Directions and amplitudes of first compressional and shear motions were measured in two-dimensional plastic models from a simulated explosion, a unidirectional-force, a single-couple and a double-couple source. Patterns for a uniform circular model are compared with those for the source located on a boundary, near a boundary, in a low-velocity zone, and near a high-velocity region.

In general, the patterns for the uniform models are consistent with theory except that weak S waves were noted from the simulated explosion. Reversals of the first motions of P due to the presence of velocity discontinuities were observed within 30° of the boundary running through the double-couple source.

Amplitude reduction in the high-velocity medium of the non-uniform models leads to the possibility of missing the first motions. Under these conditions, “first-motion” patterns may be undependable as a means of determining the source mechanism. Thus, a single couple, double couple, and radial force may produce a single-force distribution and the single force may give a single-couple pattern.

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