The PL phase is a normally dispersed train of waves of periods greater than about 10 seconds beginning at or near the time of the initial P wave and sometimes continuing at least to the time of the beginning of the Rayleigh-wave train. With adequate instrumentation the PL phase is commonly observed at distances less than about 25° from shallow shocks. In general, surface particle motion is elliptical and progressive, and amplitudes are not greater than about one-quarter those of Rayleigh waves of the same period. Comparison of PL- and Rayleigh-wave dispersion shows that both waves propagate in roughly the same near-surface wave guide. Whereas Rayleigh waves correspond to normal- (nonleaking-) mode propagation, PL waves appear to correspond to leakingmode propagation within this wave guide.

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