Ultralow Velocity Zones at the Core-Mantle Boundary Near the Caroline Hotspot

Abstract

Ultralow velocity zones (ULVZs) at the core-mantle boundary (CMB), especially beneath the surface hotspots, are essential for understanding the nature of mantle convection and thermochemical evolution in the deep Earth. Here, we utilize tangential S and ScS recordings at the F-net in Japan from deep earthquakes in the New Britain and Kermadec–Tonga subduction zones to investigate ULVZs at the CMB beneath regions near the Caroline hotspot. A signal-enhancement approach known as the Flip-Reverse-Stacking (FRS) method, which focuses on the precursor (SdS) and reverberation (ScScS) of the main ScS phase caused by the ULVZ, is applied to resolve the detailed ULVZ structures. Synthetics derived from 1-D low-velocity models are compared to observations to obtain the thickness and S-wave velocity reductions of low-velocity zones (LVZs) across our study regions. The results show that LVZs with complex topographic undulations are widely distributed at the CMB near the Caroline hotspot. In particular, robust ULVZs are detected both inside and at the western edges of the Pacific Large Low-Velocity Province (LLVP). Additionally, a local elevated thermal boundary layer within the Pacific LLVP, with a lateral extent of ∼240 × 120 km, is revealed by the large ScS-S differential travel time delays. These observations indicate significant local thermal and chemical variations at the base of the mantle, which are likely caused by internal convection within the Pacific LLVP.