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1. Characterizing Infrasound Station Frequency Response Using Large Earthquakes and Colocated Seismometers

   https://doi.org/10.1785/0120220226  

   Fee, David; Macpherson, Kenneth; Gabrielson, Thomas (February 2023, Bulletin of the Seismological Society of America)

   ABSTRACT Earthquakes generate infrasound in multiple ways. Acoustic coupling at the surface from vertical seismic velocity, termed local infrasound, is often recorded by infrasound sensors but has seen relatively little study. Over 140 infrasound stations have recently been deployed in Alaska. Most of these stations have single sensors, rather than arrays, and were originally installed as part of the EarthScope Transportable Array. The single sensor nature, paucity of ground-truth signals, and remoteness makes evaluating their data quality and utility challenging. In addition, despite notable recent advances, infrasound calibration and frequency response evaluation remains challenging, particularly for large networks and retrospective analysis of sensors already installed. Here, we examine local seismoacoustic coupling on colocated seismic and infrasound stations in Alaska. Numerous large earthquakes across the region in recent years generated considerable vertical seismic velocity and local infrasound that were recorded on colocated sensors. We build on previous work and evaluate the full infrasound station frequency response using seismoacoustic coupled waves. By employing targeted signal processing techniques, we show that a single seismometer may be sufficient for characterizing the response of an entire nearby infrasound array. We find that good low frequency (<1 Hz) infrasound station response estimates can be derived from large (Mw>7) earthquakes out to at least 1500 km. High infrasound noise levels at some stations and seismic-wave energy focused at low frequencies limit our response estimates. The response of multiple stations in Alaska is found to differ considerably from their metadata and are related to improper installation and erroneous metadata. Our method provides a robust way to remotely examine infrasound station frequency response and examine seismoacoustic coupling, which is being increasingly used in airborne infrasound observations, earthquake magnitude estimation, and other applications. 

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2. Evaluating the temporal capability of empirical climatologies for rapid long-range volcanic infrasound propagation estimates using a multidecadal data set of persistent Vanuatu volcanic eruptions

   https://doi.org/10.1093/gji/ggaf027  

   De Negri, Rodrigo_S; Matoza, Robin_S; Hupe, Patrick; Le Pichon, Alexis; Rose, Kaelynn_M; Cevuard, Sandrine; Niroa, John_J (January 2025, Geophysical Journal International)

   SUMMARY Powerful infrasound (acoustic waves $$\lt $$20 Hz) can be produced by explosive volcanic eruptions. The long-range propagation capability, over hundreds to thousands of kilometers, of atmospheric infrasound motivates the development of regional or even global scale volcano-infrasound monitoring systems. Infrasound propagation paths are subject to spatiotemporal atmospheric dynamics, which lead to deviations in the direction-of-arrival (back-azimuth) observed at sensor arrays and contribute to source location uncertainty. Here, we further investigate the utility of empirical climatologies combined with 3-D ray tracing for providing first-order estimates of infrasound propagation paths and back-azimuth deviation corrections. The intended application is in scenarios requiring rapid or pre-computed infrasound propagation calculations, such as for a volcano-infrasound monitoring system. Empirical climatologies are global observationally based function fitting models of the atmosphere, representing robust predictors of the bulk diurnal to seasonal atmospheric variability. Infrasound propagation characteristics have previously been shown to have strong seasonal and diurnal components. At the International Monitoring System infrasound station IS22, New Caledonia, quasi-continuous multiyear infrasound array detections show oscillating azimuthal variations for arrivals from volcanoes in Vanuatu, including Yasur ($$\sim$$400 km range), Ambrym ($$\sim$$670 km range) and Lopevi ($$\sim$$650 km range). We perform 3-D ray tracing to model infrasound propagation from the Ambrym and Yasur volcano locations to IS22 every six hours (00:00, 06:00, 12:00 and 18:00 UTC) for every day of 2004 and 2019 for Ambrym and Yasur, respectively and evaluate the results as compared to the multiyear observations. We assess a variety of models and parametrizations, including both empirical climatologies and hybrid descriptions; range-independent and range-dependent atmospheric discretizations; and unperturbed and perturbed range-independent empirical climatologies. The hybrid atmospheric descriptions are composed of fifth generation reanalysis descriptions (ERA 5) from the European Centre for Medium-Range Weather Forecasts below $$\sim$$80 km altitude combined with empirical climatologies above. We propose and employ simple parametric perturbations to the empirical climatologies, which are designed to enhance the stratospheric duct and compensate for missing gravity wave perturbations not included in the climatologies, and thereby better match observations. We build year-long back-azimuth deviation interpolations from the simulations and compare them with three different multiyear array detection data sets from IS22 covering from 2003 up to 2022. Through a systematic comparison, we find that the range-independent empirical climatologies can capture bulk azimuth deviation variability and could thus be useful for rapid infrasound propagation calculation scenarios, particularly during favourable sustained propagation ducting conditions. We show that the hybrid models better describe infrasound propagation during periods of weak stratospheric ducting and during transient atmospheric changes such as stratospheric wind reversals. Overall, our results support the notion that climatologies, if perturbed to compensate for missing gravity wave structure, can improve rapid low-latency and pre-computed infrasound source discrimination and location procedures. 

