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The distance dependence of regional phase discriminants

Abstract

Amplitude ratios of P and S phases recorded at regional distances have been suggested as potential discriminants for the character of seismic sources, because of the differences expected in the radiation pattern of earthquakes and well-contained explosions. The most useful reference phase for P waves beyond 200 km from the source is Pn and for S possible choices are the mantle phase Sn and the crustally guided wave Lg. Each of these phases has a different interaction with the seismic structure of the crust and mantle and such structural effects will impose their own patterns on the radiation characteristics from the source. The range dependent component in the Lg/Pn and Sn/Pn ratios have been investigated for a dense set of three-component records covering the distance range to 700 km, for an explosive shot in southern Sweden that formed part of the 1979 Fennolora experiment. A stable measure of amplitudes and amplitude ratios is provided by using the vector resultant of ground motion (the square root of the total energy). However, even for a single source there are significant variations in the amplitude ratios for Lg/Pn and Sn/Pn of a factor of 3 or more as a function of range.

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