Tsunami Facts

What is a tsunami?

A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean, such as:

• earthquakes
• landslide
• volcanic eruptions
• explosions
• meteorites

These disturbances can either be from below (e.g. underwater earthquakes with large vertical displacements, submarine landslides) or from above (e.g. meteorite impacts).

Tsunami is a Japanese word with the English translation: "harbour wave". In the past, tsunamis have been referred to as "tidal waves" or "seismic sea waves".

 The term "tidal wave" is misleading; even though a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. (Tides result from the gravitational influences of the moon, sun, and planets.) The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.

Tsunamis are also often confused with storm surges, even though they are quite different phenomena. A storm surge is a rapid rise in coastal sea-level caused by a significant meteorological event - these are often associated with tropical cyclones.

The physics of a tsunami

Tsunamis can have wavelengths ranging from 10 to 500 km and wave periods of up to an hour. As a result of their long wavelengths, tsunamis act as shallow-water waves. A wave becomes a shallow-water wave when the wavelength is very large compared to the water depth. Shallow-water waves move at a speed, c, that is dependent upon the water depth and is given by the formula:

where g is the acceleration due to gravity (= 9.8 m/s²) and H is the depth of water.

In the deep ocean, the typical water depth is around 4000 m, so a tsunami will therefore travel at around 200 m/s, or more than 700 km/h.

For tsunamis that are generated by underwater earthquakes, the amplitude (i.e wave height) of the tsunami is determined by the amount by which the sea-floor is displaced. Similarly, the wavelength and period of the tsunami are determined by the size and shape of the underwater disturbance.

As well as travelling at high speeds, tsunamis can also travel large distances with limited energy losses. As the tsunami propagates across the ocean, the wave crests can undergo refraction (bending), which is caused by segments of the wave moving at different speeds as the water depth along the wave crest varies.

What happens to a tsunami as it approaches land?

As a tsunami leaves the deep water of the open-ocean and travels into the shallower water near the coast, it transforms. If you read the "The physics of a tsunami" section, you will know that a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant.

Consequently, as the tsunami's speed diminishes, its height grows. This is called shoaling. Because of this shoaling effect, a tsunami that is unnoticeable at sea, may grow to be several metres or more in height near the coast.

The increase of the tsunami's waveheight as it enters shallow water is given by:

where h_s and h_d are waveheights in shallow and deep water and H_s and H_d are the depths of the shallow and deep water. So a tsunami with a height of 1 m in the open ocean where the water depth is 4000m would have a waveheight of 4 to 5 m in water of depth 10 m.

Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Depending on whether the first part of the tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide.

Local bathymetry may also cause the tsunami to appear as a series of breaking waves.

Tsunamis have great erosion potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of metres inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of tens of metres.

How are tsunamis measured or observed?

In the deep ocean, a tsunami has a small amplitude (less than 1 metre) but very long wavelength (hundreds of kilometres). This means that the slope, or steepness of the wave is very small, so it is practically undetectable to the human eye. However, there are ocean observing instruments that are able to detect tsunamis.

Tide Gauges

Tide gauges measure the height of the sea-surface and are primarily used for measuring tide levels. Most of the tide gauges operated by the Bureau of Meteorology's National Tidal Centre are SEAFRAME stations (Sea Level Fine Resolution Acoustic Measuring Equipment). These consist of an acoustic sensor connected to a vertical tube open at the lower end which is in the water.

The acoustic sensor emits a sound pulse which travels from the top of the tube down to the water surface, and is then reflected back up the tube. The distance to the water level can then be calculated using the travel time of the pulse. This system filters out small-scale effects like wind-waves and has the capacity to measure sea-level changes within 1mm accuracy.

The tide gauge at Cocos Island observed the tsunami on December 26th 2004 as it passed by the island, as shown in these observations made during December.

