Lowermost Mantle Structure Beneath the Central Pacific Ocean: Ultralow Velocity Zones and Seismic Anisotropy

Ultralow velocity zones (ULVZs) and seismic anisotropy are both commonly detected in the lowermost mantle at the edges of the two antipodal large low velocity provinces (LLVPs). The preferential occurrences of both ULVZs and anisotropy at LLVP edges are potentially connected to deep mantle dynamics; however, the two phenomena are typically investigated separately. Here we use waveforms from three deep earthquakes to jointly investigate ULVZ structure and lowermost mantle anisotropy near an edge of the Pacific LLVP to the southeast of Hawaii. We model global wave propagation through candidate lowermost mantle structures using AxiSEM3D. Two structures that cause ULVZ-characteristic postcursors in our data are identified and are modeled as cylindrical ULVZs with radii of similar to 1 degrees and similar to 3 degrees and velocity reductions of similar to 36% and similar to 20%. One of these features has not been detected before. The ULVZs are located to the south of Hawaii and are part of the previously detected complex low velocity structure at the base of the mantle in our study region. The waveforms also reveal that, to first order, the base of the mantle in our study region is a broad and thin region of modestly low velocities. Measurements of Sdiff shear wave splitting reveal evidence for lowermost mantle anisotropy that is approximately co-located with ULVZ material. Our measurements of co-located anisotropy and ULVZ material suggest plausible geodynamic scenarios for flow in the deep mantle near the Pacific LLVP edge. Plain Language Summary Earthquakes cause different types of seismic waves that can be used to create an image of seismically fast and slow regions within Earth's interior. Two large-scale features with relatively low seismic velocities have been identified at the base of the mantle, one beneath Africa and one beneath the Pacific Ocean, known as large low velocity provinces (LLVPs). Small-scale, thin features with extremely low velocities, known as ultralow velocity zones (ULVZs), have previously been detected just above the core-mantle boundary, often located at the edges of the LLVPs. In this study, we investigate a region of the deep mantle at the edge of the Pacific LLVP. We use recordings of earthquake waves that have sampled this region to map two distinct ULVZ regions at this boundary. We also investigate a property known as seismic anisotropy, the directional dependence of seismic wave speeds, which can be used to infer the direction of mantle flow. We outline several potential mantle flow scenarios that are consistent with our data, helping to understand flow processes at the edges of LLVP structures in the deep mantle.