We present observations of a gravity wave (GW) event over the Netherlands and Belgium in the night and early morning of 30 June 2022. This event was captured by instruments that are part of the meteorological observational network of The Netherlands and Belgium, including the operational weather radars and automatic lidar ceilometers, and by the (tower) in situ and Doppler lidar at the Cabauw atmospheric research station. Together they provide 3D information on the GW event, with detailed information on the vertical profiles from Cabauw, and the spatial extent and evolution from the observational synoptic network. Two distinct GW trains were observed, both interpreted as ducted GW that are trapped in the lowermost 500 m of the stable nocturnal boundary layer. The GWs showed large vertical velocity amplitudes up to 3 m/s, resulting in strong modulations of wind, temperature, humidity and pressure. By comparing our observations to the Copernicus European Regional ReAnalysis (CERRA) wind and temperature fields, we interpret that these GWs were generated by convergence lines over Belgium and the North Sea that was linked to an occlusion front.

Mitigation of the threat from airbursting asteroids requires an understanding of the potential risk they pose for the ground. How asteroids release their kinetic energy in the atmosphere is not well understood due to the rarity of large impacts. Here we present a comprehensive, space-to-laboratory characterization of an impact of an L chondrite, which represents a common type of Earth-impacting asteroid. Small asteroid 2023 CX1 was detected in space and predicted to impact over Normandy, France, on 13 February 2023. Observations from several independent sensors and reduction techniques revealed an unusual but potentially high-risk fragmentation behaviour. The nearly spherical 650 +/- 160 kg (72 +/- 6 cm diameter) asteroid catastrophically fragmented at a dynamic pressure of 4 MPa around 28 km altitude, releasing 98% of its total energy in a concentrated region of the atmosphere. The resulting shock wave was spherical, not cylindrical, and released more energy closer to the ground. This type of fragmentation increases the risk of substantial damage at ground level. These results warrant consideration for a planetary defence strategy for cases where a >3-4 MPa dynamic pressure is expected, including planning for evacuation of areas beneath anticipated disruption locations.

Eruptions at continental basaltic volcanoes can take and combine various forms, including lava lakes, lava flows and fountaining, explosions or structural collapses. Aside from a few well-instrumented cases worldwide, accurately reconstructing eruptive scenarios is hampered by the lack of detailed visual observations. However, volcanoes also have their own acoustic signatures composed of low-pitched inaudible sounds, called infrasounds. Here we analyze infrasound records close to Nyiragongo volcano (D.R. Congo) ( <20 km) up to Kenya ( ~800 km), which are converted using acoustic numerical modeling into time-lapse observations of the catastrophic drainage of the world’s largest lava lake on 22 May 2021. The emitted infrasounds also enable tracking of fissure openings and lava eruptions along the flank, occurring simultaneously with the lava lake drainage. This striking example supports the growing role of infrasound as a key component of volcano monitoring and early-warning systems, as it provides unique information inaccessible to other ground-based instruments. Nyiragongo volcano emits infrasound, a low-pitched, inaudible sound that enables tracking of fissure openings and lava eruptions along the flank, co-occurring with the lava lake drainage, according to an experiment that uses acoustic modeling and source scanning.

The presence of turbulence and other wind-induced pressure fluctuations are considered a nuisance in infrasound monitoring. For this reason, wind noise filters are typically in place for suppression. In this study, we establish a relation between microbarometer observations and in situ turbulence measurements at the Cabauw Atmospheric Research Site in The Netherlands. Using this relation, we forecast turbulent pressure levels using forecasted levels of turbulence kinetic energy (TKE) that are computed as part of the high-resolution (2.5 × 2.5 km grid scale) HARMONIE weather model. The estimates are compared with pressure noise levels as a function of frequency. This approach has two foreseen applications: (1) the forecasted turbulence fields could possibly help in site selection for infrasound arrays and (2) microbarometer observations could possibly be of use in the further refining of sub-grid scale turbulence schemes in weather models.

The atmosphere is changing on a wide variety of timescales. The infrasound component of the International Monitoring System (IMS ) can sense such changes, each second over periods of tens of years. Such long term measurements of atmospheric variability enable the study of climate change. Infrasonic waves passively probe the entire atmosphere. The challenge is to unravel temperature variability (long term increases and decreases) from surface based recordings. As a reference, seismic signals are used which remain unchanged as a function of time. A so-called seismoacoustic analysis uses both seismic and infrasonic signals, where changes in the recordings can be attributed to changes in the medium. The latter being temperature changes in the troposphere and stratosphere.To illustrate the contribution of the IMS to climate studies, over 15 years of IMS seismic and infrasonic recordings will be shown. The seismoacoustic analysis performed with the recordings reveals long term changes in atmospheric temperature. Simultaneously a temperature increase in the troposphere and decrease in the stratosphere can be passively sensed.

