Anisotropic loci in the mantle beneath central Peru

Seismic anisotropy of the upper mantle beneath the central part of Peru is examined by analyzing shear waves observed at broad-band stations and a temporary array of short-period stations. Shear-wave splitting is seen on various shear phases, such as direct S waves from local intermediate to deep earthquakes, ScS waves from regional deep earthquakes, and SKS waves from teleseismic earthquakes. It is inferred that the shear-wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted Nazca plate is 0.5% at most, while the anisotropy in the subslab asthenosphere (depth range of about 150-350 km) is 2% or greater. The fast shear-wave polarization direction in this depth range, as observed at two broad-band stations, is 30°-40° different from the absolute motion of the Nazca plate. This does not fit simple two-dimensional (2-D) models of olivine alignment caused by slab-induced mantle flow, and implies either the existence of a complex flow pattern in the asthenosphere underneath the Nazca plate or the presence of an unknown mechanism for the anisotropy formation. The lower mantle beneath central Peru is found to be effectively isotropic for nearly vertically propagated shear waves.