Source phase and amplitude anomalies of long‐period surface waves

In addition to the Fermat phase anomaly and the amplitude anomaly produced by geometric focusing and defocusing along the propagation path, long‐period surface waves on a laterally heterogeneous earth exhibit phase and amplitude variations due to the deviation in takeoff azimuth of the ray at the source. The resulting perturbation in source phase is generally small enough to be ignored; however, the source amplitude anomaly may be comparable to that produced by focusing and defocusing. Adding the source anomaly significantly reduces the variance of amplitude measurements performed on a large suite of synthetic vertical‐component coupled‐mode seismograms. Future tomographic analyses that incorporate amplitude measurements should account for this effect.