Climate change is increasingly predisposing polar regions to large landslides. Tsunamigenic landslides have occurred recently in Greenland ( Kalaallit Nunaat ), but none have been reported from the eastern fjords. In September 2023, we detected the start of a 9-day-long, global 10.88-millihertz (92-second) monochromatic very-long-period (VLP) seismic signal, originating from East Greenland. In this study, we demonstrate how this event started with a glacial thinning–induced rock-ice avalanche of 25 × 10 6 cubic meters plunging into Dickson Fjord, triggering a 200-meter-high tsunami. Simulations show that the tsunami stabilized into a 7-meter-high long-duration seiche with a frequency (11.45 millihertz) and slow amplitude decay that were nearly identical to the seismic signal. An oscillating, fjord-transverse single force with a maximum amplitude of 5 × 10 11 newtons reproduced the seismic amplitudes and their radiation pattern relative to the fjord, demonstrating how a seiche directly caused the 9-day-long seismic signal. Our findings highlight how climate change is causing cascading, hazardous feedbacks between the cryosphere, hydrosphere, and lithosphere.

The electrical network frequency (ENF) of the alternating current operated on the power grid is a well-known source of noise in digital recordings. The noise is widespread and appears not just in close proximity to high-voltage power lines, but also in instruments simply connected to the mains powers grid. This omnipresent, anthropogenic signal is generally perceived as a nuisance in the processing of geophysical data. Research has therefore been mainly focused on its elimination from data, while its benefits have gone largely unexplored. It is shown that mHz fluctuations in the nominal ENF (50-60 Hz) induced by variations in power usage can be accurately extracted from geophysical data. This information represents a persistent time-calibration signal that is coherent between instruments over national scales. Cross-correlation of reliable reference ENF data published by electrical grid operators with estimated ENF data from geophysical recordings allows timing errors to be resolved at the 1 s level. Furthermore, it is shown that a polarization analysis of particle motion at the ENF can detect instrument orientation anomalies. While the source of the ENF signal in geophysical data appears instrument and site specific, its general utility in the detection of timing and orientation anomalies is presented.

In August 2021, Askja volcano in Iceland returned to the spotlight after a sudden onset of rapid uplift followed decades of continuous subsidence. In this study the extended record of microgravity data from Askja between 1988 to 2017 is revisited, and new microgravity data from 2021 and 2022 are introduced, which were collected after the uplift had started. Askja caldera had been steadily subsiding since at least 1984 and was characterised by a net decrease in microgravity, potentially signalling the contraction of its magma chamber or eviction of magma either laterally or to deeper levels. The microgravity data indicate that despite ongoing subsidence between 2017 and early 2021, a significant gravity increase can be detected in the center of the caldera between 2017 and August 2021. This increase may be introduced during - or leading up to - the period of uplift. The new microgravity data also indicate that during the period of 40 cm uplift after August 2021 to fall 2022, gravity changes approach the free-air gradient, suggesting subsurface density decreases as a driving process. This process may relate to the vesiculation of magma previously emplaced in the volcano roots, a change in the hydrothermal system, or replacement of dense basaltic magma with less dense rhyolitic magma, or a combination of these processes. However, uncertainties for this period are elevated and may obscure a gravity signal expected from additional mass accumulation. The timing and high uncertainties of some campaigns make it challenging to be conclusive on the driving process behind the uplift, but future microgravity campaigns could help solve the ambiguity. The study also provides a description of potential pitfalls in microgravity campaigns and recommendations on how the reliability of microgravity data can be improved.

