P_diff Postcursors from the Base of the Hawaiian ULVZ

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 (S_diff), 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 (P_diff) postcursors likely generated by the Hawaiian ULVZ, at much shorter periods (~1 s) than those typically associated with S_diff postcursors (10–20 s). We analyze S_diff post-cursors 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 P_diff postcursor is caused by the presence of a very thin basal layer (with estimated V_S reduction of ~40%), δV_P: δV_S 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 P_diff 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.