Satellites

Satellite altimeters measure the height of the ocean surface directly by the use of electro-magnetic pulses. These are sent down to the ocean surface from the satellite and the height of the ocean surface can be determined by knowing the speed of the pulse, the location of the satellite and measuring the time that the pulse takes to return to the satellite.

One problem with this kind of satellite data is that it can be very sparse - some satellites only pass over a particular location about once a month, so you would be lucky to spot a tsunami since they travel so quickly. However, during the Indian Ocean tsunami of December 26th 2004, the Jason satellite altimeter happened to be in the right place at the right time.

The picture below shows the height of the sea surface (in blue) measured by the Jason satellite two hours after the initial earthquake hit the region southeast of Sumatra (shown in red) on December 26, 2004. The data were taken by a radar altimeter on board the satellite along a track traversing the Indian Ocean when the tsunami waves had just filled the entire Bay of Bengal. The data shown are the differences in sea surface height from previous observations made along the same track 20-30 days before the earthquake, showing the signals of the tsunami.

Picture courtesy of NASA/JPL-Caltech

The DART System

In 1995 the National Oceanic and Atmospheric Administration (NOAA) began developing the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. An array of stations is currently deployed in the Pacific Ocean. These stations give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder which detects the passage of a tsunami. (The pressure of the water column is related to the height of the sea-surface) .

The data is then transmitted to a surface buoy via sonar. The surface buoy then radios the information to the Pacific Tsunami Warning Center (PTWC) via satellite. The bottom pressure recorder lasts for two years while the surface buoy is replaced every year. The system has considerably improved the forecasting and warning of tsunamis in the Pacific.

The Indian Ocean tsunami of 26th December 2004

An undersea earthquake in the Indian Ocean on 26th December 2004 produced a tsunami that caused one of the biggest natural disasters in modern history. Over 200,000 people are known to have lost their lives.

The waves devastated the shores of parts of Indonesia, Sri Lanka, India, Thailand and other countries with waves reported up to 15 m high, reaching as far as Somalia on the east coast of Africa, 4500 km west of the epicentre. Refraction and diffraction of the waves meant that the impact of the tsunami was noticed around the world and sea-level monitoring stations in places such as Brazil and Queensland also felt the effect of the tsunami.

This animation (10.4Mb) was produced by scientists in the Bureau of Meteorology's National Tidal Centre. A numerical model was used to replicate the generation and propagation of the tsunami and it shows how the waves propagated around the world's ocean basins.

The earthquake took place at about 1am UTC (8am local time) in the Indian Ocean off the western coast of northern Sumatra. With a magnitude of 9.0 on the Richter scale, it was the largest since the 1964 earthquake off Alaska and equal fourth largest since 1900, when accurate global seismographic record-keeping began.

The epicentre of the earthquake was located about 250 km south-southeast of the Indonesian city of Banda Aceh. It was a rare megathrust earthquake and occurred on the interface of the India and Burma tectonic plates.

This was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. A megathrust earthquake is where one tectonic plate slips beneath another, causing vertical motion of the plates. This large vertical displacement of the sea-floor generated the devastating tsunami, which caused damage over such a large area around the Indian Ocean.

The earthquake was also unusually large in geographical extent. An estimated 1200 km of faultline slipped about 15 m along the subduction zone over a period of several minutes. Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the waves was in an east-west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had very few casualties despite being a populous low-lying country.

Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines. (See this travel time map). The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.

On its arrival on shore, the height of the tsunami varied greatly, depending on its distance and direction from the epicentre and other factors such as the local bathymetry. Reports have the height ranging form 2-3 m at the African coast (Kenya) up to 10-15 m at Sumatra, the region closest to the epicentre.

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Environment Clean Generations

Magnetic Waves Could Replace Microwaves

A simulation of a magnetic nanocontact shows how it causes spin waves to spread like rings on water.

The microwave technology used in applications such as mobile phones and wireless networks may be on its way to being replaced - with parts that are smaller, less expensive, and that consume less resources. Instead of microwaves, devices of the future may use spin waves, which are nanoscale magnetic waves.

