Anisotropy beneath 9 stations of the GEOSCOPE Broadband Network as deduced from shear‐wave splitting

Abstract

Polarization of SKS, ScS and S waves has been analyzed on digital 3‐components seismograms recorded at 9 stations of the recently installed GEOSCOPE broadband network. Splitting was observed for about 15 records, and is interpreted as being due to seismic anisotropy beneath the stations. From every record, it is possible to retrieve the fast direction at the station, and the time difference between the fast and slow polarization directions. Some 30 observations of quasi‐linear S‐pulses also help constrain the direction of fast velocity. Synthetic seismograms were built to test the validity of the method used to retrieve the parameters of anisotropy. Typical time‐differences are about 1 sec, corresponding, for example, to a 80 km‐thick zone of perfectly oriented pyrolite. The fast directions obtained at 3 continental stations in America (Westford, Santa Cruz, Heredia), could be related to plate tectonic processes. On the other hand, the fast directions found in 3 oceanic islands (Kipapa, Papete, La Réunion) differ strongly from the oceanic plate velocity direction there.