Poster of the Seismicity of the Nazca Plate and South America
Nazca Plate Tectonic Summary
The South America arc extends over 7,000 km from the Chilean margin triple junction offshore of southern Chile, north along the western coast of South America, to its intersection with the Panama fracture zone offshore the south coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, the region 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 70 mm/yr in the north.
Subduction zones such as the South America arc are geologically complex and generate numerous earthquakes from a variety of tectonic processes that cause deformation of the western edge of South America. Crustal deformation and subsequent mountain building in the overriding South America plate generate shallow earthquakes. Slip along the dipping interface between the two plates generates frequent and often large interplate earthquakes between depths of approximately 10 and 50-60 km. Since 1900 numerous magnitude 8 or greater earthquakes have occurred on the interface between the Nazca and South America plates, including the 1960 M9.5 earthquake in southern Chile, the largest instrumentally recorded earthquake in the world. Earthquakes can also be generated to depths greater than 600 km from internal deformation of the subducting Nazca plate. Although the rate of subduction varies little along the entire subduction zone, there are complex changes in geologic processes along the subduction zone that dramatically influence volcanic activity, earthquake generation and occurrence. For example, an extended zone of crustal seismicity in central-northern Argentina highlight a well-known ‘flat-slab’ region of this subduction zone, where the Nazca plate moves horizontally for several hundred kilometers before continuing its’ descent into the mantle. This transition in slab structure is coincident with a marked break in the Andes volcanic chain.
The earthquakes portrayed on the main map and the depth profiles are taken from two sources: (a) the Centennial earthquake catalog (Engdahl and Villaseñor, 2002) and annual supplements for the interval 1900-2007, where the magnitude floor is 5.5 globally and (b) a catalog of earthquakes having high-quality depth determinations for the period 1964-2002 and a magnitude range of 5.0=M=5.4 (Engdahl, personal communication).
The nucleation points of great earthquakes (M=8.3) are designated with a label showing the year of occurrence. Their rupture areas are shown as pale yellow polygons. Major earthquakes (7.5=M=8.2) are labeled with the year of occurrence.
The Seismic Hazard and Relative Plate Motion panel displays the generalized seismic hazard of the region (Giardini and others, 1999) and representative relative plate motion vectors of the Caribbean plate relative to the adjacent Pacific, North and South America plates using the NUVEL-1A model (DeMets, et.al., 1994) and updates (Dixon, et.al., 1998; Weber, et.al., 2001).
Pre-instrumental seismicity was obtained from the NOAA National Geophysical Data Center database of significant earthquakes. This database contains information on earthquake locations, source parameters and macro-seismic effects for earthquakes with associated reports of at least one of the following: moderate to major damage, 10 or more deaths, an estimated magnitude of 7.5 or greater, Modified Mercalli Intensities X or tsunami generation. Earthquake locations shown on the pre-instrumental map are approximate, based on macro-seismic reports and field investigations. Earthquakes for which deaths are reported in the NOAA significant earthquake database are identified by their year of occurrence. More importantly, all of the earthquakes on this map, combined with earthquake information found on the main map, play a role in understanding and addressing seismic hazards in the Caribbean basin.
Base map data sources include GEBCO 2008, Volcanoes of the World dataset (Siebert and Simkin, 2002), plate boundaries (Bird, 2003), Digital Chart of the World and ESRI.
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