It has long been recognized that Lg waves are not observed on paths traversing oceanic crust, but this has not yet been fully explained. Using normal-mode analysis and finite-difference simulations, we demonstrate that (1) the overall thickness of the crustal wave guide affects the number of normal modes in a given frequency range; in general, thinner crust accommodates fewer modes; (2) 6-km-thick oceanic crust does not allow Lg to develop as a significant phase in the frequency band 0.3 to 2 Hz because of the limited number of modes that exist; (3) in continental crust thicker than 15 km, there are usually sufficient modes that Lg is stable; (4) the shallow sediment layer plays important roles in crustal-guided wave propagation, trapping energy near the surface, separating Lg and Rg waves; (5) a 100-km-long segment of oceanic structure on a mixed ocean/continent path can block P-SV type Lg propagation. The primary reason why Lg does not travel through oceanic crust thus lies in the structure of the crustal wave guide, with the decisive factor being the crustal thickness. The detailed shape of ocean-to-continent crustal transitions can influence Lg blockage, but the general inefficiency of Lg propagation in the oceanic structure is the dominant effect.

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