Natural Hazards || Tsunami
A tsunami, also known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large amount of water, usually in a sea or a large lake. Earthquakes, volcanic eruptions and other underwater explosions (including underwater nuclear device explosions), landslides, glacier mines, meteorite impacts and other disturbances above or below water all have the potential to generate tsunamis. Unlike normal ocean waves, which originate from the wind, or tides that arise from the gravitational pull of the moon and the sun, a tsunami is generated by the displacement of water.
Tsunami waves are not like normal water currents or ocean waves, because their wavelengths are much longer. Instead of appearing as a breaking wave, a tsunami may initially be like a rapidly rising tide, and for this reason they are often referred to as tidal waves. , Although this use has not been liked by the scientific community because tsunamis are not tides in nature. Tsunamis typically consist of a series of waves ranging from minutes to hours with a so-called "inner wave train" reaching them. Wave heights of tens of meters can be generated by large events. Although the effects of tsunamis are limited to coastal areas, their destructive power can be very high and they can affect entire oceans; The 2004 Indian Ocean tsunami was one of the deadliest natural disasters in human history, killing at least 230,000 people or missing in 14 countries.
The Greek historian Thucydides suggested in his 5th century BCE history of the Peloponnesian War that tsunamis were related to submarine earthquakes, but by the 20th century, the understanding of the nature of tsunamis remained thin and much unknown. Major areas of current research include trying to determine why some large earthquakes do not generate tsunamis while others do small ones; Try to accurately predict tsunami passage in oceans; And also to predict how tsunami waves interact with specific shorelines.
Terminology
Tsunami warning in Ashensiand Indonesian in Ule Lheu, Banda Ace
Numerous terms are used in the English language to describe waves created in a body of water by the displacement of water; however, none of the terms in frequent use is entirely accurate.
Tsunami
Tsunami, which literally means "harbor wave", comes from the Japanese 津 ami, which is made up of two kanji 津 (tsu) meaning "port" and 波 (nami), meaning "wave". (For plurals, one can follow common English practice and add an S, or use an invasive plural as in Japanese.) While not entirely accurate, tsunamis only port. Not limited to, tsunami is currently the most widely accepted term. Geologist and oceanographer.
Tidal wave
Aceh, Indonesia, after the tsunami in December 2004.
Tsunamis are sometimes referred to as tidal waves. It derives from the most common presentation of the once-popular word tsunami, which is an exceptionally high tide bore. Both tsunamis and tides produce waves of water that move inland, but in the case of tsunamis, the inland speed of the water can be very high, giving an impression of incredibly high and powerful tides. In recent years, the term "tidal wave" has fallen out of favour, especially in the scientific community, because tsunamis really have nothing to do with tides, which are the gravity of the moon and sun rather than the dissolution of water. Are constructed by the bridge. Although "tidal" means "similar" [or "" includes the form or character of tides, the use of the tidal wave is discouraged by geologists and oceanographers.
Seismic sea wave
The term seismic sea wave is also used to refer to phenomena, as waves often originate from earthquake-like activities. Prior to the rise of the use of the term tsunami in English-speaking countries, scientists generally encouraged the use of the term seismic sea wave rather than the incorrect term tidal wave. However, like a tsunami, a seismic sea wave is not a completely accurate term, because of forces other than earthquakes - underwater landslides, volcanic eruptions, underwater explosions, snowflakes in land or sea, meteorite impacts, and Weather when the atmospheric pressure changes. Very fast - displacing water can produce such waves.
History
List of historic tsunamis
While Japan may have the longest history of tsunamis, the sheer devastation caused by the 2004 Indian Ocean earthquake and tsunami event marked it as the most devastating of its kind in modern times, killing about 230,000 people. Go. The Sumatran region is not unused to tsunamis, as well as earthquakes of varying magnitude regularly occur on the island's coast.
Tsunamis are often underestimated in the Mediterranean Sea and parts of Europe. Historical and current importance (in relation to risk assumptions) are the 1755 Lisbon earthquake and tsunami (caused by the Azores-Gibraltar transform fault), the 1783 Calabrian earthquake, each with tens of thousands of deaths and the 1908 Messina earthquake and tsunami in Sicily and Calabria Claimed more than 123,000 lives and is one of the deadliest natural disasters in modern Europe. Some examples of tsunamis affecting the Storage Slide and the British Isles in the Norwegian sea suggest landslides and meteorites to be mainly and less than earthquake-induced waves.
