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Paper reference: M. Richards and C. Wicks, 1990, S-P Conversion from the Transition Zone Beneath Tonga and the Nature of the 670 KM Discontinuity, Geophys. J. Int., 101, 1-35

Using more than 300 recordings of deep Tonga earthquakes from the Warramunga Seismic Array (central Australia), we have searched for short-period S-P conversions from the 670 km discontinuity arriving in the P-wave codas. The array seismograms were stacked and plotted in sections to optimize observation of S670P yielding the following results: For the northern part of Tonga (17.3°-18.4°S), S670P-phases are observed with apparent conversion depths between about 660 and 680 km. For the central part (20.2°-22.3°S), S670P is not clearly observed. For the southern part (23.1°-25.7°S), S670P is observed for conversion depths between about 660 and 700 km. Intermittent occurrence of S-P conversion may result from different characteristic source radiation patterns among the three secions. Overall uncertainties in conversion depths (resulting from unmodelled lateral velocity variations and hypocentral location errors) are perhaps a few tens of kilometres. The S-P rays must travel either within or very close to the subducted slab because of the source/receiver/slab-dip geometry, so we conclude that large (>50 km) deformations of the 670 km discontinuity do not occur beneath Tonga. Our observations appear to be inconsistent with a chemical discontinuity at 670 km, but are easily explained if the 670 km discontinuity is a strongly pressure-dependent and, perhaps, moderately endothermic phase changein both the slab and surrounding mantle.