The propagation of Love and Rayleigh waves in the Andean Region

Abstract

The distinctive pattern from the seismic source of a radiated seismic wavefield is profoundly modified on passing through an irregular region like the Andean Cordillera of South America. The crustally guided wave Lg is useful in the discrimination of seismic sources, but, even from a single source, amplitude ratios of short period waves like Lg can vary significantly. The Bolivian Andean Cordillera forms part of the Andean chain, which originated in two major orogenic cycles of the Phanerozoic, a Late Precambrian-Paleozoic Preandean Cycle and the Mesozoic-Cenozoic Andean Cycle. During the latter Cycle, four magmatic arc systems developed successively eastward: a Jurassic-Early Cretaceous arc in the Coastal Cordillera of Chile, a Mid-Cretaceous arc in the Longitudinal Valley of Chile, a Late Cretaceous-Paleogene arc in the Chilean Precordillera and the Miocene-Holocene arc in the Western Cordillera (Omarini eta/., 1991; Dorbath eta/., 1993; Scneuber et al., 1994). In the region of the low angle Main Andean Thrust, between the Eastern Cordillera (Cordillera Real) and the Subandean Ranges in northern Bolivia, there is overlap of approximately 230 km of Neogene age (Roeder, 1988). This contributes to keeping the conspicuous chloritic Permian marine sandstones near Copacabana on Lake Titicaca and the tightly folded Ordovician-Silurian diamictites, sandstones, quartzites and lutites, exposed abundantly in .. road cuts between Cochabamba and Caracollo, at elevations of from 3800 m to 4500 m above sea level. The Cordiltera Real fault system, at the southwestern border of the Cordillera Real, marks a subvertical boundary, dipping to the southwest, which, from the surface down to a depth of 140 km, separates two strongly contrasting velocity units. The depth to !!fe Moho below the Altiplano was found by Dorbath et al., (1993) to be approximately 60 km and, below the Cordillera Real, to be approximately 50 km. Further south, across southern Bolivia and northern Argentina, Wigger eta/., (1994) fo~d that the depth to the Moho below the Altiplano is approximately 72 km and, below the Cordillera Real, is approximately 65 km. The propagation of short period Love and Rayleigh waves along a profile crossing these vertically contrasting velocity units of the Altiplano and Cordillera Real in northern Bolivia has been analyzed by the finite element method. A preliminary result is that, without allowance for absorption, at a period of 2 s, 91.69 percent of the energy of the fundamental Love mode and 98.60 percent of the energy of the fundamental Rayleigh mode are transmitted.