As early as 426 BCE, the Greek historian Thucydides inquired about the causes of the tsunami in his book History of the Peloponnesian War, and first argued that a sea earthquake should be the cause.
"The cause must, in my opinion, be to look for this event in the earthquake. At the point where its tremor has been most violent, the sea has gone back, and a sudden recurrence with red force causes the earthquake. Without earthquake. Don't see how such an accident can happen. "
The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15–19) described the specific sequence of tsunamis, in which Alexandria was devastated by an earthquake, a sudden retreat of the sea, and a massive tsunami following the 365 CE tsunami.
Generation mechanisms
The main generation mechanism (or cause) of a tsunami is the displacement of a sufficient amount of water or perishable ocean. This displacement of water is rarely attributed to either earthquakes, landslides, volcanic eruptions, glacier harvesting, or meteorites and nuclear tests. The waves formed in this way are then sustained by gravity. Tides play no part in the generation of tsunamis.
Seismicity
Tsunamis can occur when the ocean floor suddenly deteriorates and holds water for a long time. Tectonic earthquakes are a special type of earthquake that is associated with Earth's crustal deformation; When these earthquakes occur under the sea, the water above the deformed area is displaced from its equilibrium position. In particular, a tsunami can occur when thrust faults associated with convergent or destructive boundaries run abruptly, resulting in water displacement, causing the vertical components of the movement involved. Movement over common (multidimensional) faults can also cause displacement of the seabed, but only the largest of such events (usually related to swelling in the outer moat) causes enough displacement to give rise to a significant tsunami. , Such as the 1977 Sumba and 1933 Sanricu events.
The energy released produces tsunami waves.
Tsunamis have a small amplitude (wave height) offshore and have a very long wavelength (often hundreds of kilometres long, while normal ocean waves only have a wavelength of 30 or 40 meters), which is why they are common. Normally unnoticed at sea, only forming a slight swell is usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallow water in the wave process described below. A tsunami can occur in any tidal state and can even flood the coastal areas at low tide.
On April 1, 1946, there was a magnitude-7.8 (Richter scale) earthquake near the Allian Islands, Alaska. It caused a tsunami that flooded Hilo on the island of Hawaii with a 14-meter-high (46 feet) high. The earthquake zone is the area where the Pacific Ocean floor is facing downward (or being pushed downward) under Alaska.
Examples of tsunamis originating at locations far away from convergent boundaries include Storage, Grand Bank 1929, Papua New Guinea 1998 (Tapin, 2001), about 8,000 years ago. The tsunamis of Grand Bank and Papua New Guinea were caused by earthquakes, which destabilized the sediments, which swept them into the sea and caused tsunamis. They separated before travelling to transoceanic distances.
The cause of Storegga sediment failure is unknown. Possibilities include an overload of sediment, earthquake or discharge of gas hydrates (methane etc.).
The 1960 Valdivia Earthquake (Mw 9.5), the 1964 Alaska Earthquake (Mw 9.2), the 2004 Indian Ocean Earthquake (Mw 9.2), and the 2011 Tohoku Earthquake (Mw9.0) are recent examples of powerful megathrust earthquakes that have caused tsunamis (known as Teltasunamis) Known as). Which can cross entire oceans? Small (Mw 4.2) earthquakes in Japan can be called tsunamis (local and regional tsunamis) that can destroy only nearby shores but can do so within minutes.
Landslides
In the 1950s, it was discovered that before the giant tsunami, it was believed to be caused by a huge submarine landslide. These rapidly displace large volumes of water, as energy is transferred more rapidly at the rate of water than by absorbing water. Their existence was confirmed in 1958, when a massive landslide in Litua Bay, Alaska, recorded the highest wave ever recorded, reaching a height of 524 meters (over 1700 feet). The wave did not go very far, as it almost completely broke. . Two people fishing in the bay were killed, but another boat managed to ride the wave.
Another landslide-tsunami occurred in 1963 when a massive landslide of Monte Toc entered the Vagont Dam in Italy. The resulting wave rose over a 262 m (860 ft) high dam over 250 meters (820 ft) and destroyed many cities. About 2,000 people died. Scientists named these waves Megatsunamis.
Some geologists claim that large landslides from volcanic islands, e.g. The Cumbre Visa, on La Palma in the Canary Islands, may be capable of generating mega tsunamis that can cross oceans, but this is disputed by many others.
