Global observation of off‐great‐circle propagation of Long‐Period surface waves

In the current generation of global dispersion maps of surface waves, the long‐wavelength structure seems to be very well determined. There is general agreement in the patterns of global phase velocity anomalies up to harmonic degree 16. However, the shorter‐wavelength structure varies significantly between published maps, and it appears that this part of the models depends strongly on the inversion technique and on the data set of surface‐wave dispersion (usually phase measurements). Polarization data depend on the lateral gradient of phase velocity and hence are more sensitive to shorter‐wavelength structure than phase data; thus, including these data should enhance resolution. In this paper, I demonstrate that polarization data of long‐period surface waves (80 s), as a function of frequency, can be reliably measured using a multitaper technique. the resulting off‐great‐circle arrival angles of the surface‐wave packets are relatively easy to interpret within a ray‐theoretical framework. Our data base of three‐component recordings is now large enough to provide useful constraints on global dispersion maps, particularly on the shorter‐wavelength parts. Apart from the phase velocity model itself, a possible misorientation of the horizontal components at each station is included in a non‐linear inversion as an additional independent model parameter. This gives a significant improvement in the fit to the data. Misorientations of more than 3° are probable for at least four of the 37 stations investigated.