Thin and intermittent ultralow‐velocity zones

Abstract

[1] 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 ∼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.

Melting at the Edge of a Slab in the Deepest Mantle

“…But, for these anomalies, the Pd portion of SPdKS and SKPdS are tightly clustered on both source and receiver sides of the raypath. Furthermore, the receiver-side Pd segments occur at the western edge of Africa and the western Pacific region where previous evidence has been shown for ULVZs (Idehara et al, 2007;Jensen et al, 2013;Rost et al, 2010). The area to the west of Africa has not been explored (Yu & Garnero, 2018) but is within the African LLSVP and may contain ULVZs.…”

Intermittent and lateral varying ULVZ structure at the northeastern margin of the Pacific LLSVP

“…A number of approaches have been utilized to image ULVZ structure or related phenomena but center on either reflections (or scattering) of energy from the structure that produce additional arrivals or anomalous travel time and/or waveform behavior of the phase of interest due to the structure. Past analyses have included precursors to PcP and ScP which reflect off the top of the ULVZ [ Vidale and Benz , ; Mori and Helmberger , ; Kohler and Vidale , ; Revenaugh and Meyer , ; Garnero and Vidale , ; Castle and van der Hilst , ; Reasoner and Revenaugh , ; Havens and Revenaugh , ; Persh et al ., ; Rost and Revenaugh , , ; Rost et al ., , , , ; Idehara et al ., ; Rost and Thomas , ]; scattering of PKP , PKKP , and SKS waves from ULVZ structural complexities [ Vidale and Hedlin , ; Wen and Helmberger , ; Thomas et al ., ; Stutzmann et al ., ; Wen , ; Ni and Helmberger , ; Niu and Wen , ; Zou et al ., ; Rost and Earle , ; Frost et al ., ]; and travel time and/or waveform anomalies of ScS [ Ni and Helmberger , , ; Wen , ; Simmons and Grand , ; Avants et al ., , ; He et al ., ; Lay et al ., ; He and Wen , ], SPdKS [ Garnero et al ., ; Garnero and Helmberger , , , ; Helmberger et al ., , ; Wen and Helmberger , ; Rondenay and Fischer , ; Thorne and Garnero , ; Thorne et al ., ; Jensen et al ., ], S and Sdiff [ Cottaar and Romanowicz , ], P and Pdiff [ Xu and Koper , ], PcP [ Simmons and Grand , ; Hutko et al ., ], PKP [ Bowers et al ., ; Luo et al ., ], and PKKP [ Rost and Garnero , ].…”

Seismic anisotropy in the lowermost mantle near the Perm Anomaly

“…Normal mode and gravity constraints suggest that the LLSVPs are high‐density structures [ Ishii and Tromp, ], and they may correspond to anomalies in both temperature and composition [e.g., Masters et al, ; McNamara and Zhong, ]. The edges of LLSVPs are often associated with structures known as ultralow velocity zones, which correspond to localized but drastic reductions in seismic velocities [ Thorne and Garnero, ; Rost et al, ], although this relationship has only been investigated in a few regions.…”