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| Enlarge ImageQUAKE-PRONE OCEAN TRENCHES—Puerto Rico, the U.S. Virgin Islands, and Hispaniola (shared by Haiti and the Dominican Republic) are located near earthquake-prone deep-ocean trenches similar to those that recently have caused earthquakes and a tsunami off Sumatra, Indonesia. A large fault that runs along Sumatra also bears striking resemblance to the Septentrional Fault Zone in Hispaniola. Deep-ocean trenches occur where two of Earth's crustal plates collide, with one thrusting down beneath the other. (Christopher Polloni, USGS and Jack Cook, WHOI Graphic Services) |
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| Enlarge ImageCARIBBEAN QUAKE AND TSUNAMI RISKS—Woods Hole geologists Uri ten Brink and Jian Lin reported a heightened earthquake risk from the Septentrional fault zone, which cuts through the highly populated region of the Cibao valley in the Dominican Republic. The possibility of earthquakes of magnitude 7.5 and higher in the offshore Puerto Rico and Hispaniola trenches pose additional risk to the region. (Illustration by Jack Cook, WHOI Graphic Services) |
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Two days before the devastating tsunami in the Indian Ocean
on Dec. 26, 2004, two Woods Hole geologists reported that similar
earthquake- and tsunami-generating conditions exist in
the Caribbean Sea.
The
Indian Ocean tsunami was caused by a magnitude-9.3 undersea
earthquake in a deep-ocean trench off the Indonesian island
of Sumatra, where two of Earth’s crustal plates collide.
Another earthquake, with a magnitude of 8.7, occurred March
28 along the same trench, about 120 miles to the south. Just
like Sumatra, the islands of Puerto Rico, the Virgin Islands,
and Hispaniola are located near earthquake-prone deep-ocean
trenches. A large fault on Hispaniola bears a striking resemblance
to a fault on Sumatra.
In a study published Dec. 24, 2004, in the Journal of
Geophysical Research, Woods Hole geologists Uri ten
Brink and Jian Lin reported a heightened earthquake risk
from the Septentrional fault zone, which cuts through the
highly populated Cibao valley in the Dominican Republic.
The potential for earthquakes of magnitude 7.5 or greater
in the trenches offshore Puerto Rico and Hispaniola pose
additional risk.
While tsunamis are rare in the Caribbean, earthquakes are
not. A dozen earthquakes of magnitude 7.0 or greater have occurred
near Puerto Rico, the Virgin Islands, and Hispaniola in the
past 500 years. Several have generated tsunamis, the most recent
in 1946 following a magnitude-8.1 earthquake off the northeast
coast of the Dominican Republic, which killed more than 1,600
people.
With nearly 20 million people now living in this tourist region
and a major earthquake occurring about every 50 years, Lin
and ten Brink say it is not a question of if another major
earthquake will happen but when.
The risks of major earthquakes in the Caribbean, and the possibility
of a resulting tsunami, although small, are real and should
be taken seriously, said Lin, a senior scientist in the WHOI
Geology and Geophysics Department, and ten Brink, a
geologist at the U.S. Geological Survey in Woods Hole and an
adjunct scientist at WHOI.
Deep-sea trenches are susceptible to earthquakes
Earthquakes typically occur near faults, or fractures, in
Earth’s crust where rock formations—driven by the
movements of the tectonic plates that make up Earth’s
surface—grind slowly past each other or collide. Stress
builds up, and at some point, stress overcomes friction, and
the rocks slip suddenly, releasing seismic energy in the form
of an earthquake. That alleviates stress in one area, but raises
it elsewhere along the fault line.
Most of these faults lie on the seafloor, so 80 percent of
earthquakes occur in the ocean. Occasionally, the sudden rupture
and movement of the crust can displace thousands of meters
of water, setting in motion great tsunami waves.
Hispaniola, Puerto Rico, and the U.S. Virgin Islands sit atop
a small crustal block that is sandwiched between the Caribbean
Plate and the North American Plate. The latter plate is being
thrust down and under the Caribbean Plate to create a deep-ocean
trench, called a subduction zone.
The Puerto Rico Trench is about 900 kilometers (560 miles)
long and 100 kilometers (60 miles) wide, and it runs roughly
parallel to the northern coast of Puerto Rico, about 75 miles
offshore. It is the deepest point in the Atlantic Ocean, descending
8,340 meters (27,362 feet) below the sea surface. The Hispaniola
Trench, which parallels the north coast of the Dominican Republic
and Haiti, and is 550 kilometers (344 miles) long and 4,500
meters (14,764 feet) deep.
Moving stresses from place to place
The two scientists studied historical earthquake data and
the geology of the northern Caribbean plate boundary, and then
used three-dimensional models to calculate the stress changes
in and near the trenches after each earthquake.
Lin said that each time an earthquake occurs on the Puerto
Rico and Hispaniola trenches, it adds stress to the Septentrional
fault zone on Hispaniola.
The region has a long history of destructive earthquakes.
Major earthquakes, greater than magnitude 7.0, have damaged
Puerto Rico 13 times since 1670, with three of the events accompanied
by destructive tsunamis.
"Our results indicate that great subduction-zone earthquakes,
which often occur in the deep trenches offshore, have the potential
to add stress or trigger earthquakes on other faults on the
nearby islands,” Lin said. “
The Puerto Rico and Hispaniola trenches are not the only tsunami
threats on the Atlantic coast of the Americas. In 1755, an
earthquake in an undersea fracture zone off Portugal generated
a giant tsunami that reached as far as the Caribbean region.
It killed more than 100,000 people, destroyed the city of Lisbon,
undermined Portugal as a rising European power, and had a profound
impact on the philosophical and religious thinking of the era.
Tsunamis
can also be generated by submarine landslides, ten Brink noted.
In 1998, a magnitude-7.1 earthquake occurred 24 kilometers
(15 miles) offshore Papua New Guinea. It was not large enough
to generate a tsunami directly, but it caused an undersea landslide
that caused a tsunami 12 meters (40 feet) high, which killed
some 2,200 people.
“We don’t want people to overreact,” said
Lin. “We just want to make them aware of the potential
risk. It is similar to understanding hurricanes or tornadoes
and being prepared to react when one is coming.”
Posted: March 25, 2005 [top] |
