Abstract

A composite P residual pattern for deep Fijian earthquakes, obtained by integrating observations made during a number of short-term surveys in eastern Queensland, displays consistent relative station delays in excess of 1 sec. Observations from selected teleseisms in other source regions indicate significant azimuthal variations in delay patterns at some stations. The main features of the observed residual field are consistent with laterally varying velocities beneath the receiving region, these occurring in the depth range of 100 to 300 km, and logically indicating differential development of an upper mantle, lowvelocity zone. The reduced velocities are believed to result primarily from increased upper mantle temperatures, with associated partial melting, and such an interpretation is consistent with apparent similarities between the distributions of the inferred regions of lower-than-average velocity and several volcanic, tectonic, and seismic features.

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