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Aspects of Pn and Pg propagation at regional distances

DEPARTMENT OF GEOSCIENCES THE PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PENNSYLVANIA 16802

Abstract

Generalized ray theory models are calculated for a moment tensor point source in two idealized structure models appropriate for the Eastern United States to study the wave propagation of regional Pn and Pg. The models are analyzed to determine useful discriminants between earthquakes and explosions. Pg is composed of multiply reflected postcritical rays trapped in the upper crustal layer. The amplitude decay with distance of Pg is found to be source dependent and may be useful as a discriminant in well-calibrated earth structures. Pg waves from dip-slip sources fall off r^–0.5 to r^–0.9 faster than those from isotropic and vertical strike-slip sources, where r is the horizontal distance. In conjunction with previously published refraction results and magnitude studies, it is shown that regional Pn along many profiles in eastern North America consists principally of high-amplitude turning rays rather than head waves as observed in the Western United States. These turning rays are as large or larger than the Pg phase explaining why Pg is not as commonly observed as a distinct phase in the east as compared with observations in the Western United States.

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