In 1981 and 18-station regional array was operated in southern Peru above the area where the dip of the subducted Nazca plate changes from virtually horizontal in the northwest to about 30¿ in the southeast. Arrival times of compressional (P) and shear (S) waves from microearthquakes recorded by this array are used in this paper to investigate the three-dimensional velocity structure of the crust and upper mantle in the region between the coast and the Cordillera Occidental. The results suggest a crustal thickness of about 40 km beneath the coast, increasing to about 70 km beneath the Cordillera Occidental. This change in thickness occurs abruptly in the northwest but is gradual in the southeast. The inverse correlation between the dip of the Moho and the dip of the slab suggests a broad-scale, causal relation between the two. At the same time, free-air gravity anomalies suggest that the steepening of the Moho in the northwest may occur only in the region under the array. Because the crust is thicker in the southeast than originally surmised, a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust. The association of these earthquakes with a shallow dipping Moho is suggestive of ongoing crustal deformation to the west of the Andes. However, other seismological and geological observations make the role of these earthquakes in crustal deformation unclear. S wave velocities in the mantle between 70- and 130-km depth above the 30¿ dipping slab are low, suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern Peru.