Tsunami

A tsunami (pronounced tsu-NAA-mee), loosely referred to as, but technically not, a tidal wave, is a series of waves (called a "wave train") generated in a body of water by a pulsating or abrupt disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and the impact of extraterrestrial bodies such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage, injuries, and loss of life due to injuries or drowning.

Name

The term "tsunami" comes from the Japanese 津波 meaning tsu (harbor) and nami (wave). The term was created by fishermen who returned to port to find the area surrounding the harbour devastated, although they hadn't been aware of any wave in the open water. Tsunami waves travel underwater, and have very long wave lengths (sometimes over 100 kilometers long), which is why they generally pass unnoticed at sea, forming only a passing "hump" in the ocean.

Tsunamis were historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing tide, rather than the sort of cresting waves that are formed by wind action upon the ocean, and which most people are familiar with. However, as they are not actually related to tides, the term is considered misleading, and its use is discouraged by oceanographers.

Causes

A tsunami can be generated by any disturbance that displaces a large mass of water, such as an earthquake, landslide or meteor impact.

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 crust deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called "faults". Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift a water column and generate a tsunami.

Large landslides and cosmic-body impacts can disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area. However if the landslide or cosmic body is large enough, it will create a megatsunami. A megatsunami is a tsunami, usually caused by a collapsing island, asteroid impact, or huge chunks of ice falling into a large body of water, and is hundreds of meters high.

Characteristics

Tsunamis act very differently from typical surf swells; they propagate at high speeds and can travel great transoceanic distances with little energy loss. A tsunami can cause damage thousands of miles from its origin, so there may be several hours between its creation and its impact on the coast, more time than it takes for seismic waves to arrive.

Tsunamis have extremely long periods, 2 minutes to over one hour, and long wavelengths, in excess of 100 km. (Compare a typical wind-generated swell one sees at a surf beach, which might be spawned by a faraway storm and rhythmically roll in, one wave after another, with a period of about 10 seconds and a wavelength of 150 m.)

Typically undersea earthquakes give rise to between 3 and 5 distinct waves (crests), the second or third of which are usually the largest.

In instances where the leading edge of the tsunami is its trough, the sea will recede from the coast half the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed 800 m. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the dry sea bottom.

In instances where the leading edge of the tsunami is its first peak, low-lying coastal areas are flooded before the higher second wave reaches them. Again, being educated about a tsunami is important, to realize that when the water level drops the first time, the danger is not yet over.

A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Since a tsunami has a large wavelength, tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s²) and the water depth. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (about 712 km/hr or 442 mi/hr) with little energy loss even for far distances, while at a water depth of 40 m, the speed is 20 m/s (about 71 km/hr or 44 mi/hr), much slower, but still difficult to outrun.

In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. While in deep water a person at the surface of the water would probably not even notice the tsunami, the wave can increase to a height of 30 m and more as it approaches the coastline and compresses. Tsunamis can cause severe destruction on coasts and islands, even at locations remote to the source event, where that event itself is not even noticable without instruments.

Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.

Megatsunamis and seiches

Evidence shows that megatsunamis, a tsunami more than 100 meters (325ft) high, are possible. These rare events are typically caused by significant chunks of an island collapsing into the ocean, and can be extraordinarily devastating to faraway coastal regions.

Related to a tsunami is a seiche, an underwater, irregular fluctuation or rhythmic rocking of the water level of a lake. Often large earthquakes produce both tsunamis and seiches at the same time and there is evidence that some seiches have been caused by tsunamis.

The highest tsunami wave ever recorded was very localized: caused by a landslide in Lituya Bay, Alaska in 1958, a tsunami more than 500 m high stripped trees and soil from the steep walls of a fjord. By the time the wave reached the open sea, however, it dissipated quickly. The height of the waves was determined more by the topography of the inlet than by the energy generated by the landslide.

Warning systems

Many cities around the Pacific, notably in Japan but also in Hawaii, have warning systems and evacuation procedures in the event of a serious tsunami. Tsunamis are predicted by various seismologic institutes around the world and their progress monitored by satellites.

Bottom pressure recorders with buoys as communication link are used to detect waves which would not be noticed by a human observer on deep water. The first rudimentary system to alert communities of an impending tsunami was attempted in Hawaii in the 1920s. More advanced systems were developed in the wake of the April 1, 1946 and May 23, 1960 tsunamis which caused massive devastation in Hilo, Hawaii. The United States created the Pacific Tsunami Warning Center in 1949, and linked it to an international data and warning network in 1965.

One system for providing tsunami warning is the CREST Project (Consolidated Reporting of Earthquakes and Tsunamis) implemented on the West coast (Cascadia), Alaska, and Hawaii of the United States by the USGS, NOAA, the Pacific Northwest Seismograph Network, and three other university seismic networks.

Tsunami prediction remains an imperfect science. Although the epicenter of a large underwater quake and the probable tsunami arrival times can be quickly calculated, it is almost always impossible to know whether massive underwater ground shifts have occurred, resulting in tsunami waves. As a result, false alarms are common.

No system can protect against a sudden tsunami. A devastating tsunami occurred off the coast of Hokkaido in Japan as a result of an earthquake on July 12, 1993. As a result, 202 people on the small island of Okushiri lost their lives, and hundreds more were missing or injured. This tsunami struck just three to five minutes after the quake and most victims were caught while fleeing for higher ground and secure places after surviving the earthquake.

