Seismological constraints on the ultralow velocity zones in the lowermost mantle from core-reflected waves

Analyses of reflected seismic phases PcP and ScP from the core–mantle boundary (CMB) enable us to constrain the physical properties of the ultralow velocity zones (ULVZs) at the base of the mantle. The CMB region beneath the western and southern Pacific is investigated in detail by using a large amount of waveform data recorded by the short-period array stations. Clear arrivals of postcursor to ScP (ScsP) are detected from several event-receiver pairs, which sample the CMB regions beneath Philippine–Kalimantan and East of Australia, indicating the existence of ULVZs under the regions. No ScP precursors (SdP and SPcP) are found in our data set except for a few events with a large epicentral distance over 45 ° . No additional arrivals related to PcP are found in the region where ScsP is observed in the ScP wavelet. Elastic properties of the ULVZ are precisely constrained by waveform modeling of both PcP and ScP which sample the CMB region under Philippine–Kalimantan. Different ULVZ structures are found between the northeastern and southwestern parts of the Philippine–Kalimantan: the ratio of shear-to-compressional velocity reductions is 2.0–2.5 and the ratio of density-to-shear velocity changes is − 0.5 to − 0.33 in the northeastern part, while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found. A certain degree of density increases and shear velocity decreases are also observed in the ULVZ beneath East of Australia. Increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ULVZ anomaly. However, other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ULVZ beneath the Philippine–Kalimantan region.