RAPID: Seismo-acoustic radiation from a local earthquake aftershock sequence: how, when, and why seismic waves cross the ground-atmosphere interface

Central Idaho experienced a rare large earthquake (M6.5, depth ~14 km) on the evening of March 31, 2020 (local time). This is the second-largest ever recorded earthquake in Idaho, and because earthquakes of this magnitude are uncommon, the prospect of recording a valuable aftershock sequence motivated Boise State University researchers to set up a sensor network the following day in an area near to the epicenter. This deployment included both seismometers (to measure seismic waves underground) and infrasound sensors (to measure pressure waves in the air). This new network greatly enhances the detection capacity to the more than 283 M2.5+ aftershocks that have already been recorded by the distant regional seismic network. In addition, this new network will significantly improve the accuracy of aftershock locations, which will allow us to understand and map the fault distribution in the area and better anticipate future earthquake hazards, including potential large earthquakes that could come in the following weeks and months. In addition to tracking the seismic radiation from these aftershocks, preliminary analyses from the monitoring of infrasound, or low frequency pressure waves in the atmosphere, has revealed the production of an incredible amount of earthquake ‘sounds’. These air waves have been identified at multiple sites across the geophysical network, and early analyses indicate that they originate in the surrounding mountains as the mountains shake during the passage of the seismic waves. Past observations of mountain-generated air waves have only been studied at long distances; therefore, scientists know very little about the process of earthquake sound generation. Thus, monitoring the Stanley, Idaho aftershock sequence by expanding and maintaining the local geophysical sensor network will provide a unique and fleeting chance to study not just this region’s earthquakes, but also the transmission of seismic energy to the air (and vice versa). An improved understanding of earthquake-generated air waves may lead to new methods of monitoring seismic hazards, including secondary hazards in mountainous regions like avalanches and rockfalls, benefiting the communities exposed to such hazards and the agencies that must respond to them.Aftershocks of Central Idaho’s recent large earthquake (March 31 2020, M6.5, depth ~14 km) provide an ephemeral and unique opportunity to understand topographic seismic-acoustic energy conversion. Motivated by the rarity of events of this magnitude in Idaho, a team of researchers from Boise State University (BSU) began deploying seismometers and infrasound sensors in the region surrounding the epicenter the following day, resulting in a sensor network with excellent spatial coverage. The BSU network includes several spatially dispersed sites including both seismic and acoustic arrays (closely-spaced clusters of sensors that can identify correlated signals and determine the wave’s propagation vector), enabling it to identify backazimuths to wave sources that can be mobile, low in amplitude, continuous, or with emergent onsets. Additionally, because some infrasound sensors and seismometers are co-located, it will be possible to determine the mutual responses of seismic and acoustic instruments to acoustic and seismic waves. Early results from the temporary BSU network include aftershocks that are not detected on permanent regional stations as well as frequent earthquake-generated infrasound. Earthquake-generated infrasound is observed at multiple sites, is not associated with rock falls or avalanches, and appears to originate in nearby mountainous topography during the passage of seismic waves. Such infrasound has previously only been observed at regional distances where atmospheric propagation effects are strong, limiting the resolution of source inferences; the BSU network does not suffer from this limitation. Maintaining and expanding the BSU seismo-acoustic network during the aftershock sequence can elucidate the conversion of wave energy between the ground and atmosphere in unprecedented detail. This improved understanding of earthquake-generated infrasound will include topics like controls on infrasound generation (e.g., dependence on earthquake magnitude and depth, topographic properties, and seismic wave type), infrasound properties (e.g., radiation patterns and duration), coupling of earthquake-generated infrasound back into the ground, instrumentation sensitivities (e.g., the response of seismometers to infrasound and of infrasound sensors to ground motion), and analytical methods for locating earthquake infrasound sources. Earthquake-associated topographic infrasound could serve as a new monitoring method in earthquake-prone mountainous areas. Due to the need to distinguish earthquake-related mountain infrasound from secondary seismic hazards like avalanches or rock falls, an improved understanding of earthquake infrasound will help communities and agencies affected by such hazards.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.