Local Infrasound Research Articles

Studying Different Mechanisms of Seismo-to-Acoustic Coupling Using Ground Motion Local to Seismoacoustic Sensors

ABSTRACT The correlation between seismic and acoustic signals can be used to quantify the influence of earthquake ground motion on the output signals of infrasound sensors and thus evaluate the performance of infrasound sensors under field conditions. However, there have been relatively few studies, and earthquakes considered in those studies are large magnitude events. There remains a need to quantify correlation mechanisms at high frequencies using small earthquakes. In this study, we utilize a collocated seismoacoustic station BJT and a six-element infrasound array DQS to study different coupling mechanisms for infrasound sensors. From 2019 to 2021, 11 and 37 small magnitude earthquakes were detected by BJT and DQS, respectively. Seismoacoustic signals from these small magnitude earthquakes are used to compute the seismoacoustic spectral ratio and the observed spectral ratios are then compared with theoretical estimates. For local seismic-to-acoustic coupling (local infrasound), the highly local pressure field induced by high-frequency ground motion and the separation between seismoacoustic sensors reduce seismic and acoustic signal correlation, which prevents the use of small earthquakes in current studies. The seismic sensitivity of the infrasound sensor, MB3a is shown to be 30f0.75 for ground acceleration or 60πf1.75 for ground velocity and may be an important source of high-frequency noise for an infrasound station using this sensor. Furthermore, an empirical relation is developed to constrain the characteristics of earthquakes that can generate coupled signals on the infrasound sensor. Our study complements previous work and provides insight into the improved interpretation of infrasound signals and methods of seismoacoustic station calibration.

Using Local Infrasound to Estimate Seismic Velocity and Earthquake Magnitudes

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.

Characterizing Infrasound Station Frequency Response Using Large Earthquakes and Colocated Seismometers

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.

Local seismo-acoustic waves, but from the other side of the world

It is well understood that shallow seismic events near the Earth surface couple readily to the atmosphere where they can be observed as infrasonic waves. Typically, the observed sequence of infrasound arrivals includes a first arrival (i.e., “local infrasound”), which corresponds to the coupling of seismic body and surface waves to atmospheric infrasound, near an infrasound array. Under favorable atmospheric ducting conditions, additional coupled arrivals arrive later in time, having propagated throughout the (slower) atmosphere for part of the propagation path. In this presentation, we show that seismo-acoustic waves can couple from very deep earthquakes that occur on the other side of the world. This is demonstrated using seismo-acoustic observations from a M8.2 earthquake in Fiji that generated infrasound throughout Europe. Our analysis shows that various phases that reflected off the inner core boundary (ICB) were detected as pressure waves. This suggests that signals that are detectable on microbarometer arrays may have a surprising origin.

Infrasound measurement system for real-time in situ tornado measurements

Abstract. Previous work suggests that acoustic waves at frequencies below human hearing (infrasound) are produced during tornadogenesis and continue through the life of a tornado, which have potential to locate and profile tornadic events and provide a range of improvements relative to current radar capabilities, which are the current primary measurement tool. Confirming and identifying the fluid mechanism responsible for infrasonic production has been impeded by limited availability and quality (propagation-related uncertainty) of tornadic infrasound data. This paper describes an effort to increase the number of measurements and reduce the uncertainty in subsequent analysis by equipping storm chasers and first responders in regular proximity to tornadoes with mobile infrasound measurement capabilities. The study focus is the design, calibration, deployment, and analysis of data collected by a Ground-based Local INfrasound Data Acquisition (GLINDA) system that collects and relays data from an infrasound microphone, GPS receiver, and an inertial measurement unit (IMU). GLINDA has been deployed with storm chasers beginning in May 2020 and has provided continuing real-time automated monitoring of spectrum and peak detection. In analysis of sampled severe weather phenomena, the signal measured from an EFU (EF-Unknown, where EF represents the Enhanced Fujita scale) tornado (Lakin, KS, USA) shows an elevated broadband signal between 10 and 15 Hz. A significant hail event produced no significant increase in infrasound signal despite rotation in the storm. The consistency of these observations with existing fixed-array measurements and real-time tools to reduce measurement uncertainty demonstrates the value of acquiring tornado infrasound observations from mobile on-location systems and introduces a capability for real-time processing and display of mobile infrasonic measurements.

