Estimation ofPKPtravel times

Abstract

A set of travel times for the core-waves PKP is derived from observations and tabulated. All values are tied to the 1968 P travel times for a surface-focus for 75° < Δ < 95° due to Herrin et al. Times for both the branches DF (110° < Δ < 180°) and GH (125° < Δ < 156°) are listed in more detail than previously, together with an extension for PKiKP waves, from 110° back to 105°. Checks on the DF branch for 120° < ° < 150° using observations from the 1965 LONGSHOT explosion provide no evidence for further revision of the first section of this branch. Special attention is given to the times of the DF branch (PKIKP) for ° > 150°. Delays at 153° and 162° of order 3 sec reported by K. Ergin in 1967 are not confirmed. There is a group of small arrivals about 1 sec earlier than predicted by the present PKIKP tables for 156° < Δ < 160°, approximately. These may indicate an error in the curvature of the present curve and, perhaps, a slight discontinuity in the inner core near a radius of 850 km. No evidence is found for a layer of any consequence with reduced P velocity in the inner core. Revised times for the branch AB (denoted by PKP₂) are based mainly on readings from the Indian Ocean earthquake of February 17, 1966 and the Jeffreys-Bullen times. For Δ > 155°, the new empirical times confirm the curvature of the Jeffreys-Bullen curve for PKP₂; however, near Δ = 152°, the observed curvature appears to be greater than that of the latter curve. The new times are of the order of one second earlier; this result is highly sensitive to the precision of the epicenter estimated for the 1966 shock. PKP₂ is traceable at least out to 185°. Depth allowances for PKP have been computed, by T. Qamar, to provide a set consistent with the model Earth used by Herrin et al in the 1968 P tables.