Armageddon Online - Tsunamis - Tidal Waves of Destruction

A tsunami (pronounced /tsʊˈnɑːmi/ or /sʊˈnɑːmi/) is a series of waves created 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, landslides 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 ("tsu", 津) and wave ("nami", 波). 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. Tsunamis are common throughout Japanese history, as 195 events in Japan have been recorded.

A tsunami has a much smaller amplitude (wave heights) offshore, and a very long wavelength (often hundreds of kilometres long), 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 (with which people are more familiar). Since they are not actually related to tides the term is considered misleading and its usage is discouraged by oceanographers. [1] Since not all tsunamis occur in harbours, however, that term is equally misleading, although it does have the benefit of being misleading in a different language.

Causes

Generation of a tsunami

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.

Submarine landslides (which are sometimes triggered by large earthquakes) 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.

Tsunamis are surface gravity waves that 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 (solitons), 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.

The geologic recorded tells us that there have been massive tsunamis in Earth's past. These tsunamis were so large that they caused landslides on the opposite coast triggering another massive tsunami or "bounce back" tsunami. An example today would be a landslide equivalent to everything west of Portland falling in to the Pacific ocean, resulting in a tsunami that hits the Chinese coast with enough force to erode the coast and trigger a landslide large enough to send a tsunami that would inundate the US west coast and wipe out Portland.

Characteristics

There is a common misconception that tsunamis behave like wind-driven waves or swells (with air behind them, as in this celebrated 19th century woodcut by Hokusai). In fact, a tsunami is better understood as a new and suddenly higher sea level, which manifests as a shelf or shelves of water. The leading edge of a tsunami superficially resembles a breaking wave but behaves differently: the rapid rise in sea level, combined with the weight and pressure of the ocean behind it, has far greater force.

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.^{[citation needed]} (In other words, it decreases linearly with distance.) This is the two-dimensional equivalent of the inverse square law, which is followed by waves which propagate in three dimensions (in a sphere instead of a circle).

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 (the time for the next wave top to pass a point after the previous one), 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 height of a tsunami wave in open water is often less than one metre, and the height is spread over the wavelength of the tsunami which is multiple kilometres. This is unnoticeable to people on ships in deep water. Because it has such a large wavelength, the energy of a tsunami mobilizes the entire water column down to the sea bed. Typical ocean surface waves in deep water cause water motion to a depth 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.

Because a tsunami behaves like a 'shallow water wave,' its speed is based on the depth of the water. Typically, a tsunami wave will travel across a deep ocean at an average speed of 400 to 500 mph.([2]). As the wave approaches land, the sea shallows and the tsunami 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 (hundreds of kilometres), 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 (720 km/h or 450 mph) with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s (about 72 km/h or 45 mph), 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 (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). 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

Even though a tsunami is generated in deep water (around 4000 m below mean sea level), 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

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.

Tsunami warning sign on seawall in Kamakura, Japan, 2004. In the Muromachi period, a tsunami struck Kamakura, destroying the wooden building that housed the colossal statue of Amida Buddha at Kotokuin. Since that time, the statue has been outdoors.

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 (bathymetry) and coastal land (topography). [4]

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. [5] 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 (Kenneally, [6]).

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 (13.5 ft) 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 (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.

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. [7] 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

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.

Japan is a nation with the most recorded tsunamis in the world. The earliest recorded disaster being that of the 684 A.D. Hakuho Quake. The number of tsunamis in Japan totals 195 over a 1,313 year period, averaging one event every 6.7 years, the highest rate of occurrence in the world. These waves have hit with such violent fury that entire towns have been destroyed. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a devastating fate.

On December 26, 2004, an undersea earthquake measuring 9.0 on the Richter scale occurred 160 km (100 mi) off the western coast of Sumatra, Indonesia. It was the fifth largest earthquake in recorded history and generated massive tsunamis, which caused widespread devastation when they hit land, leaving an estimated 230,000 people dead in countries around the Indian Ocean.

1700 - Vancouver Island, Canada

January 26 - The Cascadia Earthquake, one of the largest earthquakes on record (estimated 9.0 magnitude), ruptured 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 (including most examples of Portugal's Manueline architecture). 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.

1868 - Hawaiian local tsunami generated by earthquake

On April 2, 1868, a local earthquake with a magnitude estimated between 7.25 and 7.75 rocked the southeast coast of the Big Island of Hawaii. It triggered a landslide on the slopes of the Mauna Loa volcano, five miles north of Pahala, killing 31 people. A tsunami then claimed 46 additional lives. The villages of Punaluu, Ninole, Kawaa, Honuapo, and Keauhou Landing were severely damaged. According to one account, the tsunami "rolled in over the tops of the cocoanut trees, probably 60 feet high .... inland a distance of a quarter of a mile in some places, taking out to sea when it returned, houses, men, women, and almost everything movable." This was reported in the 1988 edition of Walter C. Dudley's book, "Tsunami!" (ISBN 0-8248-1125-9).

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 aftermath of the tsunami that struck Newfoundland in 1929.

1917 - Halifax Explosion and Tsunami

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 2½ hours to reach the Burin Peninsula on the south coast of Newfoundland, where 29 people lost their lives in various communities.

1946 - Pacific tsunami

Hawai`i residents run from an approaching tsunami in Hilo, Hawai'i

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 (specifically The Pacific Tsunami Warning Center), 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 is 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 (35 ft.). 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 (without bursting it) 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 (UTC) 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 Nariño. 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

Map of Hokkaido shown on NHK during an emergency broadcast.

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. On the day of the tsunami, an emergency bulletin from the Japan Meteorological Agency was broadcasted on NHK. The signal was transmitted in English and Japanese, the same message was repeated multiple times. This alert first begins with a alarm tone that lasts about 15 seconds. This broadcast is also archived at YouTube. [9] ^{[citation needed]}

2004 - Indian Ocean tsunami

The 2004 Indian Ocean earthquake, which had a magnitude of 9.3, triggered a series of lethal tsunamis on December 26, 2004 that killed approximately 230,000 people (including 168,000 in Indonesia alone), making it the deadliest tsunami as well as one of the worst natural disasters 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.

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 (the Krakatoa eruption, which killed 36,000 people). 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.

2006 - Kuril Islands tsunami

A 8.1-magnitude quake struck an area claimed by both Russia and Japan, but the waves near Japan did not swell higher than 23 inches. There were no immediate reports of casualties or damage. Six hours later, tsunami waves up to nearly 5-foot high caused by the quake crashed into Hawaii's shores, Crescent City, California, and Santa Cruz, California causing considerable damage in some areas.

Tsunami