Thin and intermittent ultralow-velocity zones

An area of the core-mantle boundary to the east of Australia is investigated for the existence of ultralow-velocity zones (ULVZs). High-frequency recordings of deep Vanuatu and Tonga-Fiji earthquakes are studied from the small-aperture Warramunga Seismic Array in central Australia. The Tonga-Fiji data were used in a previous ULVZ study, while earthquakes from the Vanuatu subduction zone were newly collected for this study. Core-reflected ScP waves were analyzed, which possess observable precursory and postcursory arrivals in the presence of ULVZ structure. We apply a total variation deconvolution algorithm to our data, which significantly sharpens observed signals, hence, increasing our vertical resolution and therefore allowing us to detect thinner ULVZs than previously possible. The minimum ULVZ thickness detection threshold is approximately 2-3 km with this method. This data set samples a spot at the boundary of the large low shear velocity province beneath the Pacific. The new analysis provides evidence for a 5-6 km thick ULVZ to the north of a previously detected 8.5 km thick ULVZ. A second sampled region shows evidence for an even thinner ULVZ structure, with thicknesses of similar to 3 km. These findings are largely consistent with the hypothesis that ULVZs are most likely to be found along the inside margin of large low shear velocity regions that have been attributed to dense, chemically distinct material.