Open‐vent volcanic systems with active degassing are particularly effective at producing infrasound that exhibits resonant tones controlled by the geometry of the volcano's crater. Changes in the infrasound character can thus provide constraints on a crater's lava level, which may vary dynamically in the lead‐up to an eruption. Here we show that the increasing frequency content and damping characteristics of the resonant infrasound at Volcán Villarrica (Chile) relate to lava lake position in its crater/conduit preceding its 2015 eruption. We model the acoustic response of Villarrica's crater to determine that the lake began to rise on 27 February and reached the flared upper part of Villarrica's crater before oscillating during the two days prior to the 3 March paroxysm and 1.5 km‐high lava fountain. This study demonstrates the utility of remote infrasound monitoring for future eruptions of Villarrica and other analogous open‐vent volcanoes.

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We characterize and interpret a new type of infrasound signal originating from the summit of Volcán Cotopaxi (Ecuador) that was primarily observed between September 2015 and March 2016, following the 2015 eruptive period. This infrasound waveform is a slowly decaying sinusoid with exceptional low‐frequency (fp = 0.2 Hz) and high quality factor (Q = ~10) and resembles the shape of tornillo seismic waveforms. The repeating events, occurring about once per day in early 2016, are stable in frequency content, and we attribute them to excitation of a vertical‐walled crater, with radius of about 125 m and length of 300 m. Spectral properties of the tornillo permit constraints on crater sound speed (335 m/s ± 6%) and temperature (4–32°C). The initial polarity of the tornillos is predominantly a rarefaction and could reflect repeating crater bottom collapse events (implosions) or explosion sources whose infrasound is heavily modulated by the crater's pipe‐like geometry.

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In December 2018, Mount Etna (Italy) experienced a period of increased eruptive activity that culminated in a fissure eruption on the southeast flank. After the onset of the flank eruption, the peak frequency of the summit infrasound signals decreased while resonance increased. We invert infrasound observations for crater geometry and show that crater depth and radius increased during the eruption, which suggests that the flank eruption drained magma from the summit and that eruptive activity led to erosion of the crater wall. By inverting the entire infrasound amplitude spectra rather than just the peak frequency, we are able to place additional constraints on the crater geometry and invert for, rather than assume, the crater shape. This work illustrates how harmonic infrasound observations can be used to obtain hightemporal-resolution information about crater geometry and can place constraints on complex processes occurring in the inaccessible crater region during eruptive activity. Plain Language Summary Volcanoes generate low-frequency sound waves in the atmosphere (infrasound) that can be recorded by specialized microphones. Much like giant musical instruments, the character of the sound can depend upon the shape and size of the crater. Mount Etna erupted in December 2018 with lava flowing out a fissure on the flank of the volcano. The character of the sound changed after the flank eruption. We study the change in the character of the sound in order to estimate how the shape of the volcanic crater at the summit of Mount Etna changed. Our results show that the crater got deeper and wider, which suggests that the eruption of lava on the flank of the volcano drained magma from the summit area. This work shows how infrasound observations can track changes in crater geometry and provide insight about the magma plumbing systems beneath volcanoes.

Most crustal rocks present some amount of directional dependence of seismic speeds, particularly mudstones (also termed "shales"). Hence, accurate imaging of the subsurface requires anisotropic models and integration of rock physics information in order to constrain the inherent nonuniqueness of the inversion from seismic data. Resonant ultrasound spectroscopy provides estimates of the full anisotropic stiffness tensor from resonant frequencies of geological core samples, along with a measure of intrinsic attenuation. We have developed new functionality to existing codes which enable horizontal transversely isotropic samples to be analyzed. We compared and discussed estimations of the elastic properties of mudstone samples with hexagonal symmetry; one sample was drilled parallel and one perpendicular to the layering. While spatial heterogeneity in the mudstone prevented a direct correlation of the elastic parameters of each sample, time-of-flight measurements reveal frequency dispersion of the elastic parameters that is consistent between the samples.

New Zealand experienced a wave of the Omicron variant of SARS-CoV-2 in early 2022, which occurred against a backdrop of high two-dose vaccination rates, ongoing roll-out of boosters and paediatric doses, and negligible levels of prior infection. New Omicron subvariants have subsequently emerged with a significant growth advantage over the previously dominant BA.2. We investigated a mathematical model that included waning of vaccine-derived and infection-derived immunity, as well as the impact of the BA.5 subvariant which began spreading in New Zealand in May 2022. The model was used to provide scenarios to the New Zealand Government with differing levels of BA.5 growth advantage, helping to inform policy response and healthcare system preparedness during the winter period. In all scenarios investigated, the projected peak in new infections during the BA.5 wave was smaller than in the first Omicron wave in March 2022. However, results indicated that the peak hospital occupancy was likely to be higher than in March 2022, primarily due to a shift in the age distribution of infections to older groups. We compare model results with subsequent epidemiological data and show that the model provided a good projection of cases, hospitalizations and deaths during the BA.5 wave.

Over the past two decades (2000–2020), volcano infrasound (acoustic waves with frequencies less than 20 Hz propagating in the atmosphere) has evolved from an area of academic research to a useful monitoring tool. As a result, infrasound is routinely used by volcano observatories around the world to detect, locate, and characterize volcanic activity. It is particularly useful in confirming subaerial activity and monitoring remote eruptions, and it has shown promise in forecasting paroxysmal activity at open-vent systems. Fundamental research on volcano infrasound is providing substantial new insights on eruption dynamics and volcanic processes and will continue to do so over the next decade. The increased availability of infrasound sensors will expand observations of varied eruption styles, and the associated increase in data volume will make machine learning workflows more feasible. More sophisticated modeling will be applied to examine infrasound source and propagation effects from local to global distances, leading to improved infrasound-derived estimates of eruption properties. Future work will use infrasound to detect, locate, and characterize moving flows, such as pyroclastic density currents, lahars, rockfalls, lava flows, and avalanches. Infrasound observations will be further integrated with other data streams, such as seismic, ground- and satellite-based thermal and visual imagery, geodetic, lightning, and gas data. The volcano infrasound community should continue efforts to make data and codes accessible and to improve diversity, equity, and inclusion in the field. In summary, the next decade of volcano infrasound research will continue to advance our understanding of complex volcano processes through increased data availability, sensor technologies, enhanced modeling capabilities, and novel data analysis methods that will improve hazard detection and mitigation.