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 ar e obtained at depths of foci from an analysis of travel time data of deep e arthquakes. 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 25 0 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 inferr ed from the velocity-depth data. Below this discontinuity, P velocity incre ases from 8.54 km/s at 250 km to 8.62 km/s at 320 km depth and S velocity i ncreases 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-Pe ru 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 l atest global average depth estimates of the '660 km' discontinuity reveals that this discontinuity is at a relatively shallow depth in the study regio n. Further, a velocity discontinuity at about 400 km depth with a 10% veloc ity increase seems to be consistent with travel time observations from deep earthquakes in this region. (C) 1999 Elsevier Science Ltd. All rights rese rved.