A computer model study of the propagation of the long‐range Pn phase

Abstract

Synthetic long‐range (high‐frequency) Pn phases that fit both the arrival times and, more importantly, the characteristic long coda duration of this phase are generated using a velocity‐depth model consistent with long‐range refraction and surface wave observations. These synthetics show that a crustal guided‐wave plays an important part in the character of "typical" long‐range Pn coda; in particular, the largest amplitude arrivals at ranges greater than about 8° propagate through the crustal wave guide. Comparison of a synthetic long‐range Pn signal with an observation of this phase at 8° range suggests that any P‐velocity inversion at the base of the lithosphere must be small. This result is consistent with the anomalously high Q values for Sn reported by Walker et al. [1978] since a P‐velocity inversion much smaller than the associated S‐velocity inversion means that tunneling would extract much more low‐frequency energy from long‐range Sn than it would from long‐range Pn.