WHY THE L(G) PHASE DOES NOT TRAVERSE OCEANIC-CRUST

It has long been recognized that L(g) waves are not observed on paths traversing oceanic crust, but this has not yet been fully explained. U sing normal-mode analysis and finite-difference simulations, we demons trate that (1) the overall thickness of the crustal wave guide affects the number of normal modes in a given frequency range; in general, th inner crust accommodates fewer modes; (2) 6-km-thick oceanic crust doe s not allow L(g) to develop as a significant phase in the frequency ba nd 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 L(g) is stable; (4) the shallow sediment layer plays import ant roles in crustal-guided wave propagation, trapping energy near the surface, separating L(g) and R(g) waves; (5) a 100-km-long segment of oceanic structure on a mixed ocean/continent path can block P-SV type L(g) propagation. The primary reason why L(g) does not travel through oceanic crust thus lies in the structure of the crustal wave guide, w ith the decisive factor being the crustal thickness. The detailed shap e of ocean-to-continent crustal transitions can influence L(g) blockag e, but the general inefficiency of L(g) propagation in the oceanic str ucture is the dominant effect.