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Seismic tomography of shield volcanoes can be used to better understand its structure, formation, and evolution. Previous tomographic studies on the Island of Hawai'i used body waves from earthquakes and active sources and had limited resolution in the shallow crust. In this study, we obtained the empirical Green Functions (EGFs) and empirical Green Tensors (EGTs) from cross‐correlating and stacking of multiyear seismic ambient noise recorded on the island. The EGFs/EGTs contained fundamental mode and first higher mode Rayleigh waves. The different modes were separated with a new algorithm and their group velocities were measured. Using the group arrival times, we inverted for two‐dimensional group velocity maps, which provide, for the first time, a full coverage of the Island of Hawai'i. From the group velocity maps, we inverted for a three‐dimensional shear wave velocity model, which shows strong lateral variations and yields new insights into the structure and growth of the volcanoes on the island: Kı̄lauea's East Rift Zone has prominent high velocities at all depths, whereas the current rift zones of Mauna Loa are characterized by intermediate to high velocities only at depths greater than 1 km below ground surface, which may be attributed to their relatively short history and less developed state. The flanks of the volcanoes, some cut by fault zones, displayed low velocities at over a range of depths, generally interpreted as consisting of extrusive rocks, which could be further shattered by faulting. Plain Language Summary: Better imaging the structures of the shield volcanoes on the Island of Hawai'i is important, not only for understanding their evolution, but also for forecasting and mitigating future volcanic hazards. In this study, we used the waves propagating along the Earth's surface to image the shallow structures of the entire Island of Hawai'i. Our velocity model shows that Kı̄lauea's East Rift Zone has high‐velocities, compared with the intermediate to high‐velocities of Mauna Loa's rift zones, which suggests the existence of large intrusive rock cores in Kı̄lauea's East Rift Zone. Multiple low‐velocity anomalies are imaged on the volcano flanks, interpreted as consisting of extrusive volcanic rocks. The low velocities on the flanks co‐located with the fault zones are interpreted as potentially further fractured debris produced by faulting. Overall, the new velocity model opens a window into the shallow volcano‐tectonic structures on the Island of Hawai'i, enabling a better understanding of the formation and structure of the Hawaiian shield volcanoes as well as the propagation of seismic waves in the shallow crust. Key Points: First overtone Rayleigh wave, in addition to the fundamental mode, emerged from ambient noise cross‐correlation on the Island of Hawai'iAn algorithm was developed and applied to the data set to automatically separate wave modes and pick group velocity dispersion curvesThe resulting three‐dimensional shear wave velocity model reveals distinct structures of the rift zones, fault zones, and volcano flanks [ABSTRACT FROM AUTHOR]

Liu et al. (2022, https://doi.org/10.1029/2021GL093691) used Rayleigh waves extracted from the cross‐correlation of ambient noise recorded by two stations to monitor the seismic velocity variations associated with the 2018 Kı̄lauea eruption. However, their study ignored the fact that the tremors on the Island of Hawai'i were dominated by a source at the Kı̄lauea summit before the eruption. Close inspection of the waveforms of the station pair PAUD‐STCD shows a simple, mistakenly identified wave traveling direction in Liu et al. (2022, https://doi.org/10.1029/2021GL093691). A correct wave traveling direction agrees with the noise source model, where the dominant tremor source should be at the Kı̄lauea summit. Because of the drastic change in the tremor source after the eruption, the cross‐correlation of the tremor records may reflect predominantly changes in the source rather than in the medium properties between the two stations. Plain Language Summary: More and more studies are using seismic records to understand the volcano eruption process. A very recent study monitored the dyke intrusion near the Pu'u'ō'ō right after the start of the 2018 Kı̄lauea eruption, by observing increased seismic wave arrival times. However, after re‐inspecting the data, we find their study understood the source location emitting the seismic waves wrong. Thus, their observed increased seismic wave arrival times can no longer be explained by the proposed dyke intrusion in the rift zone. We further suspect that the temporal variations of the seismic source contributed greatly to the arrival time variations. Key Points: The tremor signals appeared on the ambient noise cross‐correlation functions (CCFs) should be from the Kı̄lauea summit, not Pu'u'ō'ōThe tremor sources changed after the beginning of the eruptionThe apparent temporal variations of the CCFs related to the tremors reflected the change of noise source rather than medium velocity [ABSTRACT FROM AUTHOR]

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 eruptionA strong noise source near the Kı̄lauea summit contaminated the ambient noise cross‐correlation before its 2018 eruptionThe post‐critically reflected P waves reveal two middle crust velocity discontinuities at about 4.7 and 7.2 km beneath Hawai'i [ABSTRACT FROM AUTHOR]

The Island of Hawai'i was formed by repeated eruptions of basalts at an oceanic hotspot. Kı̄lauea, the youngest among the subaerial volcanoes of the island, erupted intensely in 2018. The eruption provided an opportunity to look into the mechanisms that operate at the volcano and associated earthquake activities, as it was recorded simultaneously, for the first time, by onshore and offshore seismometers. We used most of the publicly available seismic data during the eruption period, including temporary arrays, to build a more complete earthquake catalog during the eruption than that provided by the Hawaiian Volcano Observatory. We used a short‐time‐average/long‐time‐average method to identify potential earthquakes. The detections were associated with events and automatically picked with P‐wave and S‐wave arrivals, which were used to locate the events in a three‐dimensional velocity model. After re‐examining these earthquake events, their coda/duration magnitudes were determined. The resulting half‐year catalog contains 375,736 events with one of the highest daily earthquake numbers ever reported (6,128 on 21 June 2018). A great number of events were recorded during the caldera collapses, from its beginning until its rapid ending. The catalog also contains abundant events near the Pu'u'ō'ō vent and in the lower East Rift Zone, where an increase of seismicity in mid‐July and August indicated a step‐up in magma intrusion after the eruption. Key Points: Using most available data onshore and offshore, we developed a workflow to detect and locate earthquakes during the 2018 Kı̄lauea eruptionThe new catalog contains a total of 375,736 earthquakes between 1 March and 30 September 2018, 439% more than reported by the Hawaiian Volcano ObservatoryThe new catalog provides a foundational data set, revealing rich information to better understand the eruption processes [ABSTRACT FROM AUTHOR]