Lehmann discontinuity beneath North America: No role for seismic anisotropy

The Lehmann discontinuity in the upper mantle is often interpreted as a boundary separating anisotropic and isotropic media. We demonstrate, however, that seismic anisotropy plays, if any, a minor role in the origin of the Lehmann discontinuity. Our data are obtained with S receiver function technique from recordings of the MOMA seismograph array in the eastern USA.

Part of the data is processed with a new modification of this technique. We observe Sp phase converted from the Lehmann discontinuity at a depth around 200 km with comparable amplitudes and similar polarities in two azimuths differing by about 90°. Contrary to the observations, the polarity in the synthetic seismograms for models with azimuthal anisotropy is opposite for the azimuths differing by 90°.

Radial anisotropy with a slow vertical direction results in the Sp phase with the polarity opposite to the observed one.