Use of the parabolic equation method for local infrasound propagation predictions

Local infrasound (<150 km) propagation is strongly influenced by range- and time-dependent meteorological conditions. The Parabolic Equation computational method provides an efficient and flexible tool for predicting infrasound propagation in this regime. Over these longer distances, the influence of spatial and temporal variations in the meteorological conditions can be significant. Meteorological conditions extracted from simulations using the Weather Research and Forecasting (WRF) model are incorporated by updating the effective sound speed profile as a function of propagation range. By examining horizontal grid increments between 1 and 15 km and temporal resolutions between 4 and 60 seconds, we begin to quantify the potential variability in predictions at ranges of up to 150 km, for frequencies between 1 and 20 Hz. This procedure will inform a broader discussion of the application of the PE method to infrasound prediction. [Work funded by the Assistant Secretary of the Army (Acquisition, Logistics, and Technology) [ASA(ALT)] under 62784/T40. Distribution Statement A: Approved for public release; Distribution is unlimited.]Local infrasound (<150 km) propagation is strongly influenced by range- and time-dependent meteorological conditions. The Parabolic Equation computational method provides an efficient and flexible tool for predicting infrasound propagation in this regime. Over these longer distances, the influence of spatial and temporal variations in the meteorological conditions can be significant. Meteorological conditions extracted from simulations using the Weather Research and Forecasting (WRF) model are incorporated by updating the effective sound speed profile as a function of propagation range. By examining horizontal grid increments between 1 and 15 km and temporal resolutions between 4 and 60 seconds, we begin to quantify the potential variability in predictions at ranges of up to 150 km, for frequencies between 1 and 20 Hz. This procedure will inform a broader discussion of the application of the PE method to infrasound prediction. [Work funded by the Assistant Secretary of the Army (Acquisition, Logistics, an...

Uncertainty analysis for infrasound waveform inversion: Application to explosion yield estimation.

While the acoustic waveform inversion method is increasingly used in geophysical acoustics to constrain source parameters, the inversion results are often provided without any uncertainty analysis. This study presents a probabilistic representation for acoustic waveform inversion and method to evaluate the inversion uncertainty using ground-truth data. A posteriori probability distribution of source estimate is described by a priori waveform misfit covariance and the variance of acoustic source model. The probabilistic framework is applied to local explosion infrasound to estimate the yields of explosions and uncertainty. Estimated yields showed overall good agreement with the true yields (less than 25% errors). The uncertainty of the estimated yield is represented by the sum of the waveform inversion uncertainty and source model uncertainty. It is shown that the yield uncertainty attributed to local infrasound inversion (within 10 km) is as small as the uncertainty caused by 10% prediction errors in the acoustic source model. These results indicate that the acoustic source model uncertainty should also be considered for accurate yield estimation and that local infrasound can be a valuable tool to understand the magnitude of the source uncertainty.

Local Infrasound Variability Related to In Situ Atmospheric Observation

AbstractLocal infrasound is widely used to constrain source parameters of near‐surface events (e.g., chemical explosions and volcanic eruptions). While atmospheric conditions are critical to infrasound propagation and source parameter inversion, local atmospheric variability is often ignored by assuming homogeneous atmospheres, and their impact on the source inversion uncertainty has never been accounted for due to the lack of quantitative understanding of infrasound variability. We investigate atmospheric impacts on local infrasound propagation by repeated explosion experiments with a dense acoustic network and in situ atmospheric measurement. We perform full 3‐D waveform simulations with local atmospheric data and numerical weather forecast model to quantify atmosphere‐dependent infrasound variability and address the advantage and restriction of local weather data/numerical weather model for sound propagation simulation. Numerical simulations with stochastic atmosphere models also showed nonnegligible influence of atmospheric heterogeneity on infrasound amplitude, suggesting an important role of local turbulence.

Infrasonic Signal Observation through the Monitoring Network

The methods of observing and processing infrasonic signals from earthquake sources are discussed in this paper. The categories of earthquake infrasound are firstly discussed. Spectrum analysis as well as travel speed calculation are used to distinguish high frequency local infrasound from the acoustic-gravity wave. Monitoring network has been established to capture the signals from the atmosphere. Basing on a simplified correlation method, a triangle array made of three sensors can decide the direction of the coming signal, and locate the earthquake sources. The calculation error and steps of further improvement are also discussed at the end of the paper.

