The
Peru earthquake of September 25, 2013, occurred as thrust-faulting on
or near the thrust-interface at the boundary between the South America
plate and the subducting Nazca plate. The Nazca plate subducts beneath
the South America plate at the Peru-Chile trench offshore of western
South America, and the thrust interface between the two plates dips
east-northeast beneath the South American continent. At the latitude
of the earthquake, the Nazca plate moves to the east-northeast with
respect to the South America plate with a velocity of about 70 mm/y.
The
boundary-region between the Nazca and South American plates experiences
a large number of earthquakes. The region within 250 km of the
epicenter of the earthquake of September 25 had experienced seventeen
previous earthquakes of magnitude 6 and larger since 1973. The largest
of these, the magnitude 8.4 earthquake of June 23, 2001, occurred along
the plate boundary to the south. It killed at least 74 people and
destroyed over 17,000 homes. The magnitude 7.7 earthquake of November
12, 1996, killed at least 14 people and left 12,000 homeless. It
occurred along the plate boundary to the north of the September 25
epicenter.
Seismotectonics of South America (Nazca Plate Region)
The
South American arc extends over 7,000 km, from the Chilean margin
triple junction offshore of southern Chile to its intersection with the
Panama fracture zone, offshore of the southern coast of Panama in
Central America. It marks the plate boundary between the subducting
Nazca plate and the South America plate, where the oceanic crust and
lithosphere of the Nazca plate begin their descent into the mantle
beneath South America. The convergence associated with this subduction
process is responsible for the uplift of the Andes Mountains, and for
the active volcanic chain present along much of this deformation front.
Relative to a fixed South America plate, the Nazca plate moves slightly
north of eastwards at a rate varying from approximately 80 mm/yr in the
south to approximately 65 mm/yr in the north. Although the rate of
subduction varies little along the entire arc, there are complex changes
in the geologic processes along the subduction zone that dramatically
influence volcanic activity, crustal deformation, earthquake generation
and occurrence all along the western edge of South America.
Most
of the large earthquakes in South America are constrained to shallow
depths of 0 to 70 km resulting from both crustal and interplate
deformation. Crustal earthquakes result from deformation and mountain
building in the overriding South America plate and generate earthquakes
as deep as approximately 50 km. Interplate earthquakes occur due to slip
along the dipping interface between the Nazca and the South American
plates. Interplate earthquakes in this region are frequent and often
large, and occur between the depths of approximately 10 and 60 km. Since
1900, numerous magnitude 8 or larger earthquakes have occurred on this
subduction zone interface that were followed by devastating tsunamis,
including the 1960 M9.5 earthquake in southern Chile, the largest
instrumentally recorded earthquake in the world. Other notable shallow
tsunami-generating earthquakes include the 1906 M8.5 earthquake near
Esmeraldas, Ecuador, the 1922 M8.5 earthquake near Coquimbo, Chile, the
2001 M8.4 Arequipa, Peru earthquake, the 2007 M8.0 earthquake near
Pisco, Peru, and the 2010 M8.8 Maule, Chile earthquake located just
north of the 1960 event.
Large intermediate-depth earthquakes
(those occurring between depths of approximately 70 and 300 km) are
relatively limited in size and spatial extent in South America, and
occur within the Nazca plate as a result of internal deformation within
the subducting plate. These earthquakes generally cluster beneath
northern Chile and southwestern Bolivia, and to a lesser extent beneath
northern Peru and southern Ecuador, with depths between 110 and 130 km.
Most of these earthquakes occur adjacent to the bend in the coastline
between Peru and Chile. The most recent large intermediate-depth
earthquake in this region was the 2005 M7.8 Tarapaca, Chile earthquake.
Earthquakes
can also be generated to depths greater than 600 km as a result of
continued internal deformation of the subducting Nazca plate. Deep-focus
earthquakes in South America are not observed from a depth range of
approximately 300 to 500 km. Instead, deep earthquakes in this region
occur at depths of 500 to 650 km and are concentrated into two zones:
one that runs beneath the Peru-Brazil border and another that extends
from central Bolivia to central Argentina. These earthquakes generally
do not exhibit large magnitudes. An exception to this was the 1994
Bolivian earthquake in northwestern Bolivia. This M8.2 earthquake
occurred at a depth of 631 km, making it the largest deep-focus
earthquake instrumentally recorded, and was felt widely throughout South
and North America.
Subduction of the Nazca plate is geometrically
complex and impacts the geology and seismicity of the western edge of
South America. The intermediate-depth regions of the subducting Nazca
plate can be segmented into five sections based on their angle of
subduction beneath the South America plate. Three segments are
characterized by steeply dipping subduction; the other two by
near-horizontal subduction. The Nazca plate beneath northern Ecuador,
southern Peru to northern Chile, and southern Chile descend into the
mantle at angles of 25° to 30°. In contrast, the slab beneath southern
Ecuador to central Peru, and under central Chile, is subducting at a
shallow angle of approximately 10° or less. In these regions of
“flat-slab” subduction, the Nazca plate moves horizontally for several
hundred kilometers before continuing its descent into the mantle, and is
shadowed by an extended zone of crustal seismicity in the overlying
South America plate. Although the South America plate exhibits a chain
of active volcanism resulting from the subduction and partial melting of
the Nazca oceanic lithosphere along most of the arc, these regions of
inferred shallow subduction correlate with an absence of volcanic
activity.
More information on regional seismicity and tectonics
