Localized ultra-low velocity zones at the eastern boundary of Pacific LLSVP

Abstract

It is suggested that ultra-low velocity zones (ULVZs) are often located near the boundary of large low shear velocity zones (LLSVPs). Previous studies have provided evidence of ULVZs residing along the western and northern boundary of the Pacific LLSVP. However, so far there is no detailed report on ULVZs along the eastern boundary of the Pacific LLSVP. In this study, we collected PKP precursor waveforms from earthquakes in South America that are recorded by seismic arrays in Australia. These precursors show clear and coherent phases. Slowness and migration analysis of these precursors indicate that they originate from scatterers in the lowermost mantle east of the Pacific LLSVP. Waveform modeling suggests that these seismic scatterers are localized ULVZs, with P-wave velocity reductions of ∼3–10% and thickness of several tens of kilometers. The dimension of the ULVZs and the geological correlation of the ULVZs with the Galapagos hotspot, suggest that these ULVZs are mainly due to compositionally distinct heterogeneities, although partial melting cannot be completely ruled out. The distribution of both coherent and incoherent PKP precursors also suggests vigorous small-scale mantle convections near the edges of the LLSVP.