Underwater Earthquake Spawns Tsunami
A series of extremely powerful ocean waves, or "tsunami,"
engulfed coastlines throughout South Asia on December 26,
2004, killing at least 150,000 people and leaving millions
homeless. The event was among the worst natural disasters
in recent history.
The term "tsunami" is a Japanese word meaning "harbor
wave." Tsunamis are often referred to as "tidal waves," but
the name is misleading, as the waves are not generated by
tides. Rather, they are produced when an earthquake or
some other disturbance, such as the eruption of an
underwater volcano or a giant meteorite impact, displaces a
large mass of water. In deep water, the forceful waves can
reach speeds of up to 890 kilometers per hour (550 mph)—
the speed of a jet airplane—but they are typically only a
couple of feet tall. As a result, tsunamis often go unnoticed as
they blast through the deep ocean.
30 meters (100 feet) as it approaches shore. The biggest
tsunami on record, which struck Lituya Bay, Alaska in 1958,
reached a staggering 525 meters (1,720 feet) above sea level.
The December 26 tsunami was the result of the world's most
powerful earthquake in the past 40 years—it measured 9.0
on the Richter scale. The earthquake's epicenter—that is, the
point on Earth's surface directly above where the quake
originated—was located underwater, off the northwest coast
of the Indonesian island of Sumatra.
However, when a tsunami enters shallower waters, the
deepest part of the wave begins to hit the ocean floor,
slowing the wave down; as a result, the moving water starts
to "pile up," like cars in a traffic jam, and the tsunami keeps
growing taller. A powerful tsunami can grow to a height of
USGS
PRH/NOAA
Tsunamis are hard to spot in the deep ocean,
where they are only a couple of feet high or less.
However, when tsunamis enter shallower water,
they are forced to slow down. This causes the
water to "pile up" and form tall, destructive
waves.
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On December 26, 2004, the most powerful
earthquake in the past 40 years struck off the
northwest coast of the island of Sumatra,
sending the tsunami radiating out through the
Indian Ocean. The quake and its aftershocks
occurred along a subduction zone, where one
tectonic plate—in this case, the India Plate—is
subducting or slipping under another
plate—in this case, the Burma plate.
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Scientists have attributed the quake to slippage along a
subduction zone—a region of Earth's crust where one
tectonic plate is sliding under, or subducting, another one.
(Tectonic plates are the giant plates that make up Earth's
crust.) The sliding at a subduction zone does not happen
smoothly; instead, the plates stick to one another due to
friction, and tension builds up between them. Eventually the
tension becomes too great, and the plates slip past one
another, causing an earthquake.
The quake sent the tsunami radiating out through the Indian
Ocean. When the waves reached land, they rapidly destroyed
entire villages and tourist resorts. In Indonesia, more than
130,000 people died, and Sri Lanka suffered more than
31,000 fatalities. The combined India and Thailand death
tolls surged above 15,000, and fatalities were also reported
in Malaysia, Myanmar, the Maldives, Bangladesh and as far
west as Somalia. "It's an extraordinary calamity of such
colossal proportions that the damage has been
unprecedented," Chief Minister Jayaram Jayalalithaa of
Tamil Nadu, a state in southern India, told the Associated
Press.
Emergency relief efforts provided food, clean water and
shelter to those who lost their homes. In the wake of the
disaster, many people were living in unsanitary conditions
that health officials feared could lead to epidemics of cholera
and other water-borne diseases.
Tsunamis, because they move quickly and are difficult to
spot in deep water, often take people by surprise. However,
many experts say that the death toll of the recent disaster
could have been greatly reduced if the affected countries had
had a tsunami detection system like the one used by most
nations bordering on the Pacific Ocean. The system, called
the Deep-ocean Assessment and Reporting of Tsunamis
(DART) program, uses buoys and underwater pressure
centers stationed in the deep ocean to detect tsunamis and
determine where they're headed. This information allows
nations to issue warnings before a tsunami hits. DART is
overseen by the U.S. National Oceanic and Atmospheric
Administration (NOAA).
Many countries affected by the tsunami claim that they did
not have the money to buy tsunami-detecting devices. But
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PMEL/NOAA
Experts have argued that the death toll of
the December 26 tsunami could have been
greatly reduced if affected countries had
invested in a tsunami-detection system like
the Deep-ocean Assessment and Reporting
of Tsunamis (DART) system. DART, which is
run by National Oceanic and Atmospheric
Administration (NOAA), uses buoys and
pressure sensors stationed in the deep
ocean to detect possible signs of tsunamis.
The buoys send alert signals to satellites
when evidence indicates that a tsunami may
be on the way. A diagram of a DART station
is shown here.
Thammasarote Smith, a former senior forecaster at
Thailand's Meteorological Department, stated that—even
without such devices—more steps could have been taken to
reduce the death toll in Thailand, based on the fact that a
quake was known to have occurred. "The department had up
to an hour to announce the emergency message and
evacuate people but they failed to do so," he told The
Bangkok Post newspaper. "It is true that an earthquake is
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unpredictable but a tsunami, which occurs after an
earthquake, is predictable."
After NOAA scientists detected the underwater earthquake,
they tried to warn countries that might be at risk of a
tsunami, but they had trouble alerting the proper officials.
"We don't have contacts in our address book for anybody in
that part of the world," said NOAA director Charles
McCreery.
"Countries affected by the 2004 Indian Ocean earthquake."
Wikipedia. [accessed December 2, 2006]: en.wikipedia.org/
wiki/Countries_affected_by_the_2004_Indian_Ocean_
earthquake. Summary of damage and casualties by country.
Bibliography
Aglionby, John, Patrick Barkham and John Vidal. "Giant
Waves Kill Thousands after Calls for Warning System
Ignored." Guardian, December 27, 2004, page 1.
Choudhury, Shankhadeep and Paul Watson. "Catastrophe in
Southern Asia; At Least 13,000 Die in Tsunami." Los
Angeles Times, December 27, 2004, page A1.
"Magnitude 9.0 - Off the West Coast of Northern Sumatra."
U.S. Geological Survey, National Earthquake Information
Center and the World Data Center for Seismology, Denver,
Colorado. (December 27, 2004) [accessed December 27,
2004]: earthquake.usgs.gov/eqinthenews/2004/usslav.
"The Physics of Tsunamis." West Coast and Alaska Tsunami
Warning Center. (November 18, 2004) [accessed December
27, 2004]: www.wcatwc.gov/physics.htm.
Sipress, Alan and Peter S. Goodman. "Sea Surges from
Massive Quake Kill About 20,000 Across South Asia."
(December 27, 2004) [accessed December 27, 2004]:
www.washingtonpost.com/wp-dyn/articles/
A28574-2004Dec27.html.
Additional Resources
Kong, L. "Dart Buoys Provide Real-Time Reporting of
Tsunamis." International Tsunami Symposium 2001
Proceedings. [accessed December 27, 2004]:
www.prh.noaa.gov/itic/tsunami_events/media/factsheets/
tsunami_detection_buoy_article.pdf. A paper discussing
NOAA's Deep-ocean Assessment and Reporting of Tsunamis
(DART) project. Based on material from 2001.
Citation: "Underwater Earthquake Spawns Tsunami.” Today’s Science on File January 2005. Facts For Learning. Facts On File News Services.
http://www.2facts.com
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