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

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. Plain Language Summary Modern seismometers can record not only the ground shaking produced by earthquakes, but also the ground vibrations produced by ocean waves and magma motions. Although these small vibrations appear to be noise, they carry information about the underground structures. Because ground vibrations are dominated by waves propagating along the Earth's surface, it has been difficult to observe body waves, which propagate in the interior of the Earth, from these small vibrations, especially in volcanic regions. In this study, we found that body waves can be extracted from ambient vibrations only after the 2018 Kı̄lauea eruption on the Island of Hawai'i, where tremors caused by the magma and/or hydrothermal activities masked the body wave signals before the eruption and the volcano became quieter afterward. The body wave signals include the direct arrival and those reflected off wave speed boundaries at different depth beneath the island. The new signals could be useful to monitor magmatic activities and study the structures of the Island of Hawai'i. Additionally, the study emphasizes the importance of conducting seismic observations after volcano eruptions. Key Points Direct and post‐critically reflected P waves in ambient noise cross‐correlation on Hawai'i are visible only after the 2018 Kı̄lauea eruption A strong noise source near the Kı̄lauea summit contaminated the ambient noise cross‐correlation before its 2018 eruption The post‐critically reflected P waves reveal two middle crust velocity discontinuiti