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Abstract

We present an analysis of precursors to PP produced by underside reflections from discontinuities in the upper mantle beneath the NW Pacific. The events used for this study occur in the western Pacific Rim (New Zealand, Fiji, Tonga, Solomon, New Guinea, Philippine Islands) and are recorded at the short-period Yellowknife Array (YKA) in northern Canada. The source-receiver combination results in PP reflection points which allow us to study the upper mantle structure in a corridor from the Hawaiian Islands to the Kuril subduction zone. To detect the weak precursors in the time window between the P arrival and the PP onset and to identify them as PP underside reflections, special array techniques are used. Our analysis indicates a reflector at a depth of ~200 km beneath the northwestern Pacific. This reflector shows strong topography of some tens of kilometres on length scales of several hundred kilometres, complicating the detection of this reflector in global or regional stacks of seismograms. Different models for the impedance jump across the reflector, the thickness and the possible fine structure of the reflector are modelled using synthetic seismograms and are compared with the data. The thickness of the reflector has to be less than 7 km and the P wave impedance contrast has to be larger than 5.0 ? 6.5 per cent to be detected by this study. This corresponds to a P-velocity jump of ~4 per cent assuming the PREM density model.