The circum-Pacific region is defined by subduction
zone plate boundaries that produce 80% of
earthquakes worldwide but has over 400 active
volcanoes, earning it the title “Pacific
Ring of Fire”. More than one-quarter of
the world's great magnitude 8 or larger earthquakes
have occurred in western South America. It
is home to the 1960 magnitude 9.5 Chile megathrust
earthquake, the largest earthquake ever recorded,
and to an array of earthquakes generated by
plate boundary and intra-plate tectonic processes.
Regional tectonics are dominated by subduction
of the oceanic Nazca Plate beneath the continental
South American Plate which produced both the
5,900-km-long Peru-Chile Trench that extends
from Colombia to southern Chile, and the arc
of active volcanoes parallel to the trench.
Rates of convergence range from 5.6 cm/year
in Ecuador to 6.3 cm/yr in northern Chile.
They have recently been updated using GPS
observations.
South America has over 800 measured earthquakes
every year. By examining all earthquakes greater
than magnitude five for a 10-year period,
we see a pattern of shallow to deep earthquakes
from west to east across the subduction zone.
A cross section through Central Chile shows
how earthquakes outline classic subduction
geometry. To 70 km depth, earthquakes are
concentrated along the plate boundary but
also occur within both plates near the boundary.
Below 70 km, earthquakes occur only within
the relatively cold and therefore brittle
subducting plate. Andean volcanoes are located
above the point where the top of the Nazca
Plate has subducted to about 150 km depth
at 300 km from the trench. Melting of mantle
rocks just above the subducting plate, not
melting of the subducting plate itself, generates
the magma that supplies these volcanoes. In
Ecuador and southern Chile, the pattern of
earthquakes is similar, with the oceanic plate
diving at an angle of about 30° and volcanoes
300 to 400 km from the trench.
However, in most of Peru and at about 30°S
in Chile where the volcanoes are absent, earthquakes
reveal what is called “flat-slab” subduction.
In these segments, the oceanic plate dives
to 100 km depth hen slides along the bottom
of the South American Plate for several hundred
kilometers before resuming its descent into
Earth’s mantle. There are no active volcanoes
above the flat-slab subduction. Flat slab
regions occur where unusually thick, and therefore
buoyant, oceanic crust caps the Nazca Plate
as along the Nazca and Juan Fernandez Ridges.
Since 1906 there have been sixteen great earthquakes
of magnitude 8 or larger on the megathrust
boundary between the Nazca and South American
plates. Yellow patches indicate the area ruptured
in each great earthquake. Gaps between the
ruptures, called “seismic gaps” are particularly
vulnerable to future earthquakes because potential
energy continues to build between the locked
plates The region in northernmost Chile was
designated the “Iquique seismic gap” because
it had not had a great earthquake since 1877.
On April 1, 2014 a Magnitude 8.2 earthquake
confirmed the forecast, releasing energy in
a portion of the gap. Aside from this gap,
the Ecuador-Peru seismic gap is also conspicuous.
To explore the mechanics of subduction-zone
megathrust earthquakes, let’s examine the
1960 magnitude 9.5 Chile earthquake, the largest
instrumentally-recorded seismic event. For
great earthquakes, the displacement at the
hypocenter is just the beginning.
The initial fault break in 1960 triggered
a runaway rupture that displaced an area more
than 800 km long. The entire fault rupture
elapsed over four minutes releasing immense
seismic energy equivalent to a cascade of
10,000 magnitude seven earthquakes.. Most
of the damage was caused by the resulting
tsunami, which we can examine by looking at
a cross section
. The oceanic Nazca Plate dives beneath the
more-buoyant continental Plate. Because tectonic
plates are elastic like a spring, the leading
edge of the continental plate stores elastic
energy as the plates converge. With the megathrust
plate boundary locked by friction, the upper
plate is forced back and shortened, raising
the land surface while depressing the ocean
floor between the coast and the trench. In
1960, the force of the converging plates overcame
friction causing the leading edge of the continental
plate to lurch seaward. During the earthquake,
uplift of the seafloor formed a mound of seawater
that spread out as a tsunami while coastal
and nearby inland areas dropped.
5.8 m of uplift was measured on several offshore
islands. while subsidence by as much as 2.3 m
affected land areas near the coast. The resulting
tsunami reached heights of 15 m along the
outer coast causing over 1000 of the 1425
fatalities in Chile. The trans-Pacific tsunami
was one of the most destructive of modern
times killing 61 in Hawaii, 32 in the Philippines,
and 142 in Japan. The 1960 tsunami motivated
development of the international Pacific tsunami
warning system.
Although the largest earthquakes affecting
South America are subduction-zone megathrust
events, earthquakes within, rather than between,
the plates have caused the most fatalities
in recent history. For example. the magnitude
7.9 Chimbote, Peru earthquake in 1970 was
the most deadly earthquake in the Americas.
Fault motion that produced this earthquake
occurred within the upper brittle part of
the subducting Nazca Plate. Although the M
9.5 Chile earthquake released over 300 times
as much energy, the location of the Peru earthquake
54 km deep below the port city of Chimbote
and a massive earthquake-induced landslide
caused great loss of life. 100-million-m3
volume of rock and glacial ice broke from
Nevado Huascaran, the highest mountain in
the Peruvian Andes. The resulting debris avalanche
rushed down a glacial valley at speeds up
to 300 km/hr burying towns in rock, ice,
and mud up to 30 meters deep. The death toll
was estimated at 70,000.
In 1944, the most deadly earthquake in Argentine
history destroyed much of San Juan killing
8000 people beneath collapsed buildings, How
did Nazca – South America plate interactions
produce this magnitude 7.0 earthquake within
the South American crust almost 400 km east
of the Trench? The Sierras Pampeanas region
of northwest Argentina is within a flat-slab
segment of the Nazca – South America subduction
zone. The cross section through this region
shows earthquakes in two depth intervals;
within the brittle crusts of both the South
American Plate and underlying Nazca Plate.
As the oceanic plate rubs horizontally along
the base of the continental plate, compressive
forces build in the continental crust. These
forces eventually break the continental crust
into blocks separated by thrust faults. The
magnitude 7.0 earthquake occurred on one of
these thrust faults, rupturing from 20 km
depth up to the surface near San Juan demonstrating
that intense ground shaking from a nearby
magnitude 7 earthquake can be more destructive
than a larger earthquake at greater distance.
Western South America is a region of high
earthquake hazard because of tsunami-generating
megathrust earthquakes on the Nazca –South
America plate boundary, major earthquakes
within the subducting plate beneath populated
coastal towns, 843and major earthquakes on
faults within the continental crust. Earthquake
and tsunami education along with construction
of earthquake-resilient buildings and infrastructure
have proved to be vitally important in this
seismically active region.
