Small-scale heterogeneities in the convective upper mantle beneath circum-Pacific subduction zones: Evidence for fragments of recycled basaltic crust

We show evidence for the presence of compositional heterogeneities at 10 km scale in the convective upper mantle below the circum-Pacific subduction zones. By array-processing we examine seismograms of intermediate-depth earthquakes recorded by large-aperture seismic networks at teleseismic distances. In the P coda until about 25 s after direct P waves, we observe anomalous and significant signals arriving from the directions close to the P waves. The amplitude and duration of the signals decrease with focal depth within the range from about 180 to 300 km. We show that the signals arise mainly from S-to-P scattering in the mantle below the foci. Using the scattering theory for randomly heterogeneous media we also show that the observed features of the signals are matched by 5 to 10 km-scale density and rigidity anomalies that diminish below about 400 km depth. The rigidity anomaly is of the same sign and nearly twice as large as the density anomaly. These properties of the scattering objects could be consistent with basalt embedded in depleted mantle. The results indicate that the upper mantle transition zone beneath subducting slabs is not well mixed even by vigorous convection.