BODY WAVE TRAVEL-TIMES AND A SPHERICALLY SYMMETRICAL P-WAVE AND S-WAVE VELOCITY MODEL

The derivation of a spherically-symmetric reference earth model is a subject which has recently regained interest. After a decade of studies on lateral heterogeneity we now have the tools to approach the problem. An IASPEI Working Group on Traveltime Tables was formed in 1988 with the goal of deriving a reliable and up-to-date radial model of seismic velocities. Following the guidelines developed by that working group we have investigated the body wave traveltimes for several branches frequently reported in the ISC Bulletins and propose here a model for P-and S-wave velocities consistent with these data. Approximately 16 000 best-recorded, shallow-focus earthquakes are selected from 24 years of ISC Bulletins (1964-1987) and relocated using, among others, corrections for lateral heterogeneity. From this dataset we derive summary traveltime curves specified for each degree of epicentral distance for P, PcP-P, PKP(AB), PKP(BC), PKIKP and S. To constrain velocity at the top of the outer core, we include data for SKS and SKKS-SKS from Hales & Roberts (1970,1971). Our discrete time-distance curves ar e obtained by evaluating the shape of the residual density distribution, independently for each 1-degrees interval, to limit contamination due to the interference of other branches, precursors or late readings. In the upper mantle, the resulting model, SP6, differs from iasp91 (Kennett & Engdahl 1991) only by slightly higher P and S velocities between 410 and 660 km. In the lower mantle, new velocity profiles have lower velocity gradients. This is also true in comparison with PREM (Dziewonski & Anderson 1981). At the top of the outer core upsilon(P) is smaller than in PREM, and the velocity jump at the inner core boundary is reduced to the value of 0.62 km s-1. The new inner core radius is 1215 km. Our model predicts traveltimes that differ from the iasp91 tables considerably, although generally less than 1 s, except for SKS and SKKS for which the differences are greater.