Subduction of the Nazca Plate under Peru as evidenced by focal mechanisms and by seismicity

Abstract

The focal mechanisms of 40 earthquakes in Peru and Ecuador, together with the seismicity of the region, indicate particular features of the subduction of the oceanic plate beneath this portion of South America. At shallow depths near the coast and at foci along the contact between the subduction zone and the continental plate the focal mechanisms indicate an underthrust of the continent by the oceanic plate on a thrust plane dipping 10°–15° beneath the continent. Near this same depth but at foci within the oceanic plate, normal faults occur that correspond both to flexure of the plate and to downdip axial tension. At intermediate depths the plate continues to act as a stress guide, the axis of tension being down about 30° from the horizontal and trending to the ENE. The dip of the Benioff zone steepens notably in southern Peru near the Peru-Chile corner, and the motion of the descending slab relative to the continental plate is in a direction N40°E. Deep-focus earthquakes indicate a vertical segment of plate under axial compression at depths of 550–600 km. Numerous earthquakes also occur interior to the continent and within the continental lithosphere at depths down to 90 km. Both strike slip and reverse-type faults are found, but in either case the stress system corresponds to an E-W horizontal compression. Comparison with the seismicity is consistent with the model of an oceanic plate moving almost horizontally under the continental lithosphere in northern and central Peru and a separate, more steeply plunging segment of plate moving normal to the coast under southern Peru. Equal-area projections are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D. C. 20006. Document J75-003; $1.00. Payment must accompany order.