Long-period precursors to pP from deep-focus earthquakes: the Moho underside reflection pMP

Abstract

SUMMARY Moho underside reflection pMP is a weak precursor to the surface reflection pP. It is best observed in long-period records from deep-focus earthquakes, provided that the P coda has sufficiently low amplitudes. pMP reflection in continental or shelf regions produces a clear arrival, whereas pMP reflection at the oceanic Moho produces a smooth transition from pMP to pP. These properties are illustrated by WWSSN seismogram examples for events below the Sea of Japan, the Sea of Okhotsk, Argentina and Tonga-Fiji. Synthetic seismograms are calculated with a simple reflectivity method, and comparison with observed seismograms, corresponding to continental pMP reflection, gives estimates of crustal thickness and Moho P-velocity change; they are averages over reflection zones with a diameter of about 300 km. Application to the crust of North Korea indicates an increase of crustal thickness from about 27 to 32 km over about 300 km from N to S, accompanied by a decrease in the velocity change at the Moho from about 1.3 to 0.9 km s-'.