March 2011
Last updated at
08:43 GMT
A tsunami (pronounced sue-nahm-ee) is a series of huge waves that can cause great devastation and loss of life when they strike a coast.
Tsunamis are caused by an underwater earthquake, a volcanik eruption sub-marine rockslide, or, more rarely, by an asteroid or meteorid crashing into in the water from space. Most tsunamis are caused by underwater earthquakes, but not all underwater earthquakes cause tsunamis - an earthquake has to be over about magnitude 6.75 on the Richter scale for it to cause a tsunami. About 90 percent of all tsunamis occur in the Pacific Ocean.
Many tsunamis could be detected before they hit land, and the loss of life could be minimized, with the use of modern technology, including seismographs (which detect earthquakes), computerized offshore buoys that can measure changes in wave height, and a system of sirens on the beach to alert people of potential tsunami danger.
NOTE: If you see the water recede quickly and unexpectedly from a beach (this is called drawback), run toward higher ground or inland -- there may be a tsunami coming. Also, if you are on the coast and there is an earthquake, it may have caused a tsunami, so run toward higher ground or inland. Some beaches have tsunami warning sirens -- do not ignore them. The first wave in a tsunami is often not the largest; if you experience one abnormally-huge wave, go inland quickly -- even bigger waves could be coming soon.
The word tsunami comes from the Japanese word meaning "harbor wave." Tsunamis are sometimes incorrectly called "tidal waves" -- tsunamis are not caused by the tides (tides are caused by the gravitational force of the moon on the sea). Regular waves are caused by the wind.
The Development of a Tsunami:
A tsunami starts when a huge volume of water is quickly shifted. This rapid movement can happen as the result of an underwater earthquake (when the sea floor quickly moves up or down), a rock slide, a volcanic eruption, or another high-energy event.
After the huge volume of water has moved, the resulting wave is very long (the distance from crest to crest can be hundred of miles long) but not very tall (roughly 3 feet tall). The wave propagates (spreads) across the sea in all directions; it can travel great distances from the source at tremendous speeds.
The Size of a Tsunami:
Tsunamis have an extremely long wavelength (wavelength is the distance between the crest (top) of one wave and the crest of the next wave) -- up to several hundred miles long. The period (the time between two successive waves) is also very long -- about an hour in deep water.
In the deep sea, a tsunami's height can be only about 1 m (3 feet) tall. Tsunamis are often barely visible when they are in the deep sea. This makes tsunami detection in the deep sea very difficult.
The Speed of a
Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the
open ocean - as fast as a jet flies. It can take only a few hours for a tsunami
to travel across an entire ocean A regular wave (generated
by the wind) travels at up to about 90 km/hr.
As a tsunami wave approaches the coast (where the sea becomes shallow), the trough (bottom) of a wave hits the beach floor, causing the wave to slow down, to increase in height (the amplitude is magnified many times) and to decrease in wavelength (the distance from crest to crest).
At landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce higher tsunami waves.
In addition to large tsunami waves that crash onto shore, the waves push a large amount of water onto the shore above the regular sea level (this is called runup). The runup can cause tremendous damage inland and is much more common than huge, thundering tsunami waves.
Tsunami Warning
Systems:
Tsunami warning systems exist in many places around the
world. As scientists continuously monitor seismic activity (earthquakes), a
series of buoys float off the coast and monitor changes in sea level.
Unfortunately, since tsunamis are not very tall in height when they are out at
sea, detection is not easy and there are many false alarms. Sirens at affected
beaches may be activated -- do not ignore them!
Regular waves (caused by the wind) are very different from tsunami waves. Tsunami waves are much faster than wind-generated waves and they have a much longer wavelength (the distance from crest to crest). In the deep sea, tsunami waves are very small, but by the coast, they dwarf regular waves.
How Often do
Tsunamis Occur?
Tsunamis are very rare. There are roughly six major
tsunamis each century.
A magnitude 7.7
earthquake shook Indonesia's northwest island of Sumatra early Wednesday,
triggering a small tsunami, snapping power lines and sending panicked residents
rushing for higher ground.
There were scattered reports of injuries, but only minor damage in most areas.
The quake struck at 5:15 a.m. (2215 GMT) and was centered undersea 125 miles
northwest of the coastal town of Sibolga in Sumatra at a depth of 19 miles, the
U.S. Geological Survey said. It had earlier said the quake measured 7.8.
