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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]

During effusive volcanic crises, the eruption and propagation of lava flows pose a significant hazard to nearby populations, homes, and infrastructure. Consequently, timely lava flow forecasts are a critical need for volcano observatory and emergency management operations. Previous lava flow modeling tools are typically either too slow to produce timely forecasts, or are fast, but lack critical aspects of lava physics or important forecasting outputs. In particular, the strong thermal stratification present in laminar, high‐Prandtl number flows has generally been neglected. Bulk rheological changes have previously been computed from cell‐averaged temperatures, assuming that the flow is thermally mixed. Here, we detail the development and initial testing of Lava2d, a new two‐dimensional depth‐averaged finite volume model of lava flow propagation over natural terrain which accounts for bulk rheological changes due to thermorheological stratification. We use a novel approach to energy conservation based on tracking cooling and solidifying at the flow base and at the moving flow surface, allowing for the estimation of more realistic vertical thermorheological profiles, while maintaining computational efficiency, producing very timely model runs. We validate our approach with three examples: comparison with theoretical propagation of crust‐dominated lava flows, comparison with a large‐scale molten basalt experiment from the Syracuse University Lava Project, and efficiency testing and comparison with the initial phase of the 1984 Mauna Loa lava flows. Our model is shown to produce rapid, realistic forecasts, making it a good candidate for operationalization in active volcanic regions such as in Hawai'i. Plain Language Summary: During volcanic eruptions, lava flows pose a significant hazard to nearby populations, homes, and infrastructure. During lava flow crises, forecasts of lava flow advance, coverage, and arrival time can significantly help volcano observatories and emergency managers perform their operations. The most realistic forecasts are made from simulations of lava flows, which include a model of how lava flows cool and solidify as they travel. Although some models use realistic cooling, they are generally too slow to be used in emergencies. Most previous lava flow models have instead estimated this cooling by assuming that temperature within a lava flow does not vary significantly with depth; however, this assumption is not realistic. Real lavas are much cooler at the surface and base than in their cores. This difference has a large impact on the lava's overall speed. Here, we show the development of Lava2d, a new model of lava flow which directly incorporates these temperature differences. We test our model using theory, experiments, and comparison with a large natural lava flow, showing that our model produces realistic simulations very quickly. Key Points: Strong thermorheological stratification significantly impacts lava flow emplacement and coolingWe have developed a new depth‐averaged finite volume lava flow model (Lava2d) incorporating this stratificationThis model is capable of producing realistic inundation and advance rate forecasts on operational timescales [ABSTRACT FROM AUTHOR]