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.

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