The detection of PKIIKP and damping in the inner core

Abstract

SUMMARY - The phase PKIIKP, reflected once from the lower surface of the inner core boundary, was most probably recorded by LAS A from the Faultless underground nuclear explosion at 10°.9 angular distance. The array beam detected a P onset, with the correct slowness, within 2 sees of the theoretical travel time. The ground amplitude is 3 + 1 millimicrons at a dominant period of 1.5 sec. By comparison, the PKiKP ground amplitude from Faultless is 75 millimicrons at a dominant period of 1.1 sec. This detection again confirms the sharpness of the inner core boundary and the 1968 core travel times for PKiKP. The additional attenuation of PKIIKP over PKiKP yields, from direct and spectral measurements, Q = 4 5 0 + 1 00 for P waves in the inner core. This value sharpens the earlier estimates of I. Sacks, G. Buchbinder, A. Qamar, and A. Eisenberg and is almost independent of core structure and rigidity. On the assumption of a constant velocity in the inner core, the observed differential PKIIKP minus PKiKP travel time requires an average P velocity of 11.4 + 0.02 km/sec for an inner core radius of 1216 km.