P Waves Emerged From Ambient Noise Cross-Correlation Post the 2018 Kı̄lauea Eruption Revealing Middle Crust Velocity Discontinuities Beneath the Island of Hawai'i

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Empirical Green Functions (EGFs) obtained from ambient noise cross-correlation are important for imaging and monitoring underground structures. The EGFs on the Island of Hawai'i in different years are similar at low frequencies (0.1–0.4 Hz), but very different at high frequencies (0.4–1.0 Hz): Only the EGFs after the 2018 Kı̄lauea eruption show clear P waves. Grid search reveals a strong noise source near the Kı̄lauea summit before the eruption, which contaminated the EGFs but became silent after the eruption. Modeling of the P waves identifies the direct arrival and post-critical reflections from two velocity discontinuities at 4.7 and 7.2 km depth beneath the island, which we interpret as the base of volcanic edifices and deposits and the boundary between basaltic dikes and gabbros, respectively. The P waves in EGFs could provide valuable high-resolution constraints for monitoring deep magmatic changes and imaging the volcano structures.

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Geophysical Research Letters