Short-period S-P conversion in the mantle at a depth near 700 km

Data from 16 deep earthquakes originating in the Tonga subduction zone and recorded at the Warramunga Seismic Array (WRA) in central Australia show the presence of a distinct later arrival in the coda of P. This phase which is not predicted by smooth velocity-depth models of the Earth has been called ‘X’ in the paper. Slownesses and arrival azimuths of X are similar to those of the P-wave. It is unlikely that X was radiated as the result of multiple focal activity because a corresponding phase was not observed, neither in the coda of ScP nor on seismograms of seismic stations located at close epicentral distances. Relative to P, the variation in arrival times with focal depth of the earthquakes indicates that X is produced by S-P conversion below the source, at a depth near 700 km. Most of the observed X phases have approximately the same signal duration as the P-waves thus suggesting a transition zone near 700 km which is only a few kilometres thick. Synthetic seismograms calculated for a model with a first-order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations. As demonstrated by the synthetic seismogram modelling the amplitude of X depends on the radiation pattern of the earthquake source. Variations in the radiation pattern offer a plausible explanation that, first, X is only intermittently detected at WRA and, secondly, that X has most frequently been observed from the southern part of the Tonga subduction zone.