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SUMMARY A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the microearthquakes recorded occurred to the east of the mountain belt: on the Huaytapallana fault in the Eastern Cordillera and beneath the western margin of the sub‐Andes. Thus the sub‐Andes appear to be the physiographic province subject to the most intense seismic deformation. Focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub‐Andes appear to be as deep as 40‐50 km. The fault‐plane solutions of events in the sub‐Andean margin show thrust faulting on steep planes oriented roughly north‐south, similar to that observed in teleseismic earthquakes studied using body wave modelling. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a NE‐SW trend that agrees with the fault scarp and the east‐dipping nodal plane of two large earthquakes that occurred on this fault on 1969 July 24 and October 1. Microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with recent observations showing that beneath Peru the slab first dips at an angle of about 30° to a depth of 100 km and then flattens following a quasi‐horizontal trajectory. Fault‐plane solutions of intermediate‐depth microearthquakes have horizontal T axes oriented east‐west suggesting slab pull is the dominant force in the downgoing slab.

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