A Revised Travel-Time Table for S

Abstract

Summary The travel times of S waves have been revised using the same statistical methods and data from the same selection of earthquakes as for the 1968 Seismological Tables for P phases. Because of the large quantity of data used, significant departures are observed in the new travel times from those of Jeffreys. A corresponding velocity distribution is obtained and involves lower velocities in the upper mantle and higher velocities at depths from 700 to 1000 km. The present study has been undertaken with the aim of providing travel-time tables for S that are consistent with the Herrin (1968) tables for P, and an associated S-velocity distribution for application to other problems of seismology. The data comprise all S phases reported in ZSS and ZSC bulletins for the set of earthquakes used by Herrin et al. (1968). The source parameters as redetermined by Herrin et al. were adopted for this study to maintain consistency between their P tables and this S table. The method has been similar to that set up by Herrin and his colleagues for P. Experience of the difficulties of picking the onset of the S phase which, unlike P, emerges from a background of earlier signals, suggests that one should expect, a priori, larger standard errors. So much larger, in fact, that the likely effects on the travel times of station corrections would be negligible in comparison. For this reason, no attempt was made at this stage to isolate station corrections. The data as reported in the ISS/ISC bulletins have been adjusted to allow for difference in source parameters by correcting distances to refer to the Herrin epicentre rather than that of ISC, and by correcting the travel times as required by the change of origin time. The effect of focal depth is allowed for by adding, to both distance and time, adjustments to project the ray back to the surface. A correction for ellipticity has been made, using the tables of Bullen (1937). The adjusted data were then grouped into 1 distance ranges starting from 20.5 and estimates of the mean travel time and its standard error obtained by an iterative process, involving successive approximation to these statistics, followed by truncation of the data at two standard deviations to remove gross errors. A correction to the estimates of standard error to allow for the effect of truncation was made, using the method of Freedman (1966). The means and their standard errors quickly converged to the values shown in Table 1. For distances beyond 80.5 the observations are complicated by the presence of the phase SKS which will be discussed in a separate publication (Randall 1970).