Group-velocity tomography of South America and the surrounding oceans

This paper presents the results of a study of the dispersion characteristic s of broadband fundamental-mode surface waves propagating across South Amer ica and the surrounding oceans. Broad-band waveform data from about 765 eve nts from 1977 to 1996 recorded at 48 individual stations have produced 7000 Rayleigh-wave and 4800 Love-wave dispersion curves. We present group-veloc ity maps from 20 to 150 s period for Rayleigh waves and from 20 to 100 s fo r Love waves. Measurement uncertainties estimated from cluster analyses ave rage about 0.025-0.030 km s(-1), but are larger for the short-period Love w aves. We estimate surface resolving kernels and show that the average resol ution across South America is about 6 degrees-8 degrees for Rayleigh waves and 7 degrees-10 degrees for Love waves below 100 a period but degrades at longer periods and near the periphery of the maps. The estimated maps produ ce a variance reduction relative to the Preliminary Reference Earth Model ( PREM) of more than 90 per cent at short periods, more than 80 per cent for Rayleigh and Love waves below about 100 s period, but nearer to about 70 pe r cent at longer periods. Synthetic experiments are presented to estimate t he bias caused by azimuthal anisotropy using the recent global model of Tra mpert & Woodhouse (1996). This bias, which is worse for Love than for Rayle igh waves, may be substantial (up to 5 per cent in some places), but it is expected to be largely off the continent and is not correlated with the mai n features of the observed maps. Many known geological and tectonic structu res are observed in the group-velocity maps. Of particular note are the sig natures of sedimentary basins (e.g. Maturin-Llanos Basin, Maranon-Ucayali-M adre de Dies complex, Chaco-Tarija Basin, S. Parana Basin, the basins in th e W. Caribbean and W. Gulf of Mexico), variations in crustal thickness (e.g . Andes, Altiplano, Brazilian Highlands), continental roots (e.g. Guyana an d Guapore shields, Sao Francisco Craton), and the Galapagos thermal anomaly . Comparison of the estimated group-velocity maps with those predicted by C RUST5.1/S16B30 is qualitatively good, but there are significant differences in detail that provide new information that should help to calibrate futur e crustal and upper-mantle models of South America.