High-frequency seismic wave propagation in western South America along the continental margin, in the Nazca plate and across the Altiplano

Abstract

Summary. High-frequency shear waves (0.5–2 Hz) recorded at regional distances at WWSSN stations in western South America are classified according to their apparent velocity and frequency/amplitude character. For propagation paths crossing any given region, the observations are abundant and consistent. Sn is not observed at distances beyond about 15.5° along paths in the eastern part of the relatively young (Eocene) oceanic Nazca plate. This observation does not appear to be related to any specific tectonic or structural feature along the propagation paths. In contrast, Sn is observed at distances beyond about 25° in the older (Jurassic–Cretaceous) parts of the western Pacific and western Atlantic oceanic plates and to distances of over 40° in continental shield regions. One interpretation of these and other data is that the maximum distance over which Sn is observed increased with the thickness of the lithosphere. The disappearance of Sn in the Nazca plane is quite sharp, occurring between 15 and 16°. Such a phenomenon cannot be explained by simple attenuation mechanisms. Lateral variations in Sn propagation occur beneath the Altiplano, a high plateau in the central Andes. Two types of seismograms imply inefficient Sn propagation: one shows no Sn, implying an average low-Q along the path; the other has a complex ringing character suggesting a large amount of scattering along the path. Sn is also observed to propagate efficiently across some parts of the Altiplano, which would usually imply high-Q material in the mantle wedge that separates the South American plate from the descending Nazca plate. However, because the dip of the descending Nazca plate beneath the Altiplano is only about 30°, it is possible that these Sn waves are actually refracted along the descending plate instead of travelling mainly through the mantle wedge. Sn has a velocity of about 4.5 km/s along western South America. Sn travelling in the oceanic Nazca plate converts efficiently into Lg travelling in the continental crust where the crust thickens beneath the Andes. No reverse Lg to Sn conversion is observed. Lateral variations in Lg propagation appear to be related to the orientation of the path relative to the structural trend of the Andes rather than to anomalous regions in the continental crust. It appears that Lg propagates efficiently only when the direction of propagation is approximately parallel to the strike of the Andes.