DETECTION OF LOWER MANTLE SCATTERERS NORTHEAST OF THE MARIANNA SUBDUCTION ZONE USING SHORT-PERIOD ARRAY DATA

Short period University of Washington, northern California, and southe rn California seismic array records of a sequence of deep Mariana subd uction zone earthquakes show unexpected clear arrivals nearly 90 and 1 05 s following P. We measure slowness, arrival time, and arrival azimu th of these phases relative to P using array techniques to find where they emanate in the mantle. By combining slowness and back azimuth dev iations and the move-out with depth of the arrivals from different ear thquakes, we find their most probable origin is through S-to-P convers ion at point scatterers in the mid-to lower mantle at 25.7 degrees N 1 48.2 degrees E 1590 km and 25.5 degrees N 151.0 degrees E 1850 km resp ectively, separated by similar to 300 km. These features indicate elas tically distinct material in the middle of the lower mantle, which, if fragments of subducted slabs, are not related to contemporary subduct ion. They may represent upwelling convective material from the deep ma ntle or D '', but they are not purely thermal in nature. As chemically distinct bodies, they may represent potential reservoirs of the isoto pic and trace element components recognized geochemically.