Seismograms of Peru-Brazil deep focus earthquakes recorded on the nearby Carnegie broadband station at Cuzco, Peru (CUS) are characterized by an anomalous large-amplitude P wave arrival that follows about 1.5 s after direct P. The anomalous arrival is interpreted to be an underside wide-angle reflection from the upper surface of the descending Nazca plate somewhere in the depth range 150--400 km and in a region where the slab is wholly aseismic. Particle motion analysis of the CUS seismograms shows that the direct P wave has arrival angles as predicted from simple ray tracing. The large, impulsive anomalous phase (termed Pr) also arrives as a P wave but is of opposite polarity and about 1.5 times the amplitude of the direct P arrival. The back azimuth of Pr is about 20¿ clockwise from the path of direct P, and its angle of emergence is significantly larger. Forward modeling calculations indicate that the seismic observations are consistent with a model in which Pr originates as a P wave that travels up (inside) the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary. Results from modeling lead to the following conclusions: (1) the descending Nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth: (2) the plate is very steeply dipping (~70¿) below depths of about 150 or 200 km; and (3) the velocity discontinuity at the reflection point must be relatively sharp, probably less than about 10 km in thickness. ¿ American Geophysical Union 1990