Seismo-acoustic evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system: An example from the 2012 Tongariro eruption

The 6 August 2012 Te Maari eruption comprises a complex eruption sequence including multiple eruption pulses, a debris avalanche that propagated ~2km from the vent, and the formation of a 500m long, arcuate chasm, located ~300m from the main eruption vent.The eruption included 6 distinct impulses that were coherent across a local infrasound network marking the eruption onset at 11:52:18 (all times UTC). An eruption energy release of ~3×1012J was calculated using a body wave equation for radiated seismic energy. A similar calculation based on the infrasound record, shows that ~90% of the acoustic energy was released from three impulses at onset times 11:52:20 (~20% of total eruption energy), 11:52:27 (~50%), and 11:52:31 (~20%). These energy impulses may coincide with eyewitness accounts describing an initial eastward directed blast, followed by a westward directed blast, and a final vertical blast.Pre-eruption seismic activity includes numerous small unlocatable micro-earthquakes that began at 11:46:50. Two larger high frequency earthquakes were recorded at 11:49:06 and 11:49:21 followed directly by a third earthquake at 11:50:17. The first event was located within the scarp based on an arrival time location from good first P arrival times and probably represents the onset of the debris avalanche. The third event was a tornillo, characterised by a 0.8Hz single frequency resonance, and has a resonator attenuation factor of Q~40, consistent with a bubbly fluid filled resonator. This contrasts with a similar tornillo event occurring 2.5weeks earlier having Q~250–1000, consistent with a dusty gas charged resonator. We surmise from pre-eruption seismicity, and the observed attenuation change, that the debris avalanche resulted from the influx of fluids into the hydrothermal system, causing destabilisation and failure. The beheaded hydrothermal system may have then caused depressurisation frothing of the remaining gas charged system leading to the onset of explosive activity.

Infrasonic Signals Associated with the Aftershocks of LuShan Earthquake of April 20th, 2013

On April 20th, 2013, a strong earthquake (Ms 7.0, depth of 13 km) occurred in LuShan County, Southwestern Sichuan province, China, followed by a series of aftershocks (Ms = 3.0∼5.4). Infrasonic waves associated with the earthquake were detected and recorded by a digital infrasound recording system in our lab located in Chengdu city. Fourier analysis was used to determine the amplitude and frequency characteristics of each infrasonic recording. A certain frequency distribution range was clearly identified in the spectrograms of all analyzed shocks. Furthermore, a logarithm regression relation is also found between aftershock magnitude and peak value of corresponding infrasound events in amplitude spectra based on analysis results of more than 30 observed events. This study presents a data set and an empirical relationship. This relationship requires additional testing with observations from other earthquake aftershock sequences. These results improve our understanding of local infrasound produced by the coupling of earth surface motion to the atmosphere.

Ionospheric detection of the 25 May 2009 North Korean underground nuclear test

[1] The total electron content (TEC) measurements of the Global Navigation Satellite System (GNSS) revealed traveling ionospheric disturbances (TID) that locate North Korea's underground nuclear explosion (UNE) of 25 May 2009 to within about 3.5 km of its seismically determined epicenter. The random chance for this pattern of TIDs to register across the eleven GNSS stations is roughly 1 in 19 billion. Monte Carlo analysis of nearly 1,300 TIDs from a 7-station subset of the 11 GNSS stations supports the statistical strength of the array's signature. The UNE was also detected by seismic stations and possibly a local infrasound network of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), but no radionuclide evidence was found. Thus, global GNSS infrastructure enables mapping spatial and temporal variations of TEC that augment and complement other methods of detecting and locating clandestine UNEs.

Calibration of Acoustic Gauge in the Field Using Seismic Lg Phase and Coupled High-Frequency Local Infrasound

