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TECTONICS
Chilean quake among top 10 strongest on record

What is a tsunami?
Tokyo, Feb 28, 2010 (AFP) - A once-exotic word that has now entered everyday use as a term tinged with fear, a tsunami refers to a shock of water propagated through the sea, usually after an undersea quake. A section of seabed is thrust up or driven down by movement of the Earth's crust. The rift displaces vast quantities of water that move as waves, able to span enormous distances and sometimes with the speed of a jet plane. The 8.8-magnitude quake that slammed central Chile on Saturday killing at least 300 people sent giant waves roaring across the Pacific Ocean that forced Japan on a tsunami alert almost a day later. The word "tsunami" comes from the Japanese words for "harbour" and "wave".

When tsunamis approach a coastline, the shelving of the sea floor causes them to slow down -- but also gain in height. To those on the shore, the first sign of something amiss is an eerie retreat of the sea, which is followed by the arrival of exceptional waves. "The sea was driven back, and its waters flowed away to such an extent that the deep seabed was laid bare and many kinds of sea creatures could be seen," wrote Roman historian Ammianus Marcellus, awed at a tsunami that struck the then-thriving port of Alexandria in 365 AD. "Huge masses of water flowed back when least expected, and now overwhelmed and killed many thousands of people.... Some great ships were hurled by the fury of the waves onto the rooftops, and others were thrown up to two miles (three kilometres) from the shore." Several factors determine the height and destructiveness of a tsunami.

They include the size of the quake, the volume of displaced water, the topography of the sea floor as the waves race to the coast and whether there are natural obstacles that dampen the shock. Destruction of protective mangroves and coral reefs and the building of homes or hotels on exposed beaches are fingered as leading causes of high death tolls from tsunamis. Large quakes are the main drivers of tsunamis, but the phenomenon can also be sparked by other cataclysmic events, such as volcanic eruptions and even landslides. In 1883, a volcano shattered the Pacific island of Krakatoa, causing a blast so loud that it could be heard 4,500 kilometres (2,800 miles) away, followed by a tsunami that killed some 30,000 people.

The great tsunami of December 2004 in the Indian Ocean was caused by a monstrous 9.1-magnitude earthquake off the Indonesian island of Sumatra. It released the energy of 23,000 Hiroshima-type atomic bombs, according to the US Geological Survey (USGS). Some 220,000 people in 11 nations were killed, many of them thousands of kilometres from the epicentre. The Pacific Ocean is particularly prone to earthquakes and therefore to tsunamis. But recent research has found that, over the millennia, tsunamis have occurred in many parts of the world, including the Atlantic and Mediterranean. A global monitoring network, overseen by the UN, has been set in place to alert areas at risk.
by Staff Writers
Santiago (AFP) Feb 27, 2010
The 8.8 magnitude earthquake that slammed central Chile on Saturday is among the top 10 strongest on record, according to data from the US Geological Survey, which monitors earthquakes around the world.

Eighteen of the top 20 most powerful earthquakes, including the strongest, struck in a zone of volcanic instability that encircles the Pacific Ocean known as the "Ring of Fire."

The "Ring" stretches along the western coast of the Americas through the island nations of the South Pacific and on through Southeast Asia.

Half of the top 20 strongest earthquakes struck the Pacific coast of the Americas, and six of those -- including four in Chile -- struck south America.

The most powerful earthquake ever, a magnitude 9.5 whopper, struck Chile on May 22, 1960 not far from the epicenter of Saturday's earthquake.

The 1960 quake, known as the Valdivia quake, killed 1,655 people, injured 3,000 and left two million homeless. It triggered a tsunami that killed 61 people in Hawaii, 138 in Japan and 32 in the Philippines.

The top 20 earthquakes, according to the USGS and not counting Saturday's are:

- May 22, 1960 - south-central Chile - magnitude 9.5

- March 28, 1964 - Prince William Sound, Alaska - magnitude 9.2

- December 26, 2004 - Sumatra-Andaman Islands - magnitude 9.1

- November 4, 1952 - Kamchatka peninsula, former Soviet Union - magnitude 9.0

- August 13, 1868 - Arica, Peru (now Chile) - magnitude 9.0

- January 26, 1700 - Cascadia Subduction Zone (north-western US coast/southern British Columbia, Canada) - magnitude 9.0

- January 31, 1906 - Off the Coast of Esmeraldas, Ecuador - magnitude 8.8

- February 4, 1965 - Rat Islands, Alaska - magnitude 8.7

- November 1, 1755 - Lisbon, Portugal - magnitude 8.7

- July 8, 1730 - Valparasio, Chile - magnitude 8.7

- March 28, 2005 - Northern Sumatra, Indonesia - magnitude 8.6

- March 9, 1957 - Andreanof Islands, Alaska - magnitude 8.6

- August 15, 1950 - Assam - Tibet - magnitude 8.6

- September 12, 2007 - Southern Sumatra, Indonesia - magnitude 8.5

- October 13, 1963 - Kuril Islands, former Soviet Union - magnitude 8.5

- February 1, 1938 - Banda Sea, Indonesia - magnitude 8.5

- February 3, 1923 - Kamchatka peninsula - magnitude 8.5

- November 11, 1922 - Chile-Argentina Border - magnitude 8.5

- June 15, 1896 - Sanriku, Japan - magnitude 8.5

- October 20, 1687 - Lima, Peru - magnitude 8.5



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TECTONICS
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Kathmandu (AFP) Feb 10, 2010
As Haiti counts the cost of last month's devastating earthquake, experts are warning of the potential for an even greater disaster in another of the world's poorest countries, Nepal. Geologists say it is only a matter of time before a major earthquake hits Nepal's densely populated capital Kathmandu, where 2.5 million people live in cramped, poorly-built housing with little or no awareness o ... read more







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