Abstract

The shallow, mb = 5.3, 1986 earthquake that affected the Cuzco region reactivated the 18-km-long Chincheros-Qoricocha faults in the central part of the Cuzco fault system. Although the event has a moderate magnitude, clear surface ruptures were observed to total 3 km in length, with maximum 10-cm normal displacements. Fault kinematics related to the earthquake, analyzed where the Qoricocha fault had a maximum surface fault displacement, is in agreement with roughly N-S extension. The normal fault mechanism proposed for this earthquake is compatible with the surface kinematic analysis. Seismic activity prior to the 1986 earthquake is deduced from analysis of Holocene scarps. Trench exposures give evidence for at least three fault displacements, suggesting an average recurrence interval of the order of a few thousand years for surface-rupturing seismic events. These fault reactivations, with vertical displacement ranging from 0.6 to 2 m, were related to seismic events of Mw magnitude probably ranging from 5.8 to 6.9. Structural analyses of Holocene faults indicate that the Chincheros-Qoricocha fault activity was related to roughly N-S-trending extension, characterized by a horizontal N-S-trending σ3 axis and a vertical σ1 axis. The N-S extensional tectonics that controlled the Holocene and 1986 Chincheros-Qoricocha fault activity are the typical deformational pattern of the Peruvian High Andes and seem to be in agreement with compensated high topography.

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