Thrust and extensional faulting under the Chilean coast: 1965, 1971 Aconcagua earthquakes

Earthquakes under coastal northern Chile, from 18° S to 33° S occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a couple of Ms ≈ 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40 km depth, while the 1965 March 28 earthquake had a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P- and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 × 1027dyne cm from both P- and surface waves. The source area for this event is relatively small compared to that of other underthrust events with similar moments. Assuming a circular fault shape and a rigidity of 7 × 1011 dyne cm−2 we find a stress drop of 38 bar. The 1965 event is more complex; seismic moments of 1.8 and 1.0 × 1027 dyne cm are found from surface and body waves respectively. The stress drop estimated from P-waves and a circular fault model is close to 91 bar. The existence of earthquakes at two different depths near the Chilean coast may be interpreted in terms of a double layered Benioff zone, as has been found in Honshu, Kuriles or central Peru. Unfortunately the hypocentral determinations of events in this region are not precise enough to separate the two layers. An alternative method to demonstrate the existence of a double Benioff zone is to look for compressional events associated with the upper layer of the down-going slab. A search among available fault plane solutions is ambiguous; three shallow coastal events are found in Stauder's catalogue with horizontal pressure axes normal to the trench, but they might be intra-continental plate events.