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Why the L_g phase does not traverse oceanic crust

Institute of Tectonics University of California, Santa Cruz, California 95064

Abstract

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. 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 L_g 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 L_g is stable; (4) the shallow sediment layer plays important 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, with the decisive factor being the crustal thickness. The detailed shape of ocean-to-continent crustal transitions can influence L_g blockage, but the general inefficiency of L_g propagation in the oceanic structure is the dominant effect.