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 GH branch of PKP from deep-focus earthquakes

summary . Approximately 1000 PKP observations in the range 110° < Δ < 170° have been statistically studied to examine the existence of Bolt's GH branch of the PKP travel-time curve. Data give firm evidence for the existence of this branch from 130 to 153° but the possibility of its extending further cannot be ruled out. Observations for this branch fit the form:
Where a =(1150.3254±0.5328)s, b = (16.4907±0.3803)s, c = (−0.1316±0.0630)s.
Times for this branch are always greater by 2 s than the corresponding Gogna times from Pacific earthquakes.     

Inference of core-mantle boundary topography from ISC PcP and PKP traveltimes

We invert ISC PcP and PKP absolute and differential traveltimes in an attempt to infer the long-wavelength topography of the core-mantle boundary (CMB). The data selection and processing methods are described and evaluated. These travel-time data are very noisy and the geographic distribution of the data is highly non-uniform, inhibiting reliable inference of CMB topography. Spatial averaging enhances the coherent component of the residual variance (related to heterogeneity), however, the random component of the variance is much larger than the coherent component. We show that for PcP data the coherent signal due to mantle heterogeneity overshadows that arising from the CMB, and that the effects of mantle heterogeneity are mapped into our inferred CMB solutions. The PcP data are not correlated across the spatial averaging bins and seem to have a strong bias due to small-scale structure and/or noise. The non-uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the CMB. Spatial patterns of CMB models inferred from different phases do not agree. Amplitudes of seismically inferred CMB undulations vary greatly. The sensitivity of inferred CMB models to the processing, spatial averaging procedure, and inversion techniques are investigated. Topographic amplitudes increase strongly with increasing input residual variance. The power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher. Based on this work, we conclude that reliable inference of long-wavelength CMB topography is not likely with the current ISC data set or with a spherical harmonic expansion truncated to degree and order 6.