Abstract Local microphone field calibration can be performed using seismic signals recorded by a well calibrated seismometer. When the 29 May 2004 regional earthquake ( M w 5.1) occurred off the coast of Korea, the Pn , Pg , and Lg seismic phases produced high-frequency (1–4 Hz) local infrasound signals at the Chulwon Seismo-Acoustic Array (CHNAR), an array deployed in the Republic of Korea. Instrument-corrected waveforms of seismic signals are compared to those of local infrasound signals using time-varying coherence estimates to identify the time window and bandwidth for the calibration process. Based on this assessment, the first 6.4 sec of the Lg phase and the coupled infrasound signals from 1 to 4 Hz are used. Envelope functions for the instrument-corrected Lg phase and local infrasound signal are calculated and integrated in the time domain. The transfer function from ground velocity to atmospheric pressure perturbation is estimated in the time domain using the ratio of the integrated envelope functions. The observed transfer functions estimated in this way are compared to the theoretical transfer function based on a model relating ground velocity to pressure perturbation, which is dependent on the density of air and speed of sound at the surface. A maximum difference of 4 dB is apparent between observed transfer functions and theory for highly correlated seismic and coupled infrasound signals (>0.8). Six factors are considered as contributors to these differences, including the porous hose array attached to each acoustic sensor for noise reduction, variations in the density of air or speed of sound, local perturbations in air pressure, differences in ground velocity at each sensor, and differences in acoustic instrument sensitivity. Variations of the acoustic gauge’s sensitivities at CHNAR are identified using a laboratory calibration procedure and documented as the major contributor to the difference between observed and theoretical transfer function at CHNAR.

Infrasound observations of the 2008 explosive eruptions of Okmok and Kasatochi volcanoes, Alaska

The recent explosive eruptions of Okmok and Kasatochi volcanoes provide an opportunity to use seismic, local infrasound, distant infrasound array, and remote sensing data in concert to better monitor volcanoes in the Aleutian Arc and to better understand the source processes. The eruption of Okmok Volcano began on 12 July 2008 and included a seismically active phase that lasted continuously for about 10 h. In contrast, the eruption of Kasatochi which began on 7 August 2008 consisted of five explosive events that lasted from 26 to 68 min each and had a cumulative duration of 3.4 h. Given the event times by local seismic stations, the corresponding infrasound signals were found in the data recorded by local infrasound sensors and by distant infrasound arrays. Signals from the Okmok eruption were detected by three International Monitoring System (IMS) arrays as far away as 4400 km; signals from the Kasatochi eruption were detected at greater distances up to 5200 km away by seven infrasound arrays including the ones that detected the event at Okmok Volcano. Back azimuth propagation and a simple acoustic wave propagation model in unison with known event times were used to confirm that the planar, acoustic signals recorded at the arrays had originated from the eruptions. The infrasound array data reflected the differences in eruption styles between Okmok and Kasatochi as the signals from Kasatochi were of shorter duration, of greater amplitude, and detected over greater distances. The infrasound array data were also able to distinguish between two types of tremor episodes that occurred at Kasatochi Volcano based on atmospheric disturbance.

Volcanic eruptions, lightning, and a waterfall: Differentiating the menagerie of infrasound in the Ecuadorian jungle

In northeastern Ecuador, near Reventador Volcano, the airwaves are filled with infrasound. Here we identify the locations and characterize three distinct sources of local infrasound, including two types of infrasonic sources, which are not commonly discussed in the literature. The first of these novel sources is an intense and continuous radiator with a fixed location corresponding to San Rafael Waterfall. The signal from the river exhibits a tremor‐like envelope that is well correlated across the 3‐element infrasound network. Beyond the river, we also observe and map spatially variable sources corresponding to thunder. These transient signals have impulsive onsets, but are not well correlated across the network and are attributable to spatially‐distributed source regions. Finally, we identify plentiful infrasound corresponding to Reventador's volcanic vent that is associated with unrest. This study demonstrates the utility of dispersed infrasound networks for distinguishing variable sources and improving interpretation of mechanisms of infrasound radiators.

Infrasound Originating Near Mountainous Regions in Colorado

Abstract An array of infrasonic microphones, installed near Fraser, CO, operated from February through June 1973. This observatory, in conjunction with a similar, permanent installation at Boulder, triangulated on sources of infrasound in Colorado. We found that the lee of a group of mountains that includes Long's Peak is a preferred source region for infrasound. Heretofore, such triangulations used long propagation paths usually of hundreds of kilometers, which resulted both in poorly defined source regions and signal source characteristics. The generation of local infrasound is probable when the 500 mb winds are high. A comparison with Boulder's historical data shows that most signals occur during the winter months with durations longer than 3 h. One case study presented shows that source motion can occur (20 km over an 8 h period). The identification of a local, accessible infrasonic source region offers wide opportunities for further research (e.g., the study of an active source region using time-laps...