Tsunami Warnings and Prevention
Tag:
tsunami warnings and prevention
A tsunami cannot be prevented or precisely predicted - even if the right magnitude of an earthquake occurs in the right location. Geologists, Oceanographers and Seismologist analyse each earthquake and based upon many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunami. One of the most important systems that is used and constantly monitored are bottom pressure sensors. These are anchored and attached to buoys. Sensors on the equipment constantly monitor the pressure of the overlying water column - this can be deduced by the simple calculation of:
where
P = the overlying pressure in Newtons per metre square,
ρ = the density of the seawater= 1.1 x 103 kg/m3,
g = the acceleration due to gravity= 9.8 m/s2 and
h = the height of the water column in metres.
Hence for a water column of 5,000 m depth the overlying pressure is equal to
or about 5.7 Million tonnes per metre square.
In instances where the leading edge of the tsunami wave is the trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope of the coastal seabed is shallow, this recession can exceed many hundreds of meters. People unaware of the danger may remain at or near the shore out of curiosity, or for collecting fish from the exposed seabed. During the Indian Ocean tsunami of 26th December 2004, the sea withdrew and many people then went onto the exposed sea bed to investigate. Pictures taken show people on the normally submerged areas with the advancing wave in the background. Most people who were on the beach were unable to escape to high ground and died.
Regions with a high risk of tsunami may use tsunami warning systems to detect tsunami and warn the general population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs advise people of evacuation routes.
The Pacific Tsunami Warning System is based in Honolulu. It monitors all sesimic activity that occurs anywhere within the Pacific. Based up the magnitude and other information a tsunami warning may be issued. It is important to note that the subduction zones around the Pacific are seismically active, but not all earthquakes generate tsunami and for this reason computers are used as a tool to assist in analysing the risk of tsunami generation of each and every earthquake that occurs in the Pacific Ocean and the adjoining land masses.
As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat of all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is currently being installed in the Indian Ocean.
Computer models can predict tsunami arrival - observations have shown that predicted arrival times are usually within minutes of the predicted time. Bottom pressure sensors are able to relay information in real time and based upon the readings and other information about the seismic event that triggered it and the shape of the seafloor (bathymetry) and coastal land (topography), it is possible to estimate the amplitude and therefore the surge height, of the approaching tsunami. All the countries that border the Pacific Ocean collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures to prepare people for the inevitable tsunami. In Japan such preparation is a mandatory requirement of government, local authorities, emergency services and the population.
Some zoologists hypothesise that animals may have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. Some animals seem to have the ability to detect natural phenomena and if correct, careful observation and monitoring could possibly provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and has not been proven scientifically. There are some unsubstantiated claims that animals before the Lisbon quake were restless and moved away from low lying areas to higher ground. Yet many other animals in the same areas drowned. The phenomenon was also noted in Sri Lanka in the 2004 Indian Ocean earthquake. The following two references whilst relevant, are media and not scientific. It is possible that certain animals (e.g., elephants) may have heard the sounds of the tsunami as it approached the coast. The elephants reaction was to move away from the approaching noise - inland. Some humans, on the other hand, went to the shore to investigate and many drowned as a result.
It is not possible to prevent a tsunami. However, in some 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 tsunami. However, their effectiveness has been questioned, as tsunami often surge higher than the barriers. For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake 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 may 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 absorbed by 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 seacoast which are prone to tsunami risks. It would take some years for the trees to grow to a useful size, but such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the construction of artificial barriers.
Indian Ocean Tsunami
Tag:
indian ocean tsunami
According to an article in "Geographical" magazine (April 2008), the Indian Ocean tsunami of 26th December 2004 was not the worst that the region could expect. Professor Costas Synolakis of the Tsunami Research Center at the University of Southern California co-authored a paper in "Geophysical Journal International" which suggests that a future tsunami in the Indian Ocean basin could affect locations such as Madagascar, Singapore, Somalia, Western Australia and many others. The Boxing Day tsunami killed over 300,000 people with many bodies either being lost to the sea or unidentified. Some unofficial estimates have claimed that approximately 1 million people may have died directly or indirectly solely as a result of the tsunami.
Signs Of An Approaching Tsunami
Tag:
tsunami, tsunami photos, tsunami pictures, tsunami videos, tusunami
There is often no advance warning of an approaching tsunami. However, since earthquakes are often a cause of tsunami, any earthquake occurring near a body of water may generate a tsunami if it occurs at shallow depth, is of moderate or high magnitude, and the water volume and depth is sufficient. In Japan moderate - 4.2 Magnitude earthquakes can generate tsunami which can inundate the area within 15 minutes.
