Upper mantle anisotropy in the S.W. Pacific from earthquake travel-time analysis

Abstract

The negative P-residuals, from readings at stations on Niue, Rarotonga and the Chatham Islands of shallower events along the Tonga-Kermadec trench and below the North Island, New Zealand, exhibit a scatter too large to be accounted for solely by observation and mislocation errors. The corresponding wavepaths cover a range of propagation azimuths between N30°E and N180°E. Travel times from a large number of events reveal a variation of P-velocity with azimuth which is consistent with large scale upper mantle anisotropy in this area. The P-velocity varies from a maximum of 8.4 km s−1 along N62°E to a minimum of 7.8 km s−1 along a perpendicular axis, perhaps indicating a fossil spreading direction along N62°E. There is also a very consistent, but unexplained, variation of P-residuals with earthquake depths between 0 and 100 km; an observation warranting further study.