The character of regional S wave trains is quite complex and consists of two major elements: Sn waves which principally travel in the upper mantle and Lg waves guided in the Earth's crust. The effect of lateral heterogeneity is to modify the signals for these phases compared with the predictions for horizontally stratified models. However, complete synthetic seismogram calculations for simple, stratified crustal models for a suite of distances out to 950 km, including both Sn and Lg, provide a range of insights into the nature of the S wave train. In particular, late picks of Sn arrivals are likely to arise because the actual S onset has been missed in the P coda, and a higher amplitude surface multiple phase with an extra crustal delay has been picked instead. Such a effect was first recognized by Lapwood (1955) in a study of the ISS readings for a sequence of Japanese events recorded at regional distances, but has usually not been taken into consideration in studies using regional networks.

Lateral heterogeneity along the propagation path has the effect of inducing interconversion between Sn and Lg energy and thus alters the character of the S wave train. The lower attenuation for S in the uppermost mantle, compared with the crust, frequently leaves Sn as the most prominent phase at ranges greater than 1000 km. Long range propagation of high-frequency Lg requires a relatively undisturbed crust.

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