Searching for seismic scattering off mantle interfaces between 800 km and 2000 km depth

[1] We systematically examined seismic data from deep earthquakes in the western and northwestern Pacific, Tonga/Fiji, and South America for signs of sharp gradients in seismic properties that could represent either a mineralogical phase change or the boundary between geochemical reservoirs in the Earth’s lower mantle. This involved the stacking of short-period, vertical component teleseismic waveforms from the dense Pacific Northwest Seismic Network and searching for evidence of S-to-P conversions at sharp mantle structures between 800 and 2000 km depth. With these data, we estimate we should be able to detect seismic discontinuities with Vs> 2%, transition width <20 km, and limited local topography. Using data from over 45 earthquakes, we found no evidence of a horizontal global discontinuity, neither near 920 km nor near 1700 km, beneath the four convergent margins studied here. Consistent with previous results, we detected a deep structure with topography near 1600 km beneath the Marianas region. Because we observed the signal over a large geographical region beneath the Marianas (10N to 20N), it is likely a coherent local structural and not a point source. The absence of such a structure beneath the northwest Pacific, the Tonga/Fiji region, or Sout