Subduction reported as cause for Tuesday’s quake in Chile

The Tuesday 8.2 magnitude earthquake in northern Chile occurred as the result of thrust faulting at shallow depths near the Chilean coast. The location and mechanism of the earthquake are consistent with slip on the primary plate boundary interface, or megathrust, between the Nazca and South America plates.

At the latitude of the earthquake, the Nazca plate subducts eastward beneath the South America plate at a rate of 65 millimeters a year. Subduction along the Peru-Chile Trench to the west of Chile has led to uplift of the Andes mountain range and has produced some of the largest earthquakes in the world, including the 2010 8.8 magnitude earthquake in central Chile, and the largest earthquake on record, the 1960  9.5 magnitude earthquake in southern Chile.

The Tuesday earthquake occurred in a region of historic seismic inactivity, termed the northern Chile or Iquique seismic gap. Historical records indicate a magnitude 8.8 earthquake occurred within the Iquique gap in 1877, which was preceded immediately to the north by a magnitude 8.8 earthquake in 1868.

A recent increase in seismicity rates has occurred in this vicinity. A magnitude 6.7 earthquake with similar faulting mechanism occurred March 16 and was followed by 60-plus earthquakes of  magnitude 4.0 or more and 26 earthquakes of 5.0 or more. The March 16 earthquake was also followed by three 6.2 magnitude events on March 17, March 22, and March 23.

The spatial distribution of seismicity following the March 16 event migrated spatially to the north through time, starting near 20 degrees south and moving to about 19.5 degrees south. The initial location of the Tuesday earthquake places the event near the northern end of this seismic sequence.

The South American arc extends over 7,000 kms, from the Chilean margin triple junction offshore of southern Chile to its intersection with the Panamá 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 millimeters a year in the south to approximately 65 millimeters a year 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 kms 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 kms. 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 kms. Since 1900, numerous magnitude 8 or larger earthquakes have occurred on this subduction zone interface that were followed by devastating tsunamis, including the 1960 9.5 magnitude earthquake in southern Chile, the largest instrumentally recorded earthquake in the world.

Large intermediate-depth earthquakes (those occurring between depths of approximately 70 and 300 kms.) 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 kms. 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 magnitude 7.8 Tarapaca, Chile, event.

Earthquakes can also be generated to depths greater than 600 kms. 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 kms. Instead, deep earthquakes in this region occur at depths of 500 to 650 kms. 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 magnitude 8.2 earthquake occurred at a depth of 631 kms., which was until recently 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 degrees. In contrast, the slab beneath southern Ecuador to central Peru, and under central Chile, is subducting at a shallow angle of approximately 10 degrees 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.

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