South America's western
coast is a seismic hot
spot
By Thomas H. Maugh II,
Los Angeles Times
The same forces that
formed the Andes
Mountains produced the
magnitude 8 earthquake
that struck Peru's
southern desert region
Wednesday night: 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 3 inches per
year, one of the fastest
rates anywhere in the
world, according to
geophysicist Paul Earle
of the U.S. 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
5-mile-deep Peru-Chile
trench about 100 miles
off the coast.
The earthquakes that
result from the
subduction are called
mega-thrust quakes
because Earth's surface
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.
A magnitude-8.4 quake
struck in 2001. The
great Peru earthquake of
1868 had a magnitude of
9.0. It killed 25,000
people and caused
tsunami damage as far
away as Hawaii.
"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.
Other deadly quakes
Some of the deadliest
earthquakes to hit South
and Central America in
the past century:
June 23, 2001: Peru;
magnitude 8.4; 138
killed; sent a tidal
surge a half-mile
inland.
Jan. 25, 1999: western
Colombia, 6.1; 1,185
killed, devastated city
of Armenia.
April 22, 1991: Costa
Rica; 7.6; more than 80
killed in Costa Rica and
Panama; 30,000 along
Costa Rica-Panama border
cut off from food,
water, medical supplies
for days.
Sept. 19, 1985: central
Mexico; 8.0; more than
9,500 killed.
Feb. 4, 1976: Guatemala;
7.5; 23,000 killed.
Dec. 23, 1972:
Nicaragua; 6.2; 5,000
killed.
May 31, 1970: Chimbote,
Peru; 7.9; 66,000
killed.
May 22, 1960: Chile;
9.5; 5,700 killed.
Jan. 24, 1939: Chillan,
Chile; 7.8; 28,000
killed.
Source: U.S. Geological
Survey
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