Preliminary models of upper mantle P and S wave velocity structure in the western South America region

Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes. The inferred velocity models for the Chile-Peru-Ecuador region reveal an increase of P velocity from 8.04 km/s at 40 km to 8.28 km/s at 250 km depth, while the S velocity remains almost constant at 4.62 km/s from 40 to 210 km depth. A velocity discontinuity (probably corresponding to the L discontinuity in the continental upper mantle) at 220-250 km depth for P and 200-220 km depth for S waves, with a 3-4% velocity increase, is inferred from the velocity-depth data. Below this discontinuity, P velocity increases from 8.54 km/s at 250 km to 8.62 km/s at 320 km depth and S velocity increases from 4.81 km/s at 210 km to 4.99 km/s at 290 km depth. Travel time data from deep earthquakes at depths greater than 500 km in the Bolivia-Peru region, reveal P velocities of about 9.65 km/s from 500 to 570 km depth. P velocity-depth data further reveal a velocity discontinuity, either as a sharp boundary at 570 km depth with 8-10% velocity increase or as a broad transition zone with velocity rapidly increasing from 560 to 610 km depth. P velocity increases to 10.75 km/s at 650 km depth. A comparison with the latest global average depth estimates of the '660 km' discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region. Further, a velocity discontinuity at about 400 km depth with a 10% velocity increase seems to be consistent with travel time observations from deep earthquakes in this region. (C) 1999 Elsevier Science Ltd. All rights reserved.