The intracontinental seismicity of the Andes of Peru, Ecuador, and southern Colombia is concentrated along the eastermost flank of the Cordillera beneath the western margin of the sub-Andes. The focal depths and fault plane solutions of the largest events were constrained by comparing the observed long-period P waves with synthetic waveforms. In general, the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest-southeast and reflect crustal shortening perpendicular to the range, probably in response to the subduction of the Nazca plate to the west. Earthquakes in the sub-Andes occur at depths of between 8 and 38 km, indicating that much of the crust deforms in a brittle manner. The seismicity seems to reflect antithetic underthrusting of the Brazilian shield beneath the eastern margin of the Andes. However, the earthquakes are too deep and their nodal planes are too steep to be associated with the thin-skinned decollement of the sedimentary cover. Instead, they appear to reflect the tectonic deformaiton of the underthrust basement west of the zone of active deformation at the surface in the sub-Andes. Hence the earthquakes on the east side of the Andes may reflect the style of deformation in the hinterland of fold and thrust belts, such as in the Canadian Rockies, while much of the decollement farther east occurs aseismically. In contrast, normal faulting occurs in the high Andes on planes parallel to the strike of the mountain belt. Thus, there is a delicate balance between the compressive stress applied to the Andes in the direction of subduction, which causes thrust faulting in the sub-Andes, and the gravitational body forces acting on the topographically higher parts and the crustal root of the Andes, which may cause normal faulting at high elevation.