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3. Updated Global Reference Models of Broadband Coherent Infrasound Signals for Atmospheric Studies and Civilian Applications

   https://doi.org/10.1029/2022EA002222  

   Kristoffersen, S. K.; Le Pichon, A.; Hupe, P.; Matoza, R. S. (July 2022, Earth and Space Science)

   Abstract The International Monitoring System (IMS) infrasound network has been established to detect nuclear explosions and other signals of interest embedded in the station‐specific ambient noise. The ambient noise can be separated into coherent infrasound (e.g., real infrasonic signals) and incoherent noise (such as that caused by wind turbulence). Previous work statistically and systematically characterized coherent infrasound recorded by the IMS. This paper expands on this analysis of the coherent ambient infrasound by including updated IMS data sets with data up to the end of 2020 for all 53 of the currently certified IMS infrasound stations using an updated configuration of the Progressive Multi‐Channel Correlation (PMCC) method. This paper presents monthly station‐dependent reference curves for the back azimuth, trace velocity, and root mean squared amplitude, which provides a means to determine the deviation from the nominal monthly behavior. In addition, a daily Ambient Noise Stationarity (ANS) factor based on deviations from the reference curves is determined for a quick reference to the coherent signal quality compared to the nominal situations. Newly presented histograms provide a higher resolution spectrum, including the observations of the microbarom peak, as well as additional peaks reflecting station‐dependent environmental noise. The aim of these reference curves is to identify periods of suboptimal operation (e.g., nonoperational sensor) or instances of strong abnormal signals of interest. 

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4. Using Local Infrasound to Estimate Seismic Velocity and Earthquake Magnitudes

   https://doi.org/10.1785/0120220237  

   Macpherson, Kenneth A.; Fee, David; Coffey, Juliann R.; Witsil, Alex J. (April 2023, Bulletin of the Seismological Society of America)

   ABSTRACT Earthquake ground motions in the vicinity of receivers couple with the atmosphere to generate pressure perturbations that are detectable by infrasound sensors. These so-called local infrasound signals traverse very short source-to-receiver paths, so that they often exhibit a remarkable correlation with seismic velocity waveforms at collocated seismic stations, and there exists a simple relationship between vertical seismic velocity and pressure time series. This study leverages the large regional network of infrasound sensors in Alaska to examine local infrasound from several light to great Alaska earthquakes. We estimate seismic velocity time series from infrasound pressure records and use these converted infrasound recordings to compute earthquake magnitudes. This technique has potential utility beyond the novelty of recording seismic velocities on pressure sensors. Because local infrasound amplitudes from ground motions are small, it is possible to recover seismic velocities at collocated sites where the broadband seismometers have clipped. Infrasound-derived earthquake magnitudes exhibit good agreement with seismically derived values. This proof-of-concept demonstration of computing seismic magnitudes from infrasound sensors illustrates that infrasound sensors may be utilized as proxy vertical-component seismometers, making a new data set available for existing seismic techniques. Because single-sensor infrasound stations are relatively inexpensive and are becoming ubiquitous, this technique could be used to augment existing regional seismic networks using a readily available sensor platform. 

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5. Mapping the Sources of Proximal Earthquake Infrasound

   https://doi.org/10.1029/2020GL091421  

   Johnson, J. B.; Mikesell, T. D.; Anderson, J. F.; Liberty, L. M. (December 2020, Geophysical Research Letters)

   Abstract We recorded a M_WR3.6 earthquake in Idaho (USA) on 7 April 2020 with a network of six three‐element infrasound arrays and co‐located broadband seismometers situated within 25 km of the hypocenter. Infrasound array processing is used to identify the arrival of seismic‐to‐atmospheric coupled phases and as much as 90 s of infrasound coda. Apparent velocities ranging from seismic speeds to subhorizontal atmospheric sound speeds are attributed to a superposition of coincident waves arriving at the arrays. We find that the arriving infrasound originates from a broad range of back azimuths that deviates from epicentral back azimuth and indicates the ubiquity of secondary radiators for this relatively small earthquake. Secondary radiators, which often locate in regions of elevated topography, are identified using backprojections and earthquake initiation time. Analysis of infrasound sources from proximal earthquakes can be used to map ground shaking distributions, which are important for assessment of earthquake hazards. 

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