Short-period P wave constraints on D '' reflectivity

We present evidence of a P wave reflector in D" beneath the central and eas tern equatorial Pacific obtained from nearly 7000 short-period seismic reco rdings of earthquakes in the Tonga-Fiji and South America subduction comple xes. Both regions overlap previous observations of an S wave triplication a t similar depths. Double-array waveform stacking places the reflector 190 /- 20 km above the core-mantle boundary and allows for a velocity transitio n zone no thicker than 50 km. The data admit a wide range of compressional (v(P)) and shear wave velocities (v(S)); the 2-3% increase in v(S) suggeste d by earlier long-period S wave studies, however, necessitates only a 0.5-0 .6% increase in v(P). The data stacks represent spatial averages over large areas of the lower mantle, such that reflector intermittence and topograph y decrease stack amplitude and lower the apparent v(P) jump. Our results co nstrain these processes to act over length scales of 10-100 km.