In general, landslides generate displacements mainly in the shallower parts of the coastline, and there is speculation about the nature of large landslides entering the water. It has been shown to lead to water effects in enclosed bays and lakes, but a major tsunami has not resulted in a recorded tsunami in recorded history. Smooth locations are believed to be the large islands of Hawaii, Fogo in the Cape Verde Islands, La Reunion in the Indian Ocean, and Cumbria Visa on the island of La Palma in the Canary Islands; As with other volcanic ocean islands. The reason for this is that the bulk of relatively undeclared volcanic material is on the bodies and in some cases is considered to be the development of detachment planes. However, there is growing controversy over how dangerous these slopes really are.
Meteotsunamis
Some meteorological conditions, particularly rapid changes in barometric pressure, as seen with the passage of a front, can displace bodies of water due to waves of trains with waves, which are caused by seismic tsunamis. Are similar, but usually with less energy. These are essentially equivalent to seismic tsunamis, with the only difference being that meteorites lack the transoceanic reach of critical seismic tsunamis and that the force displacing water is maintained for a long period of time such as metiotusuni Cannot be modeled. Is continuously increasing. Despite their low energies, on shorelines, where they can be amplified by resonance, they are sometimes so powerful that they can cause localized damage and loss of life. They have been documented in many places, including the Great Lakes, the Aegean Sea, the English Channel, and the Balearic Islands, where they suffice for the local name, Risaga. In Sicily he is called Marubio and in Nagasaki Bay he is called Abaki. Some examples of catastrophic meteorites include Nagasaki on 31 March 1979 and Menorca on 15 June 2006, with subsequent losses in the tens of euros.
Meteotsunami should not be confused with stormy waves, which are local increases in sea level associated with low barometer pressure passing tropical cyclones, nor should they be confused with the setup, the temporary localization of sea level Growth that is caused by strong-medium winds. Storm surge and setup in severe weather are also dangerous causes of coastal flooding, but their dynamics are completely unrelated to tsunami waves. They are unable to propagate beyond their sources, as the waves do.
Man-made or triggered tsunamis
The ability to make tsunami waves as a tectonic weapon and to involve at least one real effort has been studied.
In World War II, New Zealand military forces introduced Project Seal, which attempted to produce a small tsunami with explosives in the area of today's Shakespeare Regional Park; Attempt failed.
There has been much speculation about the possibility of using nuclear weapons to cause a tsunami near the enemy's coastline. The idea of using traditional explosives was also considered during World War II. Nuclear testing at the Pacific Proving Ground by the United States produced poor results. Two 20 kiloton TNT (84 TJ) bombs at Operation Crossroads, one in the air and one underwater, above and below the shallow (50 m (160 ft)) water of the Bikini Atoll Lagoon. At a distance of about 6 km (3.7 mi) from the nearest island, the waves there did not exceed 3–4 m (9.8–13.1 ft) upon reaching the shoreline. Other underwater tests, mainly Hardtack I / Wahoo (deep water) and Hardtech I / Umbrella (shallow water), confirmed the results. Analysis of the effects of shallow and deep underwater explosions shows that the energy of the explosions is not easily generated like the deep, all-ocean waves that are tsunamis; Most of the energy creates steam, causes vertical fountains above water, and creates compressed waves. Tsunamis are identified by permanent large vertical displacements of very large volumes of water that do not occur in bursts.
Characteristics
When the wave enters shallow water, it slows down and its amplitude (height) increases.
The wave further slows and amplifies as it hits land. Only the largest waves crest.
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travels at high speeds, and a large amount of water's destructive force escapes from the ground and carries large amounts of debris, even That does not seem to be large even with the waves.
While everyday airwaves have a wavelength of about 100 m (330 ft) (from the crest) and a height of about 2 m (6.6 ft), a deep-sea tsunami has a large wavelength of 200 km (up to 120). Mi). Such a wave covers a distance of more than 800 kilometers per hour (500 mph), but due to the strong wavelength at any point, the wave takes 20 or 30 minutes to complete a cycle and is only 1 meter long. (3.3 feet). This makes it difficult for tsunamis to detect deep waters where ships are unable to feel their way.
The velocity of a tsunami can be calculated by obtaining the square root of the water depth in meters multiplied by the acceleration due to gravity (estimated for 10 m sec2). For example, if the Pacific Ocean is considered to be 5000 meters deep, the tsunami's velocity would be a square root of square5000 x 10 = 0050000 = ~ 224 meters per second (735 feet per second), which equals ~ 806 kilometers per second. An hour or at a speed of about 500 mph. This formula is used to calculate the velocity of shallow waves, because a tsunami behaves like a shallow wave as it reaches the surface from the summit to the summit.
