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The same
forces that formed the
Andes
Mountains
produced the magnitude 8.0 earthquake that struck
Peru’s
southern desert region last week: The collision of two
massive tectonic plates along South America’s western
coast.
The
Nazca plate under the eastern
Pacific Ocean is ramming into the larger South American plate at a
rate of about three inches a year, one of the fastest
rates anywhere in the world, according to geophysicist
Paul Earle of the US Geological Survey’s
National
Earthquake Information Center.
As the
Nazca plate dives under the coastal plate, it forces the
ground upward, forming the mountains and releasing
tremendous amounts of energy. That makes the region one
of the most seismically active in the world and the
source of frequent, massive temblors.
The two
plates are merging, a process called subduction,
offshore. That process has formed the five-mile-deep
Peru-Chile trench about 100 miles off the coast.
The
earthquakes that result from the subduction are called
mega-thrust quakes because the Earth is being thrust
upward, and they are the most powerful earthquakes on
the planet. Typically, land levels will rise a few yards
in the region of the epicenter.
In 1956,
there was a magnitude 8.2 quake just to the north of the
site of last week’s quake. In 1996, there was a
magnitude 7.7 quake just south of it. In 2001, there was
a magnitude 8.3 quake further south.
The
great Peru earthquake of 1868 had a magnitude of 9.0. It
killed several thousand people in Peru and caused
tsunami damage as far away as
Hawaii.
In the
wake of a quake that struck Wednesday afternoon, killing
at least 510 people, few buildings still stood in the
fishing city of Pisco. Many of the structures not
reduced to rubble were rickety deathtraps waiting to
fall.
President Alan Garcia predicted “a situation approaching
normality” in 10 days, but acknowledged that
reconstruction would take far longer. He said
authorities were considering nighttime curfews to
maintain order on the streets plagued by looting and
which still lack electricity.
Hopes of
finding more survivors diminished. At least 1,500 people
were injured, and Garcia said at least 80,000 people had
suffered the quake’s impact through the loss of loved
ones or destroyed or damaged homes.
Rescuers
continued to pull bodies from the rubble of the downtown
San Clemente church in Pisco, where hundreds had
gathered on Wednesday for
Mass.
The church’s domed ceiling broke apart in shaking that
lasted an agonizing two minutes.
Paul
Wooster, coordinator of the Rapid UK Rescue team from
Gloucester, England, said rescuers were using sound
detectors and infrared cameras to search mountains of
rubble. The last survivor, a man, was discovered at
midday Friday.
“People
think of the San Andreas fault as very active, but there
are places in the world like this that are much more
active,” Earle said.
Although
the 50-mile-thick Nazca plate is sliding under the South
American plate at a more or less constant rate,
relatively small sections of the convergence zone get
hung up and the subduction is halted until the pressure
builds and the blocked section snaps free, catching up
with the rest of the plate and producing an earthquake.
In
Wednesday’s quake, a section about 120 miles long broke
free, Earle said. In general, the longer the section
that breaks, the greater the magnitude of the quake. The
epicenter was about 25 miles below the surface, Earle
said. (With AP) |
