Shallow Volcano‐Tectonic Structures on the Island of Hawai'i Imaged by Multimode Rayleigh Wave Ambient Noise Tomography.

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]