Encyclopedia
A
tsunami is a series of
waves when a body of
water, such as an
ocean is rapidly displaced on a massive scale.
Earthquakes,
mass movements above or below water,
volcanic eruptions and other
underwater explosions, and large
meteorite impacts all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating.
The term
tsunami comes from the
Japanese language meaning
harbour and
wave . Although in Japanese
tsunami is used for both the singular and plural, in English
tsunamis is often used as the plural. The term was created by fishermen who returned to port to find the area surrounding their
harbour devastated, although they had not been aware of any
wave in the open water. A tsunami has a much smaller
amplitude offshore, and a very long
wavelength , which is why they generally pass unnoticed at sea, forming only a passing "hump" in the ocean.
Tsunamis have been 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 . However, since they are not actually related to tides the term is considered misleading and its usage discouraged by
oceanographers.
Causes
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the Earth’s crust can occur at
plate boundaries.
Subduction earthquakes are particularly effective in generating tsunamis. As an Oceanic Plate is subducted beneath a Continental Plate, it sometimes brings down the lip of the Continental with it. Eventually, too much stress is put on the lip and it snaps back, sending shockwaves through the Earth’s crust, causing a tremor under the sea, known as an Undersea Earthquake.
Sub-marine
landslides as well as collapses of volcanic edifices may also disturb the overlying water column as sediment and rocks slide downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift the water column and form a tsunami.
Waves are formed as the displaced water mass moves under the influence of
gravity and radiate across the ocean like ripples on a pond.
In the 1950s it was discovered that larger tsunamis than previously believed possible could be caused by landslides, explosive volcanic action and
impact events. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls. Tsunamis caused by these mechanisms, unlike the ocean-wide tsunamis caused by some earthquakes, generally dissipate quickly and rarely affect coastlines distant from the source due to the small area of sea affected. These events can give rise to much larger local shock waves , such as the landslide at the head of Lituya Bay which produced a water wave estimated at 50 – 150 m and reached 524 m up local mountains. However, an extremely large landslide could generate a
megatsunami that might have ocean-wide impacts.
Characteristics
Often referred to as "tidal waves", a tsunami does not look like the popular impression of "a normal wave only much bigger". Instead it looks rather like an endlessly onrushing tide which forces its way around and through any obstacle. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal area. The sheer weight of water is enough to pulverise objects in its path, often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before the tsunami subsides.
Tsunamis act very differently from typical surf swells: they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometres from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the
seismic wave generated by the originating event arrives. Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels. The energy per linear metre in the wave is proportional to the inverse of the distance from the source. This is the two-dimensional equivalent of the
inverse square law, which is followed by waves which propagate in three dimensions .
A single tsunami event may involve a series of waves of varying heights; so the set of waves is called a
train. In open water, tsunamis have extremely long periods , from minutes to hours, and long wavelengths of up to several hundred kilometres. This is very different from typical wind-generated swells on the ocean, which might have a period of about 10 seconds and a wavelength of 150
metres.
The actual height of a tsunami wave in open water is often less than one metre. This is often practically unnoticeable to people on ships. Because it has such a large wavelength, the energy of a tsunami mobilizes the entire water column, down to the sea bed. Ocean waves in deep water typically cause water motion to a depth approximately equal to half their wavelength. This means, ocean surface wave motion will only reach down to a depth of a few 100 m or less. Tsunamis, by contrast behave as shallow water waves in the deep ocean , for which little attenuation of water motion occurs with depth.
The wave travels across open ocean at an average speed of 500 mph. As the wave approaches land, the sea shallows and the wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of six stories or more as it approaches the coastline and compresses. The steepening process is analogous to the cracking of a tapered
whip. As a wave goes down the whip from handle to tip, the same energy is deposited in less and less material, which then moves more violently as it receives this energy.
A wave becomes a 'shallow-water wave' when the ratio between the water depth and its wavelength gets very small, and since a tsunami has an extremely 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 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 with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s , which is much slower than the speed in the open ocean but the wave would still be difficult to outrun.
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 . It's also not necessary that they are
symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding
geography.
Local geographic peculiarities can lead to
seiche or standing waves forming, which can amplify the onshore damage. For instance, the tsunami that hit Hawaii on April 1, 1946 had a fifteen-minute interval between wave fronts. The natural resonant period of
Hilo Bay is about thirty minutes. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159 inhabitants.
Tsunami wave
Ocean waves are normally divided into 3 groups, characterized by depth:
- Deep water
- Intermediate water
- Shallow water
Even though a tsunami is generated in deep water , tsunami waves are considered shallow-water waves. As the tsunami wave approaches the shallow waters of shore, its time period remains the same, but its wavelength decreases rapidly, thus causing the water to pile up to form tremendous crests, in an effect known as "shoaling".
Signs of an approaching tsunami
The following have at various times been associated with a tsunami :
- An earthquake may be felt.
- Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling.
- The water in the waves may be unusually hot.
- The water may smell of rotten eggs or of petrol or oil.
- The water may sting the skin.
- A thunderous boom may be heard followed by
- a roaring noise as of a jet plane
- or a noise akin to the periodic whop-whop of a helicopter,
- or a whistling sound.
- The sea may recede to a considerable distance.
- A flash of red light might be seen near the horizon.
Warnings and prevention
Tsunamis cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunamis.
In instances where the leading edge of the tsunami wave is its trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed many hundreds of metres. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the exposed sea bed.
In instances where the leading edge of the tsunami is its first peak, succeeding waves can lead to further flooding. 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. In a low-lying coastal area, a strong earthquake is a major warning sign that a tsunami may be produced.
Regions with a high risk of tsunamis may use
tsunami warning systems to detect tsunamis and warn the general population before the wave reaches land. In some communities on the west coast of the United States, which is prone to Pacific Ocean tsunamis, warning signs advise people where to run in the event of an incoming tsunami. Computer models can roughly predict tsunami arrival and impact based on information about the event that triggered it and the shape of the seafloor and coastal land .
One of the early warnings comes from nearby animals. Many animals sense danger and flee to higher ground before the water arrives. The Lisbon quake is the first documented case of such a phenomenon in Europe. The phenomenon was also noted in
Sri Lanka in the
2004 Indian Ocean earthquake . Some scientists speculate that animals may have an ability to sense subsonic
Rayleigh waves from an earthquake minutes or hours before a tsunami strikes shore .
While it is not possible to prevent a tsunami, in some particularly tsunami-prone countries some measures have been taken to reduce the damage caused on shore. Japan has implemented an extensive programme of building tsunami walls of up to 4.5 m high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunamis. However, their effectiveness has been questioned, as tsunamis are often higher than the barriers. For instance, the tsunami which hit the island of
Hokkaido on July 12, 1993 created waves as much as 30 m 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.
The effects of a tsunami can be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami's energy being sapped by a belt of trees such as
coconut palms and
mangroves. In one striking example, the village of Naluvedapathy in India's
Tamil Nadu region suffered minimal damage and few deaths as the wave broke up on 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 stretches of sea coast which are prone to tsunami risks. While it would take some years for the trees to grow to a useful size, such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the costly and environmentally destructive method of erecting artificial barriers.
Historical tsunamis
See also List of historic tsunamis by death toll.Tsunamis occur most frequently in the Pacific Ocean, but are a global phenomenon; they are possible wherever large bodies of water are found, including inland lakes, where they can be caused by landslides. Very small tsunamis, non-destructive and undetectable without specialized equipment, occur frequently as a result of minor earthquakes and other events.
1700 - Vancouver Island, Canada
January 26 - The
Cascadia Earthquake, one of the largest earthquakes on record , ruptures the Cascadia Subduction Zone offshore from Vancouver Island to northern California, and caused massive tsunamis across the
Pacific Northwest logged in Japan and oral traditions of the Native Americans.
1755 - Lisbon, Portugal
Tens of thousands of Portuguese who survived the great
1755 Lisbon earthquake were killed by a tsunami which followed a half hour later. Many townspeople fled to the waterfront, believing the area safe from fires and from falling debris from aftershocks. Before the great wall of water hit the harbour, waters retreated, revealing lost cargo and forgotten shipwrecks.
The earthquake, tsunami, and subsequent fires killed more than a third of Lisbon's pre-quake population of 275,000. Historical records of explorations by
Vasco da Gama and other early navigators were lost, and countless buildings were destroyed . Europeans of the 18th century struggled to understand the disaster within
religious and rational belief systems. Philosophers of the
Enlightenment, notably
Voltaire, wrote about the event. The
philosophical concept of the sublime, as described by philosopher
Immanuel Kant in the
Observations on the Feeling of the Beautiful and Sublime, took inspiration in part from attempts to comprehend the enormity of the Lisbon quake and tsunami.
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 collapse, some reaching a height of over 40 metres 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.
The
Halifax Explosion occurred on Thursday, December 6, 1917 at 9:04:35 a.m. local time in
Halifax,
Nova Scotia in
Canada, when the French munitions ship Mont-Blanc, bound for World War I France, collided with the Norwegian ship Imo chartered to carry Belgian relief supplies. In the aftermath of the collision, Mont-Blanc caught fire and
exploded. The explosion caused a tsunami, and a pressure-wave of air.
1929 - Newfoundland tsunami
On November 18, 1929, an
earthquake of magnitude 7.2 occurred beneath the Laurentian Slope on the
Grand Banks. The quake was felt throughout the Atlantic Provinces of
Canada and as far west as
Ottawa, Ontario and as far south as
Claymont, Delaware. The resulting tsunami measured over 7 metres in height and took about 2˝ hours to reach the Burin Peninsula on the south coast of
Newfoundland, where 28 people lost their lives in various communities.
