Shear-wave velocity structure of South America from Rayleigh-wave analysis

Abstract

The elastic structure beneath South America is shown by means of S-velocity maps for depths ranging from zero to 400km, determined by the regionalization and inversion of Rayleigh-wave dispersion. The traces of 128 earthquakes, occurring from 1990 to 2009, have been used to obtain Rayleigh-wave dispersion data. These earthquakes were registered by 71 seismic station located in South America. The dispersion curves were obtained for periods between 5 and 250s, by digital filtering with a combination of Multiple Filter Technique and Time Variable Filtering techniques. Later, all seismic events (and some stations) were grouped to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and inverted according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks 4°×4° in size. The shear-velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These results agree well with the geology and other geophysical results previously obtained. The obtained S-velocity models suggest the existence of lateral and vertical heterogeneity. The zones with consolidated and old structures (such as shields, cratons and massifs) present greater S-velocity values than the other zones, although this difference can be very little or negligible in some case. Nevertheless, in the depth range of 15-65km, the different Moho depths present in the study area generate the principal variation of S-velocity. Finally, a very interesting feature found in this study is the existence of a low S-velocity channel beneath South America, from 180 to 280km of depth, in which the S-velocity decreases with depth. © 2011 Blackwell Publishing Ltd.