The PF phase, described previously by Byerly as False S, is shown to be a longitudinal wave which travels with a velocity of 5.1 km./sec., and arrives at the observing station with an angle of emergence less than that observed for Pn.

The epicenter locations of earthquakes in northern California show that this phase arises only from shocks originating on continental structure, and is not oberved from earthquakes at large distances from the coast.

An examination of the Pn - relationship, and the Intensity, indicate that exceptionally shallow foci result in the generation of a PF phase. It is concluded that the very thick Franciscan formation in northern California carries considerable seismic energy from hese shallow focus earthquakes, and it is this energy which is identified as the PF phase.

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