1946 - Pacific tsunami
The April 1
Aleutian Island earthquake tsunami that killed 159 people on
Hawaii and five in
Alaska resulted in the creation of a
tsunami warning system , established in 1949 for
Pacific Ocean area countries. The tsunami is locally known in Hawaii as the
April Fools Day Tsunami in Hawaii due to people thinking the warnings were an April Fools prank.
1960 - Chilean tsunami
The magnitude-9.5
Great Chilean Earthquake the strongest earthquake ever recorded. Its
epicentre 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 metres high. The first tsunami arrived at
Hilo, Hawaii approximately 14.8 hrs after it originated off the coast of South Central Chile.
The highest wave at Hilo Bay was measured at around 10.7 m . 61 lives were lost allegedly due to people's failure to heed warning sirens. When the tsunami hit Onagawa, Japan, almost 22 hours after the quake, the wave height was 3 m above high tide. Up to 2,290 people died due to the Earthquake and tsunami.
1963 - Vajont Dam disaster
The reservoir behind the
Vajont Dam in northern
Italy was struck by an enormous
landslide. A tsunami was triggered which swept over the top of the dam and into the valley below. Nearly 2,000 people were killed.
1964 - Good Friday tsunami
After the magnitude 9.2
Good Friday Earthquake, tsunamis struck
Alaska,
British Columbia,
California and coastal
Pacific Northwest towns, killing 121 people. The tsunamis were up to 6 m tall, and killed 11 people as far away as
Crescent City, California.
1976 - Moro Gulf tsunami
On August 16, 1976 at 12:11 A.M., a devastating earthquake of 7.9 hit the island of
Mindanao,
Philippines. It created a tsunami that devastated more than 700 km of coastline bordering Moro Gulf in the North Celebes Sea. An estimated number of victims for this tragedy left 5,000 dead, 2,200 missing or presumed dead, more than 9,500 injured and a total of 93,500 people were left homeless. It devastated the cities and provinces of
Pagadian City,
Zamboanga del Sur,
Zamboanga City,
Basilan,
Sulu, Sultan Kudarat,
Maguindanao,
Cotabato City,
Lanao del Sur and
Lanao del Norte.
1979 - Tumaco tsunami
A magnitude-7.9 earthquake occurred on December 12, 1979 at 7:59:4.3 along the Pacific coast of
Colombia and
Ecuador. The earthquake and the resulting tsunami caused the destruction of at least six fishing villages and the death of hundreds of people in the Colombian province of Narińo. The earthquake was felt in
Bogotá,
Cali,
Popayán, Buenaventura and several other cities and towns in Colombia and in
Guayaquil, Esmeraldas,
Quito and other parts of Ecuador. When the
Tumaco Tsunami hit the coast, it caused huge destruction in the city of Tumaco, as well as in the small towns of El Charco, San Juan, Mosquera and Salahonda on the Pacific Coast of Colombia. The total number of victims of this tragedy was 259 dead, 798 wounded and 95 missing or presumed dead.
1993 - Okushiri tsunami
A devastating tsunami occurred off the coast of
Hokkaido in Japan as a result of a magnitude 7.8 earthquake, 80 miles offshore, on July 12, 1993. The largest waves recorded from this event were as much as 30 metres tall. As a result, 202 people on the small island of Okushiri were killed, and hundreds more were missing or injured. In addition, millions of pet birds, cats and dogs lost their lives.
2004 - Indian Ocean tsunami
The
2004 Indian Ocean earthquake, which had a magnitude of 9.5, triggered a series of lethal tsunamis on December 26, 2004 that
killed approximately 230,000 people , making it the deadliest tsunami in recorded history. The tsunami killed people over an area ranging from the immediate vicinity of the
quake in
Indonesia,
Thailand and the north-western coast of
Malaysia to thousands of kilometres away in
Bangladesh,
India,
Sri Lanka, the
Maldives, and even as far as
Somalia,
Kenya and
Tanzania in eastern
Africa. The disaster prompted a huge worldwide effort to help victims of the tragedy, with billions of dollars being raised for disaster relief.
Unlike in the
Pacific Ocean, there was no organized alert service covering the
Indian Ocean. This was in part due to the absence of major tsunami events since 1883 . In light of the 2004
Indian Ocean tsunami,
UNESCO and other world bodies have called for a global
tsunami monitoring system.
2006 - South of Java Island tsunami
A 7.7 magnitude earthquake shocked the Indian Ocean seabed on July 17,2006, 200 km south of
Pangandaran, a beautiful beach famous to surfers for its perfect waves. This earthquake triggered tsunami whose heights varied from from 2 metres at Cilacap to 6 metres at Cimerak beach, where it swept away and flattened buildings as far as 400 metres away from the coastline. More than 600 people were reported killed, with around 150 others still missing.
Other tsunamis in South Asia
