A reflector at 200 km depth beneath the northwest Pacific

We present an analysis of precursors to PP produced by underside reflection s from discontinuities in the upper mantle beneath the NW Pacific. The even ts used for this Study Occur in the western Pacific Rim (New Zealand, Fiji. Tonga. Solomon. New Guinea. Philippine Islands) and are recorded at the sh ort-period Yellowknife Array (YKA) in northern Canada. The source-receiver combination results in PP reflection points which allow LIS to study the up per mantle structure in a corridor from the Hawaiian Islands to the Kuril s ubduction zone. To detect the weak precursors in the time window between th e P arrival and the PP onset and to identify them as PP underside reflectio ns. special array techniques are used. Our analysis indicates a reflector a t a depth of similar to 200 km beneath the northwestern Pacific. This refle ctor show's strong topography of some tens of kilometres on length scales o f several hundred kilometres. complicating the detection of this reflector in global or regional stacks of seismograms. Different models for the imped ance jump across the reflector, the thickness and the possible fine structu re of the reflector are modelled using synthetic seismograms and are compar ed with the data. The thickness of the reflector has to be less than 7 km a nd 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 similar to4 per cent assuming the PREM density model.