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Direction of faulting from first-motion studies

LAMONT GEOLOGICAL OBSERVATORY, COLUMBIA UNIVERSITY,, PALISADES, NEW YORK

Abstract

The effect of a crustal layer on the surface distribution of initial motion from a crustal earthquake is examined.

For crustal earthquakes a simple modification of the extended distances given by Hodgson and Storey permits the direct comparison of data obtained from a network of near stations with data from distant stations treated by the Byerly method. The patterns of first motion expected at near and distant stations from a given geometry of faulting with different assumed crustal velocities are compared. In cases where either the fault or auxiliary plane dips at an angle less than about 60° there should be a significant difference in the first-motion pattern. Under certain plausible assumed conditions patterns resembling an explosion or implosion can be obtained.

Previously published first-motion studies are discussed in the light of the foregoing analysis, and in certain cases a modification of the interpretation is necessary. These results indicate more strike slip faulting than has been previously estimated for foci within the crust.

Footnotes

The authors are, respectively, of the Lamont Geological Observatory (Columbia University), Palisades, N.Y., and the Institut pour la Recherche Scientifique en Afrique Centrale, Lwiro, Belgian Congo.

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