Indonesia's Meteorology and Geophysics Agency and the Pacific Tsunami Warning
Center in Honolulu issued tsunami warnings following the quake, but lifted them
two hours later.
The meteorology agency said a minor tsunami that at its largest was about 16
inches high hit Banyak island, Nias island and Sibolga.
Safnil, the head of a
fishing village on Banyak, said the wave inundated homes along the beach and
damaged several boats but caused no casualties. No tsunami damage was reported
elsewhere.
A hospital on Simeulue island off the coast of Sumatra admitted 17 people for
treatment of injuries sustained in the quake, including four in critical
condition, said Capt. Ajas Siagian, a deputy police chief for the area.
Abdul Karim, a government spokesman in Simeulue, said dozens of houses collapsed
or were damaged in Teupah Selatan village. He said no larger buildings were
reported damaged, but electricity had been knocked out on the island.
"We are still collecting reports of damages and injuries ... but the situation
has returned to normal and people are going back home," he said.
Local network Metro TV reported that a dormitory for nurses partially collapsed
in Aceh province's Singkil district and one woman suffered minor injuries in the
rush to get out of the building. It said fires sparked by the quake had burned
at least 14 houses in Medan, the capital city of North Sumatra province.
Electricity was cut in Medan, Banda Aceh, the provincial capital of Aceh, and
other areas.
Paulus Prihandojo, another seismologist with the meteorology agency, said along
with the quake's relative depth, the epicenter was too far away from the major
population centers of Medan and Banda Aceh to cause major damage.
He said areas closer to the epicenter were more sparsely populated and many of
the buildings in that area were made of wood, which fares better in earthquakes
than brick and cement.
President Susilo Bambang Yudhoyono said the country had dodged what could have
been a disaster.
"Thank God it did not lead to a major tsunami and there is no significant damage,"
he said.
At least five strong aftershocks measuring up to 5.2 were recorded, the
meteorology agency said.
People in several cities along the southeastern coast of Sumatra as well as
Sinabang on Simeulue island and Gunung Sitoli on nearby Nias island poured into
the streets and rushed to higher ground after the quake.
"Rumors about a tsunami panicked villagers living near the beach," said Eddy
Effendi, a resident on Nias island. "They ran away on motorbikes and cars or by
climbing the hills. There was panic and chaos everywhere, but I don't see
serious damage or injuries in my village."
Residents in Sibolga said the shaking lasted more than a minute and utility
poles in the area were knocked down.
A 2004 tsunami triggered by a magnitude-9.2 earthquake in the same part of
Indonesia killed 230,000 people in a dozen countries on the Indian Ocean basin.
Tsunami hits north-eastern Japan after massive quake
A massive earthquake has hit the north-east of Japan, triggering a tsunami that has caused extensive damage.
Japanese television showed cars, ships and even buildings being swept away by a vast wall of water after the 8.9-magnitude earthquake.
The quake has sparked fires in several areas including Tokyo, and numerous casualties are feared.
It struck about 250 miles (400km) from the capital at a depth of 20 miles. There have been powerful aftershocks.
The tremor hit at 1446 local time (0546 GMT). Seismologists say it is one of the largest earthquakes to hit Japan for many years.
The tsunami warning was extended to the Philippines, Indonesia, Taiwan, the Pacific coast of Russia and Hawaii.
The Pacific Tsunami Warning Center said the wave could extend as far as Chile.
Tsunami waves hit Japan's Miyagi and Fukushima prefectures, officials said.
Japan's NHK television showed a massive surge of debris-filled water sweeping away buildings, cars and ships and reaching far inland.
The earthquake also triggered a number of fires, including one at an oil refinery in Ichihara city in Chiba prefecture near Tokyo, engulfing storage tanks.
There were reports of about 20 people injured in Tokyo after the roof of a hall collapsed on to a graduation ceremony.
Residents and workers in Tokyo rushed out of apartment buildings and office blocks and gathered in parks and open spaces as aftershocks continued to hit.
Bullet train services to northern Japan were halted, rapid transit in Tokyo was suspended and some nuclear power plants automatically shut down.
Prime Minister Naoto Kan said there had been no radiation leaks.
In a televised address, he extended his sympathy to the victims of the disaster and said an emergency response headquarters had been set up.
He said the earthquake had a magnitude of 8.4 while the US Geological Survey said it measured 8.9.
With Love and regards
Leader of the world tourism
Reha Uzundere
Chairman
Citation: CBSNEVS/Tripod.com,BBC NEWS
Webmaster: Reha Uzundere
