Seismic waves at the epicenter's antipode

Abstract

The antipodal region (178° ≤ Δ ≤ 180°) of a seismic wave source is investigated in detail and shown to provide a new set of remarkable data to use in the exploration of the earth's interior. Body and surface waves converge individually at antipodal distances after having sampled laterally the totality of the planet. The waves are focused and strongly amplified up to 1 order of magnitude with respect to the normal phase recorded 2° or more away. The delicate interference patterns thus formed yield information on departures from lateral homogeneity and sphericity of the core and mantle, the structure of the inner core, global dissipation characteristics of the upper mantle, and provide strong constraints on earth models. Seismograms have been synthesized that closely reproduce the phases Pdiff, PKIKP, PKIIKP, PKP(BC), PKP, and PP observed at World‐Wide Standard Seismographic Network long‐period instruments located within 5° from the antipode of the New Zealand Inangahua earthquake of May 23, 1968. Preliminary results indicate that the lower mantle and upper core are laterally homogeneous as seen by 15‐s waves, but the core‐mantle boundary region is probably laterally inhomogeneous. The inner core‐outer core boundary appears to be a sharp transition with a P wave velocity jump of the order of 0.8 km/s. The resolution of the long‐period data is poor, but the potential richness of the method when better data sets are available strongly motivated the investigation. Suggested future lines of research using antipodal observations include monitoring of inner core phases, study of focal processes of large earthquakes, and the exploration of planetary interiors.