Travel times for Pacific earthquakes

Summary. Pacific earthquakes studied by Gogna, also three important explosions in the Tuamotu archipelago, are rediscussed. The results are very consistent, but those from Tuamotu are later than Gogna's by about 1 s in the times of P about 60°. Both sets of data give PKP residuals about -5 s about 140° - 142°, indicating that the observations there referred to the neighbourhood of the cusp of the travel-time curve but the ISS had compared them with the DEF branch. The corresponding difference in the 1940 tables is about 2 s.
Analysis at intervals of 1° indicated that the cusp of PKP is about 141° instead of 143° as in the 1940 tables and the difference between it and the DEF branch at these distances is about - 5 s.
Travel times of S under the Pacific were found but need more data, especially at distances under 10°.
Times of PcP reported by Kogan and Carder were compared with those calculated from P in Gogan's explosions, and indicated a radius of the core of 3479.8 ± 1.8 km.     

The PWaves From the Amchitka Island Explosions

Summary. There is evidence that the equivalent seismic sources of the Amchitka Island explosions — Longshot, Milrow and Cannikin — depart significantly from the simple model of a point compressional-source in a layered elastic-medium. Consequently modelling the observed seismograms using standard source-models may not be the most efficient method of determining source properties. Here an alternative to modelling is used to obtain information on the seismic sources due to the explosions. Broad-band (BB) estimates of the P signals are obtained from the short-period (SP) seismograms, corrected for attenuation, and interpreted in terms of P, pP and radiation from secondary sources. the main conclusions are:
(i) BB estimates of the radiated displacement from the explosions can be obtained with only a small reduction in the signal-to-noise ratio seen on SP seismograms;
(ii) observations of differences in pulse amplitudes and spectra are not necessarily due to differences in anelastic attenuation;
(iii) P and pP at a given station may differ in shape so that notches in the signal spectrum may not be related to source depth;
(iv) there is evidence of arrivals that others have identified as due to slap-down but which could be interpreted as an overshoot to pP;
(v) direct interpretation of the estimated ground displacement is a better procedure for determining the seismic source properties of explosions than modelling SP seismograms using idealised models as a starting point.