Short-period P wave constraints on D" reflectivity

We present evidence of a P wave reflector in D" beneath the central and eastern equatorial Pacific obtained from nearly 7000 short-period seismic recordings of earthquakes in the Tonga-Fiji and South America subduction complexes. Both regions overlap previous observations of an S wave triplication at similar depths. Double-array waveform stacking places the reflector 190 ± 20 km above the core-mantle boundary and allows for a velocity transition zone no thicker than 50 km. The data admit a wide range of compressional (ν_P) and shear wave velocities (ν_S); the 2-3% increase in ν_S suggested by earlier long-period S wave studies, however, necessitates only a 0.5-0.6% increase in ν_P. The data stacks represent spatial averages over large areas of the lower mantle, such that reflector intermittence and topography decrease stack amplitude and lower the apparent ν_P jump. Our results constrain these processes to act over length scales of 10-100 km.