Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Abstract

Short period University of Washington, northern California, and southern California seismic array records of a sequence of deep Mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following P. We measure slowness, arrival time, and arrival azimuth of these phases relative to P using array techniques to find where they emanate in the mantle. By combining slowness and back azimuth deviations and the move‐out with depth of the arrivals from different earthquakes, we find their most probable origin is through S‐to‐P conversion at point scatterers in the mid‐ to lower mantle at 25.7°N 148.2°E 1590 km and 25.5°N 151.0°E 1850 km respectively, separated by ∼300 km. These features indicate elastically distinct material in the middle of the lower mantle, which, if fragments of subducted slabs, are not related to contemporary subduction. They may represent upwelling convective material from the deep mantle or D″, but they are not purely thermal in nature. As chemically distinct bodies, they may represent potential reservoirs of the isotopic and trace element components recognized geochemically.