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domingo, 9 de octubre de 2011

Geodynamic and seismotectonic context of Peru

The subduction of the Nazca plate beneath South America constitutes the major tectonic characteristic in Peru. According to magnetic and geodetic data, the Nazca plate is continuously moving north-eastward (azimuth 78 degress )with an average velocity of 6 cm/yr (DeMets et al., 1990; Norabuena et al., 1999). This rapid convergence causes a high seismic coupling along the subduction interface giving raise to the occurrence of interplate and intraplate earthquakes with different magnitudes and depths. As a result of this process, different geomorphological structures and tectocni features have been created such as: the Peru-Chile trench, the Andean cordillera, the volcanic chain and fault systems. In case of the fault systems those are also sources of seismicity but of lower magnitudes. The figure shows all these features.

-  The Peru-Chile trench: is a structure emplaced parallel to the Peruvian coast with a distance of about 160 km from the coast, bordering the contact between the Nazca plate and the South America plate.

-  The Andean cordillera: the most obvious tectonic feature product of the continuous interaction of the Nazca plate, covers an area of 7,500 km, with heights of 6,000 m and higher. The volcanic chain is located in the southern part of Peru from 14º N to 25º S in Chile. Finally, fault systems are result of the deformation in the continental crust due to subduction process.

Different authors have proposed that the subduction zone could be divided into three zones according to its seismic activity (North, Central and South), this zones might be separated by two prominent geomorphologic features on the subduction plate: the Mendana fracture zone (10ºS) and the Nazca ridge (15ºS). The Nazca ridge location is considered as a barrier of rupture propagation as observed in the last Pisco earthquake (Mw8.0) in 2007 (Perfettini et al., 2010; Sladen et al., 2010). However, Okal et al. (2006), using tsunami simulations for the 1868 and 1687 events suggest that the Nazca ridge appears more like a hurdle than a barrier serving as a rupture barrier for certain events such as the 1604 event for the south and 1746 event to the north. Moreover, this feature is considered as a transition between a flat subduction zone to the north and a subduction steeply dipping to the south, with different azimuth of the shore line and seismicity activity (Okal et al., 2006; Sladen et al., 2010). The Mendana fracture zone is considered as a transition zone with a difference crustal age of about 10 Myr between north of the Mendana fracture zone and off the central and south of Peru (Muller et al., 1997).

- The volcanic chain: Located in the south of Peru from 14º S to 25º S in Chile. This chain is distributed along the Andean cordillera following an aparent linearity on NW-SE direction. Among the main volcanoes located in southern Peru: Coropuna (6425 m), Sabancaya (5795 m), Misti (5825 m), Ubinas (5672 m.), Chachani (3745 m.), Huaynaputina (4800 m.), Tutupaca (5806 m.), Yucamane (5508 m.). In northern and central Peru volcanic activity have disappeared approximately 8 Ma, due to changes in the form of the subduction process (Moroco 1980).

- The fault systems: are result of constant deformation process in the continental crust due to the subduction process. These faults are present in great number, from North to South along Sub-Andean zone and the eastern side of the Andes Mountains creating major folds in the contact with the Brazilian Shield. The main fault systems located in Peru are: Alto Mayo (AM), Satipo (SA), Madre de Dios (MD),  in less proportion located on the high Cordillera and the Altiplano: the Cordillera Blanca (CB), Huaytapallana (HU), Tambomachay (TM). All fault systems owe their origin to a heterogeneous distribution of tensional and compressional efforts within thecontinent (James, 1978).

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