VELOCITY STRUCTURE OF THE COASTAL REGION OF SOUTHERN PERU FROM SEISMIC-REFRACTION WIDE-ANGLE REFLECTION DATA

Refraction information from three separate experiments in southern Peru has been integrated to produce two reversed profiles and five unreversed profiles of travel-time data for the coastal region of the area. A fan of records NW of Atico, at ranges beyond the Pn crossover show azimuthal variability in their arrival times and waveform. These are interpreted to indicate local and large variations in the velocity structure in this region and impose limitations of the possible interpretation of the results. The two reversed profiles and one split profile sample about 1000 km of the coast, from Lima tb the Chilean border. The data have been fitted with two-dimensional models with plane dipping interfaces, and have been found to be well represented by such models. Since profiles sample different azimuths, one obtains a quasi-three-dimensional picture of the region which includes gently dipping (about 5 degrees to the NE), interfaces. The reversed profiles are best fit by a model which includes: (a) a thin upper crustal layer with velocities of 5.4-6.0 km/s; (b) a fairly thick 6.5 km/s layer; (c) a relatively thick layer of unusually high crustal velocity, 7.3 km/s, which is commonly found among the first arrivals and may be interpreted from later arrivals in all the record sections; and finally, (d) a mantle velocity of 8 km/s.