Pdiff Postcursors from the Base of the Hawaiian ULVZ

Abstract

Ultra-low velocity zones (ULVZs) are anomalous regions on the core–mantle boundary. In the past decade, several ULVZs, and particularly the Hawaiian ULVZ, have been studied using postcursors to core-diffracted S waves (Sdiff), which constrain their shear-wave velocity reduction. Simultaneous observations of P-wave velocity would allow us to better constrain the origin of ULVZs. We present the first observations of core-diffracted P-wave (Pdiff) postcursors likely generated by the Hawaiian ULVZ, at much shorter periods (∼1 s) than those typically associated with Sdiff postcursors (10–20 s). We analyze Sdiff postcursors from previous studies in two different period ranges for comparison. The observed delay times can be explained by a P-wave velocity reduction of 20%–30%. If we assume that the Pdiff postcursor is caused by the presence of a very thin basal layer (with estimated VS reduction of ∼40%), δVP:δVS ranges from 1:1.3 to 1:2. Iron enrichment is able to explain this ratio, whereas partial melt cannot. We also observe an anomalously high slowness for the Pdiff postcursors, which could suggest that the postcursors are not caused by the ULVZ but rather by scattering anomalies at 2400 km depth. We prefer the ULVZ hypothesis.