Scientists greatly worried at rapid increase in Earthquakes around the World
Pastor Dale Morgan
World*
Five years ago, 250,000 lives were wiped out. Experts expect another a
huge quake under Indonesia.
Reuters
January 06, 2010
The devastating 2004 Indian Ocean tsunami, caused by a major earthquake
under the seafloor north of Aceh in Sumatra, struck five years ago
today, killing more than 250,000 people. Scientists say another massive
undersea earthquake is long overdue beneath the Mentawai islands in
Indonesia and could trigger another deadly tsunami any time.
Here is some of the science behind the process.
How tsunamis occur
In the Sumatra area, tectonic plates meet in a subduction zone -- a
place where the boundaries of one plate are forced beneath the other
plate. The Indo-Australian plate is sliding northeastward (about 2.8
inches a year) and dipping under the Eurasian plate, along a fault line
called the Sunda megathrust which runs southwest from Myanmar down
Indonesia toward Timor.
Tremendous geological strain builds over many decades until a section of
the megathrust gives way. This rupture causes the oceanic plates beneath
Sumatra to lurch forward suddenly, by many yards, in a big earthquake.
If the ocean floor ruptures, it suddenly moves a massive amount of
water. This is what happened in the earthquake that caused the deadly
Indian Ocean tsunamis of December 2004.
Major quakes that rupture the ocean floor are usually shallow quakes
occurring at a depth of less than 44 miles. The quake that caused the
2004 tsunami was about 20 miles below the seafloor.
Tsunamis rise up
On the ocean surface, tsunamis start as a ripple capable of passing
under a ship unnoticed, but they become giants as they approach land and
the ocean becomes shallow.
A tsunami is not a single wave, but a series of waves. The waves can
travel across the ocean at speeds of up to 620 miles an hour, the speed
of a jet aircraft.
The vast size of the Pacific Ocean and the large earthquakes associated
with the Ring of Fire combine to produce deadly tsunamis in the
Asia-Pacific. A tsunami can travel across the Pacific Ocean in less than
a day.
As the waves approach land, the ocean recedes dramatically, exposing
reefs as the waves draw the water out.
As the trough of the wave drags along the sea floor, slowing it down,
the crest rises up dramatically and sends a giant wall of whitewater
onto land. The first wave may not be the biggest.
The destructive force of a tsunami comes not from the height of the
wave, but from the volume of water moving.
It is as if the ocean floods the coast, smashing everything in its path,
and then just as quickly recedes.
Many people who survive the initial wave impact are washed out to sea as
the tsunami recedes.
World's worst
* The 2004 Indian Ocean tsunami was the world's most deadly, killing
about 250,000 people, with a wave height about 100 feet.
* The world's biggest tsunami, caused by a magnitude 8 quake that caused
a massive landslide, hit the remote Lituya Bay in Alaska on July 9,
1958. As the wave swept through Lituya Bay, it was forced to rise up,
reaching an estimated height of 1,720 feet on the other side of the bay,
becoming a mega-tsunami. The sparsely populated bay was devastated, but
damage was localized.
* The Krakatau island volcanic eruption of 1883 generated giant waves
reaching heights of 125 feet, killing some 30,000 people. It was the
most violent volcanic eruption in modern history.
Sources: Singapore-based Earth Observatory; School of Earth Sciences,
University of Melbourne; Australia Pacific Tsunami Warning Center,
Hawaii; Tsunami Research Center, USC