While there remains the potential for sudden devastation from a tsunami, warning systems can be effective. For example if there were a very large subduction zone earthquake (magnitude 9.0) off the west coast of the United States, people in Japan, for example, would have a little more than 12 hours (and likely warnings from warning systems in Hawaii and elsewhere) before any tsunami arrived, giving them some time to evacuate areas likely to be affected.

History

Although tsunamis occur most frequently in the Pacific Ocean, they are known to occur anywhere. Many ancient descriptions of sudden and catastrophic waves exist, particularly in and around the Mediterranean. Thousands of Portuguese who survived the great 1755 Lisbon earthquake were killed by a tsunami which followed a few minutes later. Before the great wave hit, the harbor waters retreated, revealing lost cargo and forgotten shipwrecks. In the North Atlantic, the Storegga Slide is a major incident.

Santorini

At some time between 1650 BC and 1600 BC (still debated), the volcanic Greek island Santorini blew up in a violent eruption, causing a 100m to 150m high tsunami that devastated the north coast of Crete, 70km (45 miles) away, and would certainly have eliminated every timber of the Minoan fleet along Crete's northern shore.

26 August 1883 Krakatoa explosive eruption

The island volcano of Krakatoa in Indonesia exploded with devastating fury in 1883, blowing its underground magma chamber partly empty so that much overlying land and seabed collapsed into it. A series of large tsunami waves was generated from the explosion, some reaching a height of over 40 meters above sea level. Tsunami waves were observed throughout the Indian Ocean, the Pacific Ocean, the American West Coast, South America, and even as far away as the English Channel. On the facing coasts of Java and Sumatra the sea flood went many miles inland and caused such vast loss of life that one area was never resettled but went back to the jungle and is now the Ujung Kulon nature reserve.

22 May 1960 Chilean tsunami

The Great Chilean Earthquake, the largest earthquake ever recorded, off the coast of South Central Chile, generated one of the most destructive tsunamis of the 20th century. It spread across the entire Pacific Ocean, with waves measuring up to 25 meters high. When the tsunami hit Onagawa Japan almost 22 hours after the quake, a tide gauge recorded a wave height of 10 feet above high tide. The number of people killed by the earthquake and subsequent tsunami is estimated to be between 490 to 2,290.

27 March 1964 Good Friday tsunami

After the magnitude 9.2 Good Friday Earthquake, tsunamis struck Alaska, British Columbia, California and coastal Pacific Northwest towns, killing 122 people. The tsunamis were up to 6 m tall that killed 11 people as far away as Crescent City, California.

26 Dec 2004 Indian Ocean tsunami

Animation of the 2004 Indonesian Tsunami

The magnitude 9.0 2004 Indian Ocean earthquake triggered a series of lethal tsunamis on December 26, 2004, with over one hundred and fifty thousand fatalities, ranging from those in the immediate vicinity of the quake in Indonesia, Thailand and the north-western coast of Malaysia to people thousands of kilometres away in Bangladesh, India, Sri Lanka, the Maldives, and even as far as Somalia in eastern Africa. The death toll from this event makes it the deadliest tsunami in recorded history.

Unlike the Pacific Ocean, there is no organised alert service covering the Indian Ocean. This is in part due to the absence of major tsunami events since 1883 and an emphasis on developing a tropical cyclone warning system.

The tsunami has sparked the largest ever relief efforts, gathering $2 billion dollars in contributions so far.

Other historical tsunamis

Other tsunamis that have occurred include the following:

The 1755 Lisbon earthquake, along with the resulting tsunami and fires, led to near total destruction of the Portuguese capital.
One of the worst tsunami disasters engulfed whole villages along Sanriku, Japan, in 1896. A wave more than seven stories tall (about 20 m) drowned some 26,000 people.
1946: An earthquake in the Aleutian Islands sent a tsunami to Hawaii, killing 159 people (only five died in Alaska).
1958: A very localized tsunami in Lituya Bay, Alaska was the highest ever recorded: more than 500 m (1500 ft) above sea level. It did not extend much beyond the outlet of the fjord in which it occurred, but did kill two people in a fishing vessel.
1976: August 16 (midnight) a tsunami killed more than 5000 people in the Moro Gulf region (Cotabato city) of the Philippines.
1983: 104 people in western Japan were killed by a tsunami spawned from a nearby earthquake.
July 17, 1998: A Papua New Guinea tsunami killed roughly 3,000 people. A 7.1 magnitude earthquake 15 miles offshore was followed within 10 minutes by a tsunami about 12 m tall. The villages of Arop and Warapu were destroyed.

Future threats

In 2001, scientists predicted that a future eruption of the unstable Cumbre Vieja volcano in La Palma (an island of the Canary Islands) could cause a supergiant undersea landslide. The next volcanic eruption is expected in the second half of the 21st century, but this is not necessarily the eruption that causes an immediate landslide. In this potential landslide the western half of the island (weighing perhaps 500 billion tonnes) would catastrophically slide into the ocean. Such a landslide would cause a 100 m megatsunami to devastate the coast of northwest Africa, with a 30-50 m tsunami reaching the east coast of North America 7-8 hours later causing massive coastal devastation and the deaths of perhaps millions of people, threatening Miami, suburbs of New York, and parts of Boston, and all coastal cities in between. ([1], [2]).

Earthquakes in Western America could generate tsunamis that could threaten Hawaii and Japan.

External links

References

The Big Bang that Triggered A Tragedy, by Richard Macey, The Sydney Morning Herald, 1 January 2005, p 11 - quoting Dr Mark Leonard, seismologist at Geoscience Australia