The reason for the Japanese name "Harbor Wave" is that sometimes fishermen from a village went out, and no unusual wave came while fishing in the sea, and to find their village devastated by a huge wave Come back to the ground.
As the tsunami approaches the coast and the water becomes shallow, the ripple of the wave narrows and its speed decreases to 80 kilometers per hour (50 mph). Its wavelength decreases below 20 kilometers (12 mi) and its amplitude increases according to Green's law. Since the wave still has a very long duration, the tsunami may take minutes to reach full height. Except for very large tsunamis, the approaching wave does not break but appears like a high-speed tidal bore. Open bays and coastlines adjoining very deep waters can shape a tsunami-like a step-wave with a broken front.
When a tsunami wave hits the edge of the peak, a temporary rise in sea level is called a run-up. The run-up is measured in meters above a reference sea level. A large tsunami can cause multiple waves over a period of several hours, which is the critical time between wave cracks. The first wave to reach the shore may not have the most runs.
About 80% of tsunamis occur in the Pacific Ocean, but they are possible where there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic eruptions, glacier mines, and dialects.
Drawback
An illustration of the rhythmic "faults" of surface water associated with the wave. It follows that a very large flaw can counteract the arrival of a very large wave.
All waves have a positive and negative peak, namely a ridge and a trough. In the case of a propagation wave similar to that of a tsunami, either may come first. If the first part of the coast is the ridge, the first impact of a huge breaking wave or sudden flood will be seen on the land. However, if the first part to come is a trough, then there will be a shortage, as the shoreline fills dramatically to expose the normally submerged areas. Drawbacks can exceed hundreds of meters, and people unaware of the danger sometimes live near the shore to calm their curiosity or collect fish from exposed seabeds.
A typical wave duration for a catastrophic tsunami is about 12 minutes. This means that if the drawback phase is the first part of the incoming wave, the ocean will perish, with areas below sea level after 3 minutes. During the next 6 minutes, the trough of the tsunami wave forms into a ridge, and during this time the sea fills and destruction occurs on land. During the next 6 minutes, the tsunami wave changes from a ridge to a pool, draining floodwater and flooding again. It can sweep the victims and dump debris at some distance from the ground. The process is repeated as soon as the next wave arrives.
Scales of intensity and magnitude
As with earthquakes, several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events.
Intensity scales
The first scales used to measure tsunami intensity were the Seidenberg-Ambresse scale used on the Mediterranean Sea and the Imamura-Ida intensity scale, which was used in the Pacific Ocean. The latter scale was modified by Soloviev, who calculated the intensity of the tsunami according to the formula
Where is the average wave height along the nearest coast? This scale, known as the Soloviev-Imamura tsunami intensity scale, is used as the main parameter for tsunami size in the Global Tsunami Catalog compiled by NGDC / NOAA and the Novosibirsk Tsunami Laboratory.
In 2013, following the intensively studied tsunamis in 2004 and 2011, a new 12 point scale was proposed, the Integrated Tsunami Intensity Scale (ITIS-2012), aimed at revised ESI2007 and EMS earthquake intensity scales. But had to be closely matched.
Magnitude scales
The first scale that calculates a magnitude for a tsunami rather than an intensity at a particular location was the ML scale proposed by Murty and Loomis based on potential energy. The laxity in calculating the potential energy of a tsunami means that this scale is rarely. Used. Abe introduced the scale of magnitude of a tsunami, calculated from,
Where h is the maximum tsunami-wave amplitude (in meters) measured by a tide gauge at a distance of R from the subcenter, a, b and D are constants and are used to match the mount scale with the moment magnitude scale. Is made to make possible from.
Warnings and predictions
Tsunami warning sign
Deficiencies can serve as a brief warning. Those who see the deficiencies (many survivors report a sucking sound together), can only survive when they immediately run for higher ground or seek the upper floors of nearby buildings. In 2004, ten-year-old Tilly Smith of Surrey, England, was at the Mychao beach in Phuket, Thailand with her parents and sister, and after learning about the tsunami at school recently, she told her family that The tsunami may be imminent. Her parents warned others before the wave arrived, saving dozens of lives. She credited her geography teacher Andrew Kearn.
In 2004, the lack of a tsunami in the Indian Ocean was not reported on the African coast or any other eastern-facing coasts as far as it reached. The reason for this was that the wave moved down the eastern side of the fault line and down the western side. The Western pulse affected coastal Africa and other western regions.
