts to create a global tsunami warning system, tsunami detection devices have been placed around the world on ocean floors at depths of up to 5,400 meters. Scientist have extensively mapped the ocean floor with the aim of finding out where potential tsunami-inducing areas are.

Tying the system together is a communication network that can relay data to a central data base and then disseminate the information quickly to places where the tsunamis may strike.

The world’s largest tsunami research center is run by Oregon State University. Composed of a hanger-size building with a 111-meter-long, 90-centimeter-deep wave pool, it is used mainly to study the effect of large tsunamis on buildings and infrastructure in coastal areas. Tests involve crashing waves on models in various landscapes with the goal of helping planners figure out where to place buildings, plan evacuation routes and design buildings and infrastructure. .

1755 Lisbon earthquake and tsunami

Tsunami Warning

If a very low tide occurs after an earthquakes, it means that a tsunami is approaching fast. During a tsunami the waves can keep coming. If you make it to high ground stay there for at least six hours.

A tsunami warning system for the Pacific Ocean has existed for decades. Headquartered in Hawaii, it issues warnings to 26 nations and was set up in 1965 after devastating tsunamis in Chile and Alaska in the early 1960s. The system analyzes earthquake data from several seismic networks. The information is fed into a computer and models predict were tsunamis might form and strike, and predictions and warnings are dispatched to affected areas. As new data comes in the information is updated.

Hawaii is outfit with sirens to warn people of approaching tsunamis.

Where Tsunamis Occur

About 85 percent are tsunami are produced in the Pacific Ocean. Most are generated in the Pacific rim because more undersea earthquakes are generated there than anywhere else. More than 800 tsunamis have been generated in the Pacific in the last century. Some 22 percent of these were generated off of Japan

Tsunami occur much less frequently in the Atlantic Ocean and Indian Ocean. But they the do occur there. One in 1945 killed several hundred people in Bombay. Another in 1762 ravaged a large part of what is now Bangladesh and other parts of the Bay of Bengal.

Between 1992 and late 2004 there were 17 major tsunamis, with 11 of them in the Pacific. They killed a total of 4,000 people.

Tsunamis in Japan

More than 800 tsunamis have been generated in the Pacific in the last century. Some 22 percent of these were generated off of Japan According to geological evidence catastrophic tsunamis like the one that hit Sumatra, Thailand and Sri Lanka in 2004, strike Japan every 400 or 500 years.

There are worries that a large tsunami on the Pacific coast could kill tens of thousands of people and cause hundreds of billions of dollars in damage. A large earthquake off the east coast of Japan could produce a huge tsunami and people would only have minutes to evacuate even if they are given a warning when the earthquake occurs.

Large Tsunamis in Japan

An earthquake in Tohuku in A.D. 869 is believed to have been the strongest earthquake in historical times. Based on evidence of a tsunami produced by the quake along the Pacific coast it was estimated to have a magnitude of 9 on the Richter scale. About a 1,000 people are thought to have been killed by a tsunami produced by the quake that dumped water three kilometers inland in the Sendai area.

On January 27, 1700, a large tsunami hit a 1,000 kilometer section of Japan’s Pacific coast. It crested at five meters and washed hundred of meters inland and washed up rivers more than two kilometers. The tsunami was generated by a massive earthquake off the American Pacific Northwest coast.

The highest recorded tsunami caused by an offshore earthquake occurred off Ishigaki Island in the Ryukyu chain on April 24, 1771. According to the Guinness Book of Records. it tossed an 830-ton block of coral more than 1½ miles inland and may have been as high as 279 feet.

Unzen, a large volcano on Kyushu near Nagasakai, erupted catastrophically in 1792. An earthquake triggered by the eruption and the collapse of a lava dome sent an entire mountain side sliding into the ocean. The ensuing 100-meter-high tsunami submerged coastal villages, killing about 15,000 people.

Tsunamis engulfed the city of Shimabara with water reaching as far inland as the gates of the city castle. More than 43 square miles of the Shimabara peninsula was covered by water. The waves then traveled across the bay, washing away nearly 6,000 houses and 1,600 fishing boats along another 75-miles section of coastline.

Tsunamis in 1896 and 1933 that struck the northeast coast of Japan around Sanriku killed thousands of people. The one on June 15, 1896 killed 27,120 people in Iwate Prefecture and other areas and occurred after a large earthquake at sea. Fishermen who had been out at sea didn’t eve notice the tsunami when it slipped under their boats. When they returned home they found their villages destroyed and their family members dead or disappeared. There was little warning. The other one was produced by the Sanriku Offshore Earthquake on March 3, 1933, which measured 8.1 on the Richter scale. Most of the 3,008 dead and many of the 7,479 injured were victims of the tsunami.

An 8.2 earthquake in Hokaido in 1952 and an 8 earthquake in 1843 caused tsunamis between four and seven meters (See Tsunamis).

Tsunamis in Japan in Recent Years

In 1960, 100 people were killed and 42 went missing after a tsunami generated by the Great Chilean earthquake struck Japan. That earthquakes measured 9.5 on the Richter scale, the largest ever recorded. The tsunami reached Japan 22 hours after the earthquake occurred.

A tsunami caused by an earthquake in Miyagi Prefecture in northeast Honshu in May 1983 killed 100 people, including a group of elementary school children having picnic on the beach and were swallowed up by a tsunami.

