SURFACE RUPTURE ASSOCIATED WITH A 5.3-MB EARTHQUAKE - THE 5 APRIL 1986 CUZCO EARTHQUAKE AND KINEMATICS OF THE CHINCHEROS-QORICOCHA FAULTS OF THE HIGH ANDES, PERU

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 magn itude, clear surface ruptures were observed to total 3 km in length, w ith maximum 10-cm normal displacements, Fault kinematics related to th e earthquake, analyzed where the Qoricocha fault had a maximum surface fault displacement, is in agreement with roughly N-S extension, The n ormal fault mechanism proposed for this earthquake is compatible with the surface kinematic analysis, Seismic activity prior to the 1986 ear thquake is deduced from analysis of Holocene scarps. Trench exposures give evidence for at least three fault displacements, suggesting an av erage recurrence interval of the order of a few thousand years for sur face-rupturing seismic events. These fault reactivations, with vertica l 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 activ ity was related to roughly NS-trending extension, characterized by a h orizontal N-S-trending sigma(3) axis and a vertical sigma(1) axis, The N-S extensional tectonics that controlled the Holocene and 1986 Chinc heros-qoricocha fault activity are the typical deformational pattern o f the Peruvian High Andes and seem to be in agreement with compensated high topography.