Arrival time inversion of local earthquakes in the northeast Hikurangi subduction zone has indicated high Vp/Vs at the plate interface, but the thickness of the anomalous zone is poorly constrained by the P and S travel-times. We investigate this plate interface zone further by modeling S to P converted phases, using synthetic seismograms for upgoing rays of earthquakes in the subducting plate mantle. The modeling indicates a low velocity layer at the plate interface, which is generally 1--2 km thick, and has Vp of 5.0--5.35 km/s and Vp/Vs of 2. This is consistent with a subducting sediment channel with near-lithostatic fluid. The low rigidity indicates a weak zone, which will have a strong influence on the distribution of deformation. The modeling approach shows promise in defining rheological parameters at the plate interface, and mapping their variation across the seismogenic zone of the subduction thrust. ¿ 1999 American Geophysical Union