The source of infrasound associated with long‐period events at Mount St. Helens

Abstract

[1] During the early stages of the [2004][2005][2006][2007][2008] Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the ''valve'' sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.Citation: Matoza, R.

Joint analysis of infrasound and seismic signals by cross wavelet transform: detection of Mt. Etna explosive activity

“…Johnson and Aster (2005), studying the variability of the ratio between seismic and acoustic energies, made inferences about explosive dynamics inside the volcanic vents and conduits at Karymsky and Erebus. Matoza et al (2009) performed finite difference simulation of the seismo-acoustic wave field to investigate the source mechanism of the repetitive long-period events at St. Helens. More recently, Ichihara et al (2012) highlighted how temporal variations in cross correlation function between seismic and infrasonic signals can allow visually detecting changes in eruptive activity.…”

“…Hagerty et al, 2000;Ripepe et al, 2001;Johnson and Aster, 2005;Petersen and McNutt, 2007;Matoza et al, 2009). Notably, it was demonstrated how, in particular conditions (e.g.…”

Distinguishing high surf from volcanic long‐period earthquakes

“…Numerous source models exist to explain LP signals at volcanoes. These often invoke pressure fluctuations that result from the unsteady movement of bubbly magma [e.g., Chouet, 1996;Kumagai et al, 2002] or steam [Waite et al, 2008] through volcanic conduits and sometimes extend to include the venting of pressurized fluids into the atmosphere and generation of infrasound [Matoza et al, 2009]. Periodic strain changes induced by the oceanic microseism have also been cited as a trigger for hydrothermal LP events [Stich et al, 2011].…”

Single-Station Seismo-Acoustic Monitoring of Nyiragongo's Lava Lake Activity (D.R. Congo)

“…Combining these two approaches could bring important constraints on the studied sources processes, i.e., the so-called seismo-acoustic sources generated in the vicinity of the boundary between the solid Earth and the atmosphere (Arrowsmith et al, 2010). It is now well-known that variations of (near-) surface activity at active volcanoes, either effusive or explosive, can be efficiently monitored by analyzing the coupled infrasound and seismic signals (e.g., Matoza et al, 2009Matoza et al, , 2010Ichihara et al, 2012;Richardson and Waite, 2013;Ulivieri et al, 2013). The seismicity characteristics of Nyiragongo and, more generally, of the entire Virunga Volcanic Province (VVP), are not very well known because of the lack of a permanent, local monitoring infrastructure until recently (Pagliuca et al, 2009;Oth et al, 2017).…”