Upper mantle anisotropy and coupled‐mode long‐period surface waves

SUMMARY We investigate surface wave propagation effects caused by P and S‐wave velocity anisotropy and its orientation in the upper mantle. We compare coupled‐mode synthetic seismograms, constructed from sums of free oscillations for different types of upper mantle models, in order to understand better the observable aspects of anisotropy and its fast direction in the earth. Our calculations suggest that upper mantle anisotropy, which is not transversely isotropic, of few per cent can generate significant waveform anomalies in long‐period seismic records of 0–20 mHz. Those anomalies are termed ‘quasi‐Love’ waves and are caused by spheroidal—toroidal free‐oscillation coupling. Large quasi‐Love waveforms are difficult to produce with levels of isotropic heterogeneity consistent with global tomographic models of the mantle. Coupled‐mode waveform anomalies in long‐period seismic records at a single station can be used to diagnose the presence of anisotropic structure in the upper mantle. Because 0 S l , — 0 T l coupling pairs have weak sensitivity to aspherical wavenumbers s < l — l , and | l — increases with frequency for f > 4 mHz, smoother/rougher structure generates longer/shorter‐period fundamental‐branch waveform anomalies. Anisotropy in our models is assumed to have hexagonal symmetry, that is, a single axis of symmetry. However, the symmetry axis can be arbitrarily oriented. The waveform anomalies associated with spheroidal‐toroidal coupling are sensitive to the orientation of the fast‐velocity direction of upper mantle anisotropy. In particular, for the same level of velocity perturbation, coupled‐mode waveform anomalies increase as the fast axis of anisotropic structure rotates from vertical to horizontal. The orientation of the fast axis can influence both phase shifts and coupled‐mode waveforms of long‐period surface waves. The coupled‐mode waveform anomalies predicted by our theoretical calculations are observed in the data and seem to diagnose the presence of upper mantle anisotropy which is not transversely isotropic.