Long-period energy preceding the PKPDF branch on long-period seismograms was identified as resulting from diffraction from the PKP caustic B. This conclusion is based on the measurements of the travel times, and, therefore, the slope of the travel times, on the amplitude decrease, and the very rapid dispersion of the periods of one earthquake of intermediate focal depth. Data from three deepfocus earthquakes support this interpretation. Thus, the long-period precursors are not due to the same cause as short-period precursors and are not produced by discontinuities in the transition zone between the inner and outer cores nor by structure in the inner core.

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