In most well-observed earthquakes one direction of first motion of the P wave is strongly dominant over the other. An analysis of 68 earthquakes, each with 100 or more observations, has suggested the following polarity rule: first motions are dominantly compressional for shallow earthquake sources in general, dilatational for shallow sources located beneath ocean trenches, and dilatational for deep sources. This rule applies globally with scattered exceptions. It is inferred that the source mechanism includes a component of volume change not allowed for in the usual shear model; the polarity rule is interpreted on the hypothesis that earthquakes are caused by sudden polymorphic transitions.

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