In July 1993, an earthquake hit Okushiri Island, off southwestern Hokkaido. It measured 8.1 on the Richter scale and produced a 10-meter tsunami that washed away many homes and buildings . The tsunami 232 people, destroyed 1,410 houses and caused $500 million in damage. Some victims were killed in high water, landslides and collapsed buildings caused by the tsunami.

On September 26, 2003, a powerful earthquake struck eastern Hokkaido. Measuring 8.0 on the Richter scale, the quake was located 50 miles off shore and 30 miles below the surface of the sea. A total of 573 people were hurt; 41,000 were evacuated; an oil refinery caught fire; and a train derailed. The main earthquake was followed by a strong earthquake measuring 7.1 on the Richter scale. Much of the damage was caused by a tsunami that reached a height of 1.2 meters in Kushiro in Hokkaido. The tsunami hit about one hour after the earthquake. One man told the Daily Yomiuri, “When a big wave approached the port, I thought it would swallow me and my car whole.”

About two dozen Japanese were killed in Thailand and Sri Lanka by the tsunami in 2004.

In September 2004, a series of powerful under sea earthquakes off of Honshu produced tsunami waves 86 centimeters high in Wakayama. . A powerful and shallow earthquake off of northern Japan produced tsunami waves several centimeters high.

Tsunami Warnings in Japan

Japan has a fairly sophisticated tsunami warning system. Tsunami alerts are issued in television and radio when large earthquakes occur at sea in the vicinity of Japan. More often than not the warnings turn out to false alarms, sometimes very costly ones that cost millions of dollars in lost productivity if a large area is evacuated.

In November 2006, tsunami warnings were shown repeatedly on televison after an 8.1 earthquake occurred in the Russian-held islands north of Hokkaido. The tsunamis ended up being only a 40 or 50 centimeters high at their highest and few people—even those living in area deemed most dangerous—heeded the warnings and headed for higher ground or shelters. A larger tsunami generated by the same earthquake traveled clear across the North Pacific and produce a 1.8 wave in California and damaged docks and overturned one boat.

In January 2007, an 8.2 earthquake occurred off the coast of Hokkaido about 100 kilometers from the spot 7.9 earthquake occurred in November 2006. . tens of thousands of residents were ordered to seek higher ground in the event of a tsunami. Tsunami warning were issued on television and radio but were widely ignored. The tsunami that hot Hokkaido was only around 20 centimeters high at its highest. It was predicted to be a meter high.

Tsunami in Japan Caused by Chilean Earthquake in 2010

In February 2010, the Pacific Coast of Japan was struck by a small tsunami generated by a powerful earthquake 17,000 kilometers—half a world away— in Chile that reached Japan the day after the earthquake occurred. The largest tsunami, a 120-centimeter-high surge was observed at Kuji Port in Iwate Prefecture. A 110-centimeter-high tsunami was seen at Sendai Port in Miyagi and Shibusho Port in Kagoshima Prefecture . Damage was minor: a few building were flooded . Perhaps most remarkably the tsunami traveled 20 kilometers up a river that emptied into Sendai Bay. Tsunami expert Hitoshi Tanaka of Tohuku University told the Yomiuri Shimbun, “In the case of a gently inclining river, a tsunami can travel upriver for long distances and cause flood damage inland. Residents living near rivers have to be careful.”

Tsunami waves generated by the Chilean earthquake kept coming for 20 hours. They were triggered when the seabed and water were raised by a massive amount of energy, created by movement along a 600-kilometer-long fault near Chile.

The greatest damage was suffered by oyster and scallop producers in Miyagi and Iwate Prefectures, where the back surge from the tsunami waves flipped over rafts used to oyster and scallop cultivation. . In some cases the ropes that held the rafts were severed by the tsunami and the rafts floated away. On one group of islands in Miyagi Prefecture , 90 percent of the 180 rafts used to raise nori seaweed were destroyed.

The first major tsunami warning in 17 years was issued after the Chilean quake. Advisories, messages and bulletins were broadcast all day long on television with maps showing areas expected to be struck and estimates of the size of tsunamis expected. The Japan Meteorological Agency predicted tsunamis up to three meters in high, which are capable of being quite destructive. In some places train operations were halted and evacuation orders were given. After the event the Japan Meteorological Agency apologized for not only inaccurately predicting the height of waves but also forecasting earlier landfalls than actually occurred.

One survey found that only 6.2 percent of those requested to seek shelter from the tsunami actually did so. Of the 340,000 residents in 36 cities, towns and villages in Aomori, Iwate and Miyagi Prefecture asked to evacuate, only 21,000 sought shelter in designated areas. The others either stayed at home or went somewhere else. On restaurant workers who continued working after the warning was issued told the Yomiuri Shimbun, “The bank here is very high and there’s an evacuation center nearby so we could flee to safety right away if we had to . We were safe when when tsunami caused by [another] Chile earthquake hit here 50 year ago.” Also worrisome was the fact that many of those that did evacuate returned home after the first wave, a dangerous move considering that the most dangerous waves often appear after the first one.

Image Sources: 1) Hector Garcia blog 2) U.S. Geological Survey 3) Earthquake Image Archives M. Yoshimine, Tokyo Metropolitan University; Wikimedia Commons

Text Sources: New York Times, Washington Post, Los Angeles Times, Daily Yomiuri, Times of London, Japan National Tourist Organization (JNTO), National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

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© 2009 Jeffrey Hays

Last updated January 1 2013