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.
more »
« less
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.
more »
« less
- Award ID(s):
- 2024208
- NSF-PAR ID:
- 10436822
- Date Published:
- Journal Name:
- Bulletin of the Seismological Society of America
- Volume:
- 113
- Issue:
- 4
- ISSN:
- 0037-1106
- Page Range / eLocation ID:
- 1434 to 1456
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
more » « less
Abstract Ground shaking caused by earthquakes is accompanied by seafloor and sub‐seafloor formation fluid pressure variations in offshore areas, but there have been few collocated observations of these signals. In this work, we report seismic and high‐sampling‐rate fluid pressure records of the 2021 Mw 8.2 Alaska earthquake by the Ocean Networks Canada (ONC) NEPTUNE observatory at an epicentral distance of ∼2,200 km in the northeast Pacific Ocean. The system comprises observatory nodes in various tectonic environments, with each node including buried broadband seismometers, seafloor pressure sensors, and, at two nodes, borehole pressure sensors. Seismic and tsunami waveforms of the Mw 8.2 earthquake were documented in detail. Seismic seafloor pressure variations (
P sf) were dominated by Rayleigh waves of periods between 5 and 50 s, with peak amplitudes of 3–4 kPa at most sites. Waveform similarity and the linear scaling betweenP sfand vertical ground acceleration indicate forced acceleration of the water column being dominant in governingP sfduring long‐period surface‐wave arrivals, with an additional component of elastic oscillation occurring at higher frequencies (>0.1 Hz) causing extra pressure signals. Analysis of formation pressure variations due to various types of ocean loading of distinctly different frequencies (e.g., tides, tsunami, and infragravity waves) shows stable one‐dimensional vertical loading efficiencies that depend on lithology at each borehole site, with loading response being strongly influenced by the presence of free gas at shallow depths within the Cascadia accretionary prism. Inter‐site comparisons of seismic and seafloor pressure waveforms demonstrate a key role of sediment thickness in the amplification of surface wave amplitudes. -
more » « less
Abstract Seismic ground motion creates low-frequency atmospheric sound (infrasound) that is detectable at remote sensor arrays. However, earthquake infrasound signal analysis is complicated by interference between multiple waves arriving at sensors simultaneously, reducing the accuracy and detail of ground motion detection. Here we show that individual waves in complicated wavefields can be resolved by recording infrasound on large-N arrays and processing with CLEAN beamforming. Examining both a local (ML3.5, purely tropospheric infrasound propagation) and regional earthquake (ML6.5, upper-atmospheric returns), we detect infrasound from tens of km away and up to several hundred km away respectively. Source regions span arcs of approximately 90°, indicating that although detection bias does occur (most likely from atmospheric winds) the recorded infrasound sources are widely dispersed and not simply epicentral. Infrasound-based remote detection of ground motion over wide areas can complement point measurements by seismometers and spur innovations in earthquake research and real-time hazard monitoring.
-
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.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0120220237
