A search for the oceanic Lg phase

Abstract

The Lg phase has been shown previously to be a collection of higher-mode surface waves guided by the continental crust (Knopoff, Schwab & Kausel). A simple scaling between continental and oceanic crustal thicknesses suggests that a search for an oceanic Lg phase should be made in the period range from 1 to 2s. In a search for SH polarized Lg arrivals over oceanic paths, we found that in addition to the fundamental mode, seismo-grams at relatively short ranges in the Pacific showed the presence of only the first higher mode with group velocities on the steep portion of the dispersion curve rather than at the group velocity minimum as expected. Numerical model analysis indicates that, contrary to the continental case, there is no strong confluence of stationary phases of higher-mode crustal waves in the appropriate period range to produce Lg wave packets; this is due to small but significant differences in scaled crustal structures. Further, lateral variations in the thickness of oceanic sediments are sufficient to scatter most of the crustal surface-wave energy within a relatively short distance. Even were this thickness uniform, attenuation in the sediments would be strong enough to absorb the Lg stationary phases in a short distance.