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Sequencing Postcursors of P and S Core-diffracted Waves: Implications for the Hawaiian Mega-ULVZ Properties

Description: 

Mantle plumes beneath major oceanic hotspots appear to be rooted in unusually large structures near the core-mantle boundary, which have dramatically reduced seismic wavespeeds. The origin of these large ultralow velocity zones, referred to as mega-ULVZs, remains controversial partly due to lack of constraints on the relative reduction in shear vs. compressional wavespeeds (i.e. the δlnVs/δlnVp ratio). This ratio can give clues into the compositional makeup of the mega-ULVZs. Here, we conduct a joint analysis of core-diffracted P- and S-waves (Pdiff and Sdiff) to infer the δlnVs/δlnVp ratio necessary to explain new seismic observations of Pdiff and Sdiff postcursors produced by wavefield interactions with the Hawaiian mega-ULVZ. Using three-dimensional wavefield simulations of various mega-ULVZ models, we model delay times and amplitudes of postscursor signals for both Pdiff and Sdiff. We find that the amplitude of Pdiff postcursors is dramatically lower relative to Sdiff postcursors due to wavefront healing and differences in wavelength. We constrain δlnVs/δlnVp ratios for the Hawaiian mega-ULVZ to be in the 1:1.3-2 range, corroborating previously reported values based on analysis of higher frequency waves. This inferred range is best explained by the enrichment of solid iron-rich (Mg,Fe)O, with no requirement for partial melting based on our accompanying mineral physics modeling. However, we also show that inferred δlnVs/δlnVp ratios depend strongly on the assumed volume of the mega-ULVZ, which needs to be better constrained by complementary seismic datasets.

Session: Earth’s Structure from the Crust to the Core - IV

Type: Oral

Date: 4/16/2025

Presentation Time: 04:45 PM (local time)

Presenting Author: Doyeon

Student Presenter: No

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