Institute For Coupled Modes
Department of Geology and Geophysics
P.O. Box 208109, Yale University
New Haven, Connecticut 06520
poster/oral:
We apply the Born approximation for the influence of seismic anisotropy on the coupling of free oscillations to generate synthetic seismograms. These synthetics capture faithfully the ``quasi-Love'' forward-scattered surface wave using path-integrals from source to receiver. These Love-to-Rayleigh scattered waves can be used to flag the presence and location of strong lateral gradients of anisotropy in the upper mantle, associated with, mantle flow variations, regional terrane sutures, or other lateral structures. Anisotropy restricted to the lithospheric lid leads to only weak fundamental-mode quasi-Love scattered waves at ~100-sec period, but potentially stronger scattering at periods <50 sec. Anisotropy restricted to the asthenospheric low velocity zone leads to stronger fundamental-mode quasi-Love scattered waves at ~100-sec period, but weaker scattering at periods <50 sec. The coupling effects of azimuthal P- and S-anisotropy with cos 2\eta dependence tend to cancel. Unlike conventional tomography, strong gradients of anisotropic properties along the path can be localized by the relative timing of Love, Rayleigh, and scattered waves on a single three-component record. Compared with fully-coupled synthetic seismograms computed with free oscillations in a 2-D lateral structure model, PI synthetics are subject to bias up to 2-3 degrees of arc (~300 km) in the location of these gradients along the path. This is attributable in our test examples to off-path refraction of the Love and Rayleigh raypath segments.