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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. Earth’s Upper Crust Seismically Excited by Infrasound from the 2022 Hunga Tonga–Hunga Ha’apai Eruption, Tonga

   https://doi.org/10.1785/0220220252  

   Anthony, Robert E.; Ringler, Adam T.; Tanimoto, Toshiro; Matoza, Robin S.; De Angelis, Silvio; Wilson, David C. (December 2022, Seismological Research Letters)

   Abstract Records of pressure variations on seismographs were historically considered unwanted noise; however, increased deployments of collocated seismic and acoustic instrumentation have driven recent efforts to use this effect induced by both wind and anthropogenic explosions to invert for near-surface Earth structure. These studies have been limited to shallow structure because the pressure signals have relatively short wavelengths (<∼300 m). However, the 2022 eruption of Hunga Tonga–Hunga Ha’apai (also called “Hunga”) volcano in Tonga generated rare, globally observed, high-amplitude infrasound signals with acoustic wavelengths of tens of kilometers. In this study, we examine the acoustic-to-seismic coupling generated by the Hunga eruption across 82 Global Seismographic Network (GSN) stations and show that ground motion amplitudes are related to upper (0 to ∼5 km) crust material properties. We find high (>0.8) correlations between pressure and vertical component ground motion at 83% of the stations, but only 30% of stations show this on the radial component, likely due to complex tilt effects. We use average elastic properties in the upper 5.2 km from the CRUST1.0 model to estimate vertical seismic/acoustic coupling coefficients (SV/A) across the GSN network and compare these to recorded observations. We exclude many island stations from these comparisons because the 1° resolution of the CRUST1.0 model places a water layer below these stations. Our simple modeling can predict observed SV/A within a factor of 2 for 94% of the 51 non-island GSN stations with high correlations between pressure and ground motion. These results indicate that analysis of acoustic-to-seismic coupling from the eruption could be used to place additional constraints on crustal structure models at stations with collocated seismic and pressure sensors. Ultimately, this could improve tomographic imaging models, which rely on methods that are sensitive to local structure. 

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3. Long-Period Ground Motions from Past and Virtual Megathrust Earthquakes along the Nankai Trough, Japan

   Viens, L. (April 2019, Bulletin of the Seismological Society of America)

   Long‐period ground motions from large (Mw≥7.0) subduction‐zone earthquakes are a real threat for large‐scale human‐made structures. The Nankai subduction zone, Japan, is expected to host a major megathrust earthquake in the near future and has therefore been instrumented with offshore and onshore permanent seismic networks. We use the ambient seismic field continuously recorded at these stations to simulate the long‐period (4–10 s) ground motions from past and future potential offshore earthquakes. First, we compute impulse response functions (IRFs) between an ocean‐bottom seismometer of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) network, which is located offshore on the accretionary wedge, and 60 onshore Hi‐net stations using seismic interferometry by deconvolution. As this technique only preserves the relative amplitude information of the IRFs, we use a moderate Mw 5.5 event to calibrate the amplitudes to absolute levels. After calibration, the IRFs are used together with a uniform stress‐drop source model to simulate the long‐period ground motions of the 2004 Mw 7.2 intraplate earthquake. For both events, the residuals of the 5\\% damped spectral acceleration (SA) computed from the horizontal and vertical components of the observed and simulated waveforms exhibit almost no bias and acceptable uncertainties. We also compare the observed SA values of the Mw 7.2 event to those from the subduction‐zone BC Hydro ground‐motion model (GMM) and find that our simulations perform better than the model. Finally, we simulate the long‐period ground motions of a hypothetical Mw 8.0 subduction earthquake that could occur along the Nankai trough. For this event, our simulations generally exhibit stronger long‐period ground motions than those predicted by the BC Hydro GMM. This study suggests that the ambient seismic field recorded by the ever‐increasing number of ocean‐bottom seismometers can be used to simulate the long‐period ground motions from large megathrust earthquakes. 

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4. 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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5. Conditioned Simulation of Ground-Motion Time Series at Uninstrumented Sites Using Gaussian Process Regression

   https://doi.org/10.1785/0120210054  

   Tamhidi, Aidin; Kuehn, Nicolas; Ghahari, S Farid; Rodgers, Arthur J.; Kohler, Monica D.; Taciroglu, Ertugrul; Bozorgnia, Yousef (September 2021, Bulletin of the Seismological Society of America)

   ABSTRACT Ground-motion time series are essential input data in seismic analysis and performance assessment of the built environment. Because instruments to record free-field ground motions are generally sparse, methods are needed to estimate motions at locations with no available ground-motion recording instrumentation. In this study, given a set of observed motions, ground-motion time series at target sites are constructed using a Gaussian process regression (GPR) approach, which treats the real and imaginary parts of the Fourier spectrum as random Gaussian variables. Model training, verification, and applicability studies are carried out using the physics-based simulated ground motions of the 1906 Mw 7.9 San Francisco earthquake and Mw 7.0 Hayward fault scenario earthquake in northern California. The method’s performance is further evaluated using the 2019 Mw 7.1 Ridgecrest earthquake ground motions recorded by the Community Seismic Network stations located in southern California. These evaluations indicate that the trained GPR model is able to adequately estimate the ground-motion time series for frequency ranges that are pertinent for most earthquake engineering applications. The trained GPR model exhibits proper performance in predicting the long-period content of the ground motions as well as directivity pulses. 

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