In August 2021, Askja volcano in Iceland experienced a sudden onset of rapid uplift that followed decades of continuous subsidence. We review the extensive microgravity record from Askja, revisiting data recorded between 1988 and 2017, and presenting new microgravity data from 2021 and 2022 that were collected after the uplift had started. Based on our findings, we provide a comprehensive set of recommendations that should be followed for optimal microgravity data collection and treatment in volcano monitoring. Without such standards, it becomes increasingly challenging to interpret the microgravity results in terms of volcanic processes. At Askja, from 1988 – 2016, exponentially decaying surface subsidence was accompanied by a microgravity decrease, potentially signaling the contraction of its magma chamber, eviction of magma to deeper levels, or other density-decreasing processes. Following this, between 2016 and 2021, a gravity increase occurred in the center of the caldera which effectively annuls the microgravity decrease detected between 1988 and 2016. This increase took place either during subsidence or leading up to and during the uplift, and may potentially be explained by mass accumulation below the caldera. After August 2021, gravity changes follow the free-air gradient, despite continuing deformation with a total uplift of up to 40 cm, suggesting subsurface density decreases as a driving process. Such a process could be envisaged as the previously emplaced intrusion before 2021 that is now undergoing magma vesiculation, is causing a change in the hydrothermal system, or represents the replacement of dense basaltic magma with less dense rhyolitic magma. However, uncertainties for these data are large (50μGal) and small mass intrusions contributing to the uplift may remain undetected. The driving mechanism for the uplift remains enigmatic and future microgravity campaigns will help shed light on its nature.

Results from nine microgravity campaigns from Kı̄lauea, Hawaiʻi, spanning most of the volcano's 2008–2018 summit eruption, indicate persistent mass accumulation at shallow levels. A weighted least squares approach is used to recover microgravity results from a network of benchmarks around Kı̄lauea's summit, eliminate instrumental drift, and restore suspected data tares. A total mass of 1.9 × 10 11 kg was determined from these microgravity campaigns to have accumulated below Kı̄lauea Caldera during 2009–2015 at an estimated depth of 1.3 km below sea level. Only a fraction of this mass is reflected in surface deformation, and this is consistent with previously reported discrepancies between subsurface mass accumulation and observed surface deformation. The discrepancy, amongst other independent evidence from gas emissions, seismicity, and continuous gravimetry, indicate densification of magma in the reservoirs below the volcano summit. This densification may have been driven by degassing through the summit vent. It is hypothesized that during the final years of the summit eruption, magma densification resulted in a buildup of pressure in the reservoirs that may have contributed to the lower East Rift Zone outbreak of 2018. The observed mass accumulation beneath Kı̄lauea could not have been detected through other techniques and illustrates the importance of microgravity measurements in volcano monitoring.

We report on the extraction of deep ocean travel time variations from time-lapse cross-correlations between a hydrophone station and a three-component broadband seismometer. The signals we cross-correlate in this study result from repeated activity by the Monowai seamount, one of the most active submarine volcanoes of the Tonga-Kermadec ridge. In particular, we introduce a specific workflow to exploit repetitive hydroacoustic underwater source activity, which we detail to such an extent that it serves as an example (or "cookbook"). For this reason, we have made the source code publicly available. The workflow proposed in this study (a) overcomes differences in instrument sensitivity and sample rates, (b) involves the selection of eligible cross-correlations based on a source activity criterium as well as slowness analysis, and (c) extracts the travel time variations in distinct frequency bands. In our case, the two frequency bands are 3-6 and 6-12 Hz. We find that the estimated travel time variations in both frequency bands consist of a complex periodic pattern superimposed on a robust linear trend. This linear trend is decreasing, which we attribute to increasing water temperatures along the propagation path of the hydroacoustic signals.

The 15 January 2022 climactic eruption of Hunga volcano, Tonga, produced an explosion in the atmosphere of a size that has not been documented in the modern geophysical record. The event generated a broad range of atmospheric waves observed globally by various ground-based and spaceborne instrumentation networks. Most prominent was the surface-guided Lamb wave (≲0.01 hertz), which we observed propagating for four (plus three antipodal) passages around Earth over 6 days. As measured by the Lamb wave amplitudes, the climactic Hunga explosion was comparable in size to that of the 1883 Krakatau eruption. The Hunga eruption produced remarkable globally detected infrasound (0.01 to 20 hertz), long-range (~10,000 kilometers) audible sound, and ionospheric perturbations. Seismometers worldwide recorded pure seismic and air-to-ground coupled waves. Air-to-sea coupling likely contributed to fast-arriving tsunamis. Here, we highlight exceptional observations of the atmospheric waves.

SUMMARY The ambient infrasonic noise field is complex due to the interference of spatially distributed infrasound sources. Microbaroms are one of the most dominant omnipresent infrasonic sources within this wavefield. These microbaroms are generated by nonlinear ocean surface wave interactions, and have a characteristic and continuous signature within the infrasound spectrum. Under noisy conditions, microbaroms can mask infrasonic signals of interest, such as infrasound from volcanoes or explosions, which limits detection and identification of such sources. This study performs an infrasonic climatology for infrasound array I23FR, using five years of data between 2015–2020. The array is located on the Kerguelen Islands, within the Southern Ocean, and is part of the International Monitoring System for the verification of the Comprehensive Nuclear-Test-Ban Treaty. The climatology analysis addresses the expected ambient noise levels, propagation paths and potential sources within the vicinity of an infrasound sensor. Time- and frequency-domain beamforming methods have been applied to analyse the infrasonic wavefield from the I23FR observations. A recently introduced method is applied to compute so-called soundscapes, to be compared with beamform results. Although the comparison indicates a disagreement in amplitude, there is a good agreement in directionality and frequency between both.

