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, which was until recently the largest deep-focus earthquake instrumentally recorded (superseded in May 2013 by a M8.3 earthquake 610 km beneath the Sea of Okhotsk, Russia), and was felt widely throughout South and North America.
