A high-resolution map of Hawaiian ULVZ morphology from ScS phases

Abstract

We use core reflected ScS waves sensitive to a broad region of the core-mantle boundary beneath Hawaii to create the first high-resolution map of the Hawaiian ultralow-velocity zone (ULVZ). Positive ScS-S differential times are used to identify regions of strong slow velocity anomalies in the lowermost mantle, and the presence of pre/post-cursors around the main ScS phase confirms the sharp top of a basal ULVZ layer. Pre/post-cursor arrivals are mapped into a volume across their region of sensitivity to produce a detailed image of ULVZ morphology. The variability observed across the ULVZ can be interpreted in terms of varying height or velocity reduction, but the large range of velocity variations required to explain observations suggests that most variability reflects varying layer thickness. The Hawaiian ULVZ is observed to be a large-scale regional feature of varying height (∼5-25 km) extending across a wide area along the edge of the Pacific large low-velocity province (LLVP). Variability in previous models of the Hawaiian ULVZ can be explained by studies imaging different parts of this strongly variable, large-scale feature. Maximum ULVZ thicknesses (no taller than 30 km) are found in a flat-topped, steep-sided region on the order of 1000 km in diameter located west of Hawaii. This feature is coincident with the previously identified Hawaiian mega-ULVZ, and is interpreted to represent the root of the Hawaiian plume, which is offset from the volcanic surface expression. A tall, asymmetric ridge of ULVZ material is observed to the east of Hawaii. Both regions of maximum ULVZ heights are bounded by the Pacific LLVP to the south, and fast seismic velocity anomalies interpreted as slab remnants to the north. This points to a potential geodynamical scenario where dense ULVZ material is pushed by subducted slab remnants into thicker piles against LLVP margins.