A period of 18 years of infrasonic recordings was analyzed from a microbarometer array (I18DK) in northwestern Greenland, near Qaanaaq. A huge number of infrasonic detections, over 700,000, have been made in I18DKs soundscape during the Arctic summers. Simultaneously identified were both calving events from marine‐terminating glaciers and discharge related acoustics from a land‐terminating glacier. This infrasonic activity is correlated to sea‐surface and atmospheric temperature, respectively. Inter‐yearly to daily variations were retrieved showing a strong variability in infrasonic detection rates and hence glacier activity. The highest number of infrasonic detections were found in recent years from the land‐terminating glacier. The latter is supported by actual discharge measurements and partly by a discharge model. It is concluded that monitoring infrasound from glaciers can complement other techniques to remotely and passively get insights into glacier dynamics with high temporal and spatial resolution.

In order to reduce the observation of wind and turbulence on infrasound sensors, wind noise reduction filters are in place at most infrasound stations. The use of such filters is essential to obtain low background noise levels, which in turn facilitates detection of low signal-to-noise (SNR) infrasound signals. Most filters operate by spatially integrating the pressure field in the vicinity of an infrasound sensor. While the turbulent pressure (partially) de-correlates over the spatial length scale of the filter, the infrasound wave remains coherent. Infrasound arrays that are part of the International Monitoring System make use of advanced pipe array structures that have been designed for long-term deployments. The response is of these systems is stable and well understood. In contrast, many experimental infrasound arrays have relied on the use of porous hoses for wind noise reduction. Porous hoses appear to be efficient, yet cost-effective solutions for short term deployments. Over longer timescales, however, it is known that the response of the hoses can vary significantly and that the hoses can degrade over time. In this work, we investigate the varying response of the porous hoses at the De Bilt Infrasound Array in the Netherlands, using a reference infrasound sensor without hoses. Since a weather station is co-located with the infrasound array, this allows us to study the relationship between the response of the hoses and various meteorological parameters. It is found that under dry conditions, the hoses act as a low-pass filter with a corner frequency around 1.8 Hz, which is consistent with earlier work. We shows that the higher frequency signals with sufficient SNR can be reasonably well reconstructed after a deconvolution step. Under wet conditions however, the hoses become highly absorptive. This can affect observations down to 0.1 Hz. The excess attenuation can be attributed to the presence of rain and/or dew. Although these effects appear to be reversible to some degree, this work shows that care must be taken in the interpretation of data from infrasound arrays that make use of porous hoses.

Geophysical studies and real-time monitoring of natural hazards, such as volcanic eruptions or severe weather events, benefit from the joint analysis of multiple geophysical parameters. However, typical geophysical measurement platforms still provide logging solutions for a single parameter, due to different community standards and the higher cost per added sensor. In this work, the Infrasound and Environmental Atmospheric data Recorder (INFRA-EAR) is presented, which has been designed as a low-cost mobile multidisciplinary measurement platform for geophysical monitoring. In particular, the platform monitors infrasound but concurrently measures barometric pressure, accelerations, and wind flow and uses the Global Positioning System (GPS) to position the platform. Due to its digital design, the sensor platform can be readily integrated with existing geophysical data infrastructures and be embedded in geophysical data analysis. The small dimensions and low cost per unit allow for unconventional, experimental designs, for example, high-density spatial sampling or deployment on moving measurement platforms. Moreover, such deployments can complement existing high-fidelity geophysical sensor networks. The platform is designed using digital micro-electromechanical system (MEMS) sensors embedded on a printed circuit board (PCB). The MEMS sensors on the PCB are a GPS, a three-component accelerometer, a barometric pressure sensor, an anemometer, and a differential pressure sensor. A programmable microcontroller unit controls the sampling frequency of the sensors and data storage. A waterproof casing is used to protect the mobile platform against the weather. The casing is created with a stereolithography (SLA) Formlabs 3D printer using durable resin. Thanks to low power consumption (9 Wh over 25 d), the system can be powered by a battery or solar panel. Besides the description of the platform design, we discuss the calibration and performance of the individual sensors.

