S-wave travel times for a spherically averaged earth |
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| Authors: | Robert Uhrhammer |
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| Institution: | Seismographic Station, University of California, Berkeley, California 94720, USA |
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| Abstract: | Summary. Shear-wave travel times in a spherically averaged earth are estimated using 'differential' S minus P ( S – P ) travel-time measurements and detailed statistical procedures. Fourteen earthquakes and 48 stations are specially selected, yielding 302 S - P times for 6° < Δ < 111°. Analysis of variance techniques are used to estimate simultaneously azimuthally varying source and station adjustments while constructing an S – P travel-time model. A method of weighting the equations of condition based on the distribution of stations and epicentres is developed to reduce the effects of systematic errors due to non-random sampling of the Earth. The resulting S - P travel times are added to the 1939 Jeffreys–Bullen and the 1968 Herrin P travel times as a function of distance to obtain shear-wave travel-time models. Confidence intervals for the models are estimated from the variance of the observed S – P travel times.
The standard error for a single observed S – P travel time (6° < Δ < 111°) is 2.1 s and the residual distribution is not significantly different from a normal distribution at the 95 per cent confidence level. For 30° < Δ < 80° the mean S travel time is 1.3 s later than the corresponding mean for Jeffreys–Bullen tables, which is significant at the 95 per cent confidence level. |
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