Poster of the San Agustin, Colombia Earthquake of 30 September 2012 - Magnitude 7.3
The September 30, 2012 M 7.3 earthquake west of Isnos, Colombia, occurred as a result of normal faulting deep within the subducting Nazca slab. The earthquake ruptured a fault in the interior of the inclined subduction zone that dips to the east-southeast beneath South America, having begun its decent into the mantle at the South America trench offshore of Colombia and Ecuador. The event resulted from stresses generated by the slow distortion of the subducting plate as it descends through the mantle, rather than on the thrust interface that constitutes the boundary between the Nazca and overlying South America plates; the latter is active only near the Earth's surface, while the subducting Nazca plate generates intraplate earthquakes to depths of 200 km or more in this region. At the latitude of this event, the Nazca plate moves east-northeast with respect to the South America plate at a rate of approximately 60 mm/yr.
Deep earthquakes in this region of the Nazca plate are not uncommon; there have been 13 similar events deeper than 100 km over the past 40 years, within 500 km of the September 30 2012 earthquake. The largest was a magnitude 7.1 earthquake at a depth of 206 km in August of 2010, 380 km to the south of the September 30 event.
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