If the first part of a tsunami to reach land is a trough (draw back) rather than a crest of the wave, the water along the shoreline may recede dramatically, exposing areas that are normally always submerged. This can serve as an advance warning of the approaching tsunami which will rush in faster than it is possible to run. If a person is in a coastal area where the sea suddenly draws back (many survivors report an accompanying sucking sound), their only real chance of survival is to run for high ground or seek the high floors of high rise buildings.
In the 2004 tsunami that occurred in the Indian Ocean drawback was not reported on the African coast or any other western coasts it inundated, when the tsunami approached from the east. This was because of the nature of the wave - it moved downwards on the eastern side of the fault line and upwards on the western side. It was the western pulse that inundated coastal areas of Africa and other western areas.
80% of all tsunamis occur in the Pacific Ocean, but are possible wherever large bodies of water are found, including inland lakes. They may be caused by landslides, volcanic explosions, bolides and seismic activity.
Tsunami Causes
Tag:
tsunami causes
A tsunami can be generated when converging or destructive plate boundaries abruptly move and vertically displace the overlying water. It is very unlikely that they can form at divergent (constructive) or conservative plate boundaries. This is because constructive or conservative boundaries do not generally disturb the vertical displacement of the water column. Subduction zone related earthquakes generate the majority of all tsunamis.
On 1 April 1946 a Magnitude 7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 m high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.
Examples of tsunami being generated at locations away from convergent boundaries include - Storegga during the Neolithic era, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). In the case of the Grand Banks and Papua New Guinea tsunamis an earthquake caused sediments to become unstable and subsequently fail. These slumped and as they flowed down slope a tsunami was generated. These tsunami did not travel transoceanic distances.
It is not known what caused the Storegga sediments to fail. It may have been due to overloading of the sediments causing them to become unstable and they then failed solely as a result of being overloaded. It is also possible that an earthquake caused the sediments to become unstable and then fail. Another theory is that a release of gas hydrates (methane etc.,) caused the slump.
The "Great Chilean earthquake" (19:11 hrs UTC) 22 May 1960 (9.5 Mw), the 27th March 1964 "Good Friday earthquake" Alaska 1964 (9.2 Mw), and the "Great Sumatra-Andaman earthquake" (00:58:53 UTC) 26 December 2004 (9.2 Mw), are recent examples of powerful megathrust earthquakes that generated a tsunami that was able to cross oceans. Smaller (4.2 Mw) earthquakes in Japan can trigger tsunami that can devastate nearby coasts within 15 minutes or less.
In the 1950s it was hypothesised that larger tsunamis than had previously been believed possible may be caused by landslides, explosive volcanic action e.g., Santorini, Krakatau, and impact events when they contact water. 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 at a rate faster than the ocean water can absorb it. They have been named by the media as "mega-tsunami."
Tsunami caused by these mechanisms, unlike the trans-oceanic tsunami caused by some earthquakes, may 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 1958, which produced a wave with an initial surge estimated at 524 m. However, an extremely large gravitational landslide might generate a so called "mega-tsunami" that may have the ability to travel trans-oceanic distances. This though is strongly debated and there is no actual geological evidence to support this hypothesis.
Tsunami Terminology
Tag:
tsunami terminology
The term tsunami comes from the Japanese meaning harbor and wave. For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in Japanese. Tsunamis are common throughout Japanese history; approximately 195 events in Japan have been recorded.
A tsunami has a much smaller amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 mm above the normal sea surface. A tsunami can occur at any state of the tide and even at low tide will still inundate coastal areas if the incoming waves surge high enough.
Tsunamis are often referred to popularly as tidal waves. This term is inaccurate because tsunamis are not related to tides and its use is discouraged by geologists and oceanographers; however, it is worth noting that the term tsunami is no more accurate because tsumanis are not limited to harbours.
Tsunami
Tag:
tsunami, tsunami photos, tsunami pictures, tsunami videos, tusunami
A tsunami is a series of waves created when a body of water, such as an ocean, is rapidly displaced. Earthquakes, mass movements above or below water, some volcanic eruptions and other underwater explosions, landslides, underwater earthquakes, large asteroid impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami. The effects of a tsunami can be devastating due to the immense volumes of water and energy involved. Since meteorites are small, they will not generate a tsunami.
The Greek historian Thucydides was the first to relate tsunamis to submarine quakes, but understanding of the nature of tsunamis remained slim until the 20th century and is the subject of ongoing research.
Many early geological, geographic, oceanographic etc; texts refer to "Seismic sea waves" - these are now referred to as "tsunami."
Some meteorological storm conditions - deep depressions causing cyclones, hurricanes; can generate a storm surge which can be several metres above normal tide levels. This is due to the low atmospheric pressure within the centre of the depression. As these storm surges come ashore the surge can resemble a tsunami, inundating vast areas of land. These are not tsunami. Such a storm surge inundated Burma (Myanmar) in May 2008.