For almost ten years, it has been theorized that spin waves could be propagated using magnetic nanocontacts. Recently, scientists from the University of Gothenburg and the Royal Institute of Technology, Sweden, became the first people to demonstrate that the theory meshes with observable phenomena.

 

The study began two years ago, when the researchers started construction of their magnetic nanocontacts. Using one of the world's three advanced spin wave microscopes, at the University of Perugia in Italy, they were able to visualize the movement of the spin waves created by those contacts. The waves rippled out through a thin film of nickel-iron alloy, which was three nanometers thick.

According to the scientists, these results have opened the way for a new field of research known as "magnonics," that utilizes spin waves. The technology could reportedly be integrated into traditional microwave-based electronic circuits, although the added magnonic components would be much better suited to miniaturization than their present-day microwave counterparts.

"I believe that our results will signal the start of a rapid development of magnonic components and circuits," said the University of Gothenburg's Prof. Johan Akerman. "What is particularly exciting is that these components are powered by simple direct current, which is then converted into spin waves in the microwave region. The frequency of these waves can be directly controlled by the current. This will make completely new functions possible."

Below is a video depicting a simulation of six wave-producing magnetic nanocontacts placed in a circle, illustrating how the contacts can be placed in any pattern.

by "environment clean generations"

The 10 Most Destructive Tsunamis In History

A tsunami hit the north-east coast of Japan after a magnitude 9.0 earthquake on March 11, 2011.

HE EARTHQUAKE AND SUBSEQUENT tsunami that devastated Japan have shown just how vulnerable modern society is to the power of Mother Nature.  While tsunamis were largely unknown to the wider public before the hugely destructive 2004 Boxing Day Tsunami, they have occurred many times in the past.

Tsunamis can be generated by any significant displacement of water in oceans or lakes, though are most commonly created by the movement of tectonic plates under the ocean floor, during an earthquake. But they can also be caused by volcanic eruptions, glacial carving, meteorite impacts or landslides.

Tsunamis have occurred often throughout history. So frequently in Japan, in fact, that they invented the word specifically for the phenomenon: 'tsu' meaning harbour and 'nami' meaning wave.  

"It's actually quite frightening to think that this [Japanese tsunami] event is smaller than the 2004 Indian Ocean tsunami, smaller even than the 1960 Chilean tsunami, yet the damage to Japan's people and economy is still profound," says Professor James Goff, co-director of the Australian Tsunami Research Centre and Natural Hazards Research Lab at the University of New South Wales. "It's a horrendous tragedy, caused by a completely unpredictable event."

Because little historical data exist on the size of tsunami waves, how many occur in one event, or how far they advance on shore, scientists rank them according to how much damage they wreak. However, assessing just how much damage a single tsunami event causes may take many months to years; and it may be some time before this recent disaster can be rated on a historical scale.

Here are the 10 worst tsunamis of all time:

1.    Sumatra, Indonesia - 26 December 2004

 

  The 9.1 magnitude earthquake off the coast of Sumatra was estimated to occur at a depth of 30 km. The fault zone that caused the tsunami was roughly 1300 km long, vertically displacing the sea floor by several metres along that length. The ensuing tsunami was as tall as 50 m, reaching 5 km inland near Meubolah, Sumatra. This tsunami is also the most widely recorded, with nearly one thousand combined tide gauge and eyewitness measurements from around the world reporting a rise in wave height, including places in the US, the UK and Antarctica. An estimated US$10b of damages is attributed to the disaster, with around 230,000 people reported dead.

2.    Lisbon, Portugal - 1 November 1755

 

A magnitude 8.5 earthquake caused a series of three huge waves to strike various towns along the west coast of Portugal and southern Spain, up to 30 m high, in some places. The tsunami affected waves as far away as Carlisle Bay, Barbados, where waves were said to rise by 1.5 m. The earthquake and ensuing tsunami killed 60,000 in the Portugal, Morocco and Spain.