A tsunami cannot be accurately predicted, even if the intensity and location of the earthquake are known. Geologists, oceanographers, and seismologists analyze each earthquake and may or may not issue tsunami warnings based on several factors. However, there are some warning signs of imminent tsunamis, and automated systems can warn immediately after an earthquake to save lives. One of the most successful systems uses a lower pressure sensor, which is connected to the boys, which continuously monitors the pressure of the water column.
Areas with high tsunami risk typically use a tsunami warning system to warn the population before the wave hits the ground. On the west coast of the United States, which suffers from the Pacific Ocean tsunami, warning signs indicate evacuation routes. In Japan, the community is well-educated about earthquakes and tsunamis, and tsunami warning signs along Japanese coastlines are reminiscent of natural hazards as well as a network of warning sirens, usually on top of ambient hills.
The Pacific Tsunami Warning System is located in Honolulu, Hawaii. It monitors the seismic activity of the Pacific Ocean. The magnitude and other information of a sufficiently large earthquake triggered a tsunami warning. While sub-regions around the Pacific are seismically active, not all earthquakes produce tsunamis. Computers help to analyze the tsunami risk of every earthquake in the Pacific Ocean and its adjacent terrain.
As a direct result of the Indian Ocean tsunami, the tsunami threat to all coastal areas is being re-evaluated by national governments and the United Nations Disaster Reduction Committee. A tsunami warning system is being installed in the Indian Ocean.
One of the deep water used in the DARTtsunami warning system
One of the deep water used in the DARTtsunami warning system
Computer models can usually predict tsunamis to occur within minutes of arrival time. Lower pressure sensors can relay information in real time. Based on these pressure readings and other seismic information and seafloor size (bathymetry) and coastal topography, the models estimate the amplitude and height of the near tsunami. All Pacific Rim countries cooperate in tsunami warning systems and most regularly practice evacuation and other procedures. In Japan, such preparation is mandatory for the government, local authorities, emergency services, and populations.
Some zoologists hypothesize that some animal species have the ability to feel Rayleigh waves arising from earthquakes or tsunamis. If true, monitoring their behavior may provide advance warning of earthquakes, tsunamis, etc. However, the evidence is controversial and not widely accepted. There are unfounded claims about the Lisbon earthquake that some animals fled to higher ground, while many other animals from the same area drowned. The incident was also noted by media sources in Sri Lanka in the 2004 Indian Ocean. [6] [65] It is possible that some animals (eg, elephants) may have heard the sound of a tsunami as it approached the coast. The elephants' response was to move away from the nearby noise. In contrast, some people went ashore to investigate and many drowned as a result.
On the west coast of the United States, warnings are made on television and radio through the National Weather Service using an emergency weather system, in addition to sirens.
Forecast of the possibility of tsunami attack
Kunihiko Shimazaki (University of Tokyo), a member of the Earthquake Research Committee of the Headquarters of the Government of Japan's Earthquake Research Promotion, stated the plan for a public announcement of the possibility of a tsunami attack on 12 May 2011 at the Japan National Press Club. This forecast also includes tsunami altitude, area of attack, and the probability of occurrence within 100 years. The forecast will integrate recent interdisciplinary scientific knowledge and the subsequent integration of the 2011 Tohoku earthquake and tsunami. The announcement will be available from 2014, as planned.
Mitigation
Tsunami barrier
A beach in Tsu, Japan
In some tsunami-prone countries, earthquake engineering measures have been taken to reduce the damage caused by earthquakes.
Japan, where tsunami science and response measures first began after a disaster in 1896, has sometimes produced more detailed countermeasures and response plans. The country has built several tsunami walls up to 12 meters (39 ft) to protect populated coastal areas. Floodgates of 15.5 meters (51 ft) high and channels have been built to redirect water from the tsunami that has come in other areas. However, their effectiveness has been questioned, as tsunamis often cross obstacles.
The Fukushima Daiichi nuclear disaster was directly affected by the 2011 Tohoku earthquake and tsunami, when the waves crossed the height of the plant's sea wall. The Evette prefecture, which is a high-risk area from the tsunami, had 25 km (16 mi) long total tsunami barriers in coastal cities. The 2011 tsunami toppled over 50% of the walls and caused catastrophic damage.
Okushiri, the Hokkaido tsunami that struck Okushiri Island in Hokkaido within two to five minutes of the July 12, 1993 earthquake, generating waves 30 meters (100 ft) high, as high as a 10-story building. The port city 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 been able to slow down and reduce the height of the tsunami, but it did not prevent major destruction and loss of life.
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