Indonesia Tsunamis 1883
Tag:
tsunami, tsunami history, tsunami photos, tsunami pictures, tsunami videos, tusunami
The
volcanic explosion of Krakatoa is one of the most impressive natural
disasters ever recorded in history. On August 26th 1883, the island
volcano of Krakatoa exploded with devastating fury, blowing its
underground magma chamber partly empty so that much overlying land and
seabed collapsed into it. The great majority of the island simply was
destroyed as it sank to the ocean floor. The volcanic disturbance
triggered a series of large tsunami waves, some reaching a height of
over 40 meters above sea level. Although no one is known to have been
killed as a result of the initial explosion, the tsunamis it generated
had disastrous results, killing over 36,000 people, and wiping out a
number of settlements, including Telok Batong in Sumatra, and Sirik and
Semarang in Java.
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. Ships as far away as South Africa rocked
as tsunamis hit them, and the bodies of victims were found floating in
the ocean for weeks after the event. There are even numerous documented
reports of groups of human skeletons floating across the Indian Ocean
on rafts of volcanic pumice and washing up on the east coast of Africa
up to a year after the eruption.
Portugal and Much of Europe Tsunamis 1755
Tag:
tsunami, tsunami history, tsunami photos, tsunami pictures, tsunami videos, tusunami
On
November 1st 1755, one of the biggest earthquakes in history occurred
in the Atlantic Ocean just of the coast of the Portuguese capital city
of Lisbon. The total duration of shaking lasted ten minutes and was
comprised of three distinct jolts. Scientists estimate that the
earthquake was in the range of 9.0 on the Richter scale, which caused
extensive damage throughout Lisbon. Surprisingly, the events that
unfolded from this disaster has been well-documented.
After the earthquake, survivors rushed to the open space of the
docks for safety and watched as the water receded, revealing the sea
floor, littered by lost cargo and old shipwrecks. About 35 minutes
after the initial earthquake, an enormous tsunami engulfed the
Portuguese harbor and the city’s downtown. Two other tsunamis followed
to add to more devastation to the already suffering area. Effects from
the earthquake and tsunamis were far reaching. The worst damage
occurred in the south-west of Portugal, which included Lisbon. The
tsunami reached, with less intensity, the coast of Spain, France, Great
Britain, Ireland, Belgium and Holland. In Madeira and in the Azores
islands damage was extensive and many ships were in danger of being
wrecked. In total, over 100,000 people were killed, with most
fatalities incurred in Lisbon, where over a third of the population
were instantaneously wiped out. This tragic disaster served as the
impetus for earthquake research in the world.
Japan Tsunamis 1896
Tag:
tsunami, tsunami history, tsunami photos, tsunami pictures, tsunami videos, tusunami
On
June 15 1896, an earthquake occurred of the coast near the Japanese
port city of Sanriku. The earthquake, which measured 7.2 on the
Richter scale, triggered the formation a massive tsunami that
devastated the city killing over 26,000 people. The tsunami waves
reached an intimidating height of 25 meters (80 feet) as it crashed
upon a crowd that had gathered in a city to celebrate a religious
festival. The tsunami was also observed across the Pacific: In Hawaii,
wharves were demolished and several houses were swept away. In
California, a 9.5 feet wave was observed, according to the San
Francisco Chronicle of June 16, 1896. This Sanriku tsunami served as
an impetus for tsunami research in Japan.
What is unusual about this disaster is that the size of the tsunami
was much larger than would be expected from the size of the earthquake,
7.2 on the Richter scale. At the time of the Sanriku tsunami
earthquake, a weak shock was felt, followed by an extremely slow
shaking that lasted about 5 minutes. Approximately 35 minutes after the
earthquake, the large tsunami arrived at the Sanriku coast.
Chile and Pacific Nations Tsunamis 1960
Tag:
tsunami, tsunami history, tsunami photos, tsunami pictures, tsunami videos, tusunami
On
May 22 1960, the biggest earthquake ever recorded at the time occurred
just of the coast of South central Chile, a nation of South America.
The earthquake measured 9.5 on the Richter scale with swarms of
aftershock earthquakes that measured as large 8.0 that followed. The
earthquakes triggered the creation of tsunami, which was responsible
for most of the ensuing devastation and death.
The tsunami, together with the coastal subsidence and flooding,
caused tremendous damage along the Chile coast, where about 2,000
people died. The waves spread outwards across the Pacific. Fifteen
hours after the earthquake, tsunami waves flooded Hilo, on the island
of Hawaii, where they built up to thirty feet and caused 61 deaths
along the waterfront. Seven hours after that the waves flooded the
coastline of Japan where ten-foot waves caused 200 deaths. Tsunami
waves also caused damage in the Marquesas, in Samoa, and in New
Zealand.
