Abstract

Two crustal models, from the Pacific Ocean to the Brazilian shield, have been derived for southern Peru and northern Chile. These crustal sections satisfy both the seismic and major features of the gravity anomaly when using velocity-density relations from Woollard, and Steinhart and Smith.

In general, the crustal sections show thick (over 10 km) sedimentary-metamorphic material with velocity about 4.5 km per sec under the altiplano, a 5.7- to 6.1-km-per-sec refractor pinching out near the Peru-Chile trench with a maximum thickness under the altiplano, and a 6.8- to 7.0-km-per-sec refractor becoming deeper and thicker under the Andean altiplano. The crustal thickness is about 12 km under the ocean basin, about 76 km under the altiplano, and 40 km (inferred) under the shield.

There is an abrupt lateral structural variability of the two crustal sections that is typical for the area between (at least) 12° and 30° S. lat, as determined from the regional extent of the gravity anomalies. The gravity anomaly map at sea level is dominated by a broad negative anomaly, −400 mgal, easterly offset from the line of highest Andean mountains. The Peru-Chile trench is reflected by a narrow negative anomaly (about −200 mgal extreme) that is separated from the negative Andean anomaly by a narrow, sharp, and relatively positive anomaly occasionally reaching values of about +20 mgal in the vicinity of the shore. The two crustal models account satisfactorily for observed differential time delays between seismic stations near the shoreline and stations on the altiplano. Computations of pressure differential as a function of depth between seismically derived crustal columns under the the Pacific Ocean basin (without removing the water layer) and the Andean altiplano reveal an excess of pressure under the altiplano with respect to the neighboring ocean from the surface to a depth of 55 km, with a maximum (1.5 to 1.7 kb) between 5- and 15-km depth. Below the 55-km depth and down to at least 80 km, the excess of pressure is under the ocean. This differential pressure distribution with depth apparently provides an extra mechanism of stress accumulation influencing the earthquake spatial distribution along the ocean-continent transition zone in western South America.

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