Detection of a ULVZ in the Central Pacific using high frequency Sdiff postcursors

Abstract

In recent decades, ultra-low velocity zones (ULVZs) – thin patches of strongly reduced seismic velocity atop the core-mantle boundary (CMB) – have been inferred from observations of a variety of seismic phases. One such phase is Sdiff and its out-of-plane postcursory energy, Sdiff+. In this study, we present a high quality dataset of Sdiff and Sdiff+ which shows evidence of a ULVZ in the central Pacific, roughly between Hawaii and Marquesas. The observed Sdiff+ have an unusually short dominant period (5–12 seconds) compared to previous Sdiff+ observations, which is indicative of a ULVZ that is approximately 10–15 km thick. We analyse this high frequency dataset using the 2D Wavefront Tracker (2DWT) Bayesian inversion methodology to generate a probabilistic ensemble of ULVZ models. As a result of the uniazimuthal coverage of data, there is a strong southwest-northeast trade-off, but there is a slight preference for the ULVZ to be located just outside the Pacific large low velocity province (LLVP) boundary, roughly centred 20° north of Marquesas and 20° southeast of Hawaii. The 2DWT inversion of Sdiff+ travel times suggest that the ULVZ can be approximated as a cylindrical structure; either as a larger and weaker anomaly (radius 280 km, dVs -20%) close to the LLVP, or a smaller and stronger anomaly (radius 180 km, dVs -30%) further away from the LLVP. Unlike previous broad-scale ULVZs modelled with Sdiff+, this ULVZ is thinner, might lie outside of an LLVP, and lacks a potential relationship to a mantle plume.