3.    Krakatau, Indonesia - 27 August 1883

 This tsunami event is actually linked to the explosion of the Krakatau caldera volcano. Multiple waves as high as 37 m were propagated by the violent eruptions and demolished the towns of Anjer and Merak. The sea was reported to recede from the shore at Bombay, India and is said to have killed one person in Sri Lanka. This event killed around 40,000 people in total; however, as many as 2,000 deaths can be attributed directly to the volcanic eruptions, rather than the ensuing tsunami.

4.    Enshunada Sea, Japan - 20 September 1498
An earthquake, estimated to have been at least magnitude 8.3, caused tsunami waves along the coasts of Kii, Mikawa, Surugu, Izu and Sagami. The waves were powerful enough to breach a spit, which had previously separated Lake Hamana from the sea. There were reports of homes flooding and being swept away throughout the region, with a total of at least 31,000 people killed.

5.    Nankaido, Japan - 28 October 1707
A magnitude 8.4 earthquake caused sea waves as high as 25 m to hammer into the Pacific coasts of Kyushyu, Shikoku and Honshin. Osaka was also damaged. A total of nearly 30,000 buildings were damaged in the affected regions and about 30,000 people were killed. It was reported that roughly a dozen large waves were counted between 3 pm and 4 pm, some of them extending several kilometres inland at Kochi.

6.    Sanriku, Japan - 15 June 1896
This tsunami propagated after an estimated magnitude 7.6 earthquake occurred off the coast of Sanriku, Japan. The tsunami was reported at Shirahama to have reached a height of 38.2 m, causing damage to more than 11,000 homes and killing some 22,000 people. Reports have also been found that chronicle a corresponding tsunami hitting the east coast of China, killing around 4000 people and doing extensive damage to local crops.

7.    Northern Chile - 13 August 1868
This tsunami event was caused by a series of two significant earthquakes, estimated at a magnitude of 8.5, off the coast of Arica, Peru (now Chile). The ensuing waves affected the entire Pacific Rim, with waves reported to be up to 21 m high, which lasted between two and three days. The Arica tsunami was registered by six tide gauges, as far off as Sydney, Australia. A total of 25,000 deaths and an estimated US$300 million in damages were caused by the tsunami and earthquakes combined along the Peru-Chile coast.

8.    Ryuku Islands, Japan - 24 April 1771
A magnitude 7.4 earthquake is believed to have caused a tsunami that damaged a large number of islands in the region; however, the most serious damage was restricted to Ishigaki and Miyako Islands. It is commonly cited that the waves that struck Ishigaki Island was 85.4 m high, but it appears this is due to a confusion of the original Japanese measurements, and is more accurately estimated to have been around 11 to 15 m high. The tsunami destroyed a total of 3,137 homes, killing nearly 12,000 people in total.

9.    Ise Bay, Japan - 18 January 1586
The earthquake that caused the Ise Bay tsunami is best estimated as being of magnitude 8.2. The waves rose to a height of 6 m, causing damage to a number of towns. The town of Nagahama experienced an outbreak of fire as the earthquake first occurred, destroying half the city. It is reported that the nearby Lake Biwa surged over the town, leaving no trace except for the castle. The Ise Bay tsunamis caused more than 8000 deaths and a large amount damage.

10.    Crete, Greece - 21 July 365
This famous, ancient tsunami event was reported throughout the Mediterranean region. It is estimated that the earthquake that caused it would have been at least magnitude 8, occurring somewhere in the Greek Islands. Knossis, Gortis and eight other settlements were completely destroyed on Crete. Reports say the greats waves began with a large drop in the water level, leaving ships aground and sea animals beached. In the northern Adriatic, rivers flowed in reverse and the sea retreated. Numerous coasts around the Mediterranean were flooded, and a total of 5700 deaths occurred; however, these were restricted to Greece and Turkey.
 

by "environment clean generations"