Fine‐scale ultralow‐velocity zone structure from high‐frequency seismic array data

Abstract

Variability of core‐mantle boundary (CMB) structure is imaged at the kilometer to tens of kilometers scale by intensive waveform modeling of the ScP phase. Recordings of earthquakes in the Tonga‐Fiji subduction zone from the small‐aperture (10 km), high‐frequency (∼2–4 Hz) Alice Springs Array in central Australia are used to document anomalous ScP waveform variations due to the presence of an ultralow‐velocity zone (ULVZ) at the CMB. These data expand coverage from previous results using the Warramunga Array, which is located slightly to the north, and indicate the existence of a geographically confined ULVZ at the CMB, roughly 100 km in lateral extent, south of New Caledonia and east of Australia. Synthetic modeling indicates seismic velocity reductions of ∼8% and ∼24% for P wave and S waves, respectively, and a well‐constrained density increase of ∼10% in a ∼8.5 km thick layer above the CMB. This ULVZ patch is well explained by an elevated iron content of partially molten mantle material, likely present as divalent oxidized iron in the low spin state: The properties of such an iron‐enriched assemblage are consistent with both an elevated density and bulk modulus of this region, as are dictated from our best fit seismic results. Furthermore, a positive velocity gradient within the ULVZ may be present, which need not be associated with a decrease in melt content within this region but could rather be associated with a shift in the textural distribution of melt within the ULVZ.