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 (νP) and shear wave velocities (νS); the 2-3% increase in νS suggested by earlier long-period S wave studies, however, necessitates only a 0.5-0.6% increase in νP. 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 νP jump. Our results constrain these processes to act over length scales of 10-100 km.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Geophysical Research: Solid Earth|
|State||Published - Jan 1 1999|