In the days following the January 12, 2010 M-w 7 Haiti earthquake the shaking intensity near the epicenter was overestimated and the spatial extent of the potentially damaging shaking was underestimated. This was due to the lack of seismometers in the near-source region at the time of the earthquake. Besides seismic waves, earthquakes generate infrasound, i.e., inaudible acoustic waves in the atmosphere. Here we show that infrasound signals, detected at distant ground-based stations, can be used to generate a map of the acoustic intensity, which is proportional to the shaking intensity. This is demonstrated with infrasound from the 2010 Haiti earthquake detected in Bermuda, over 1700 km away. Wavefront parameters are retrieved in a beamforming process and are backprojected to map the measured acoustic intensity to the source region. The backprojection process accounts for horizontal advection effects due to winds and inherent uncertainties with regard to the time of detection and the back azimuth resolution. Furthermore, we resolve the ground motion polarity in the epicentral region and use synthetics generated by an extended infrasound source model to support this result. Infrasound measurements are conducted globally for the verification of the Comprehensive Nuclear-Test-Ban Treaty and although the network was designed to provide global coverage for nuclear explosions in the atmosphere, it is shown in this paper that there is also global coverage for the estimation of acoustic shaking intensity. In this study, we lay the groundwork that can potentially make infrasoundbased ShakeMaps a useful tool alongside conventional ShakeMaps and a valuable tool for earthquake disaster mitigation in sparsely monitored regions. (C) 2021 The Author(s). Published by Elsevier B.V.

A method is introduced to reconstruct microbarom soundscapes in absolute values. The soundscapes are compared to remote infrasound recordings from infrasound array I23FR (Kerguelen Island) and in situ recordings by the INFRA-EAR, a biologger deployed near the Crozet Islands. The reconstruction method accounts for all-acoustic contributions, divided into evanescent microbaroms (detectable directly above the source) and propagating microbaroms (detectable over long ranges). It is computed by integrating acoustic intensities over the ocean surface, convolved with the transfer function quantifying the propagation losses and propagation time. The reconstructed soundscapes are found within 2.7 dB for 85% of the measurements in the microbarom band of 0.1-0.3 Hz. Infrasonic soundscapes are essential for understanding the ambient infrasonic noise field and are a basic need for applications, such as atmospheric remote sensing, natural hazard monitoring, and verification of the Comprehensive Nuclear-Test-Ban Treaty. Plain Language Summary Microbaroms are omnipresent sources of low-frequency, inaudible sound, that is, infrasound. They have a characteristic and continuous signature within the infrasound spectrum and are often classified as ambient noise. The microbarom signals can be divided into a direct signal, only detectable close by the source, and a propagating signal, which travels over large distances. Under noisy conditions, microbaroms can mask infrasonic signals of interest, such as infrasound from volcanoes or explosions. Insights in the ambient noise field improve natural hazards monitoring and the verification of the Comprehensive Nuclear-Test-Ban Treaty. In this study, a method for the reconstruction of the microbarom source field is introduced. The method reconstructs omnidirectional soundscapes in absolute numbers. This is a significant improvement to previous approaches, which accounted for the normalized loudest source region only. The reconstructed soundscapes are compared with microbarom recordings by microbarometer arrays and the INFRA-EAR, a miniature sensor deployed as a biologger near the Crozet Islands. The comparison shows a statistical agreement for 85% of the time between the modeled and observed soundscapes.

Seabirds are amongst the most mobile of all animal species and spend large amounts of their lives at sea. They cross vast areas of ocean that appear superficially featureless, and our understanding of the mechanisms that they use for navigation remains incomplete, especially in terms of available cues. In particular, several large-scale navigational tasks, such as homing across thousands of kilometers to breeding sites, are not fully explained by visual, olfactory or magnetic stimuli. Low-frequency inaudible sound, i.e., infrasound, is ubiquitous in the marine environment. The spatio-temporal consistency of some components of the infrasonic wavefield, and the sensitivity of certain bird species to infrasonic stimuli, suggests that infrasound may provide additional cues for seabirds to navigate, but this remains untested. Here, we propose a framework to explore the importance of infrasound for navigation. We present key concepts regarding the physics of infrasound and review the physiological mechanisms through which infrasound may be detected and used. Next, we propose three hypotheses detailing how seabirds could use information provided by different infrasound sources for navigation as an acoustic beacon, landmark, or gradient. Finally, we reflect on strengths and limitations of our proposed hypotheses, and discuss several directions for future work. In particular, we suggest that hypotheses may be best tested by combining conceptual models of navigation with empirical data on seabird movements and in-situ infrasound measurements.