Parameters of the Infrasonic Signal Generated in the Atmosphere by a Powerful Volcano Explosion

Abstract

The purpose of this work is to represent the results of performing regression analysis to fit the distance and the amplitude of the infrasonic signal generated by the explosion of St. Helens volcano, and to estimate a few signal and atmospheric parameters. The pressure amplitude in the explosion wave generated at the beginning of St. Helens volcano eruption was measured at 13 stations in the 0.9 – 39-Mm distance range; based on these data, an attempt has been made to perform a regression analysis to fit amplitude and distance. The regression based on the assumption that the infrasound propagation takes place in a waveguide where it is subject to attenuation is determined to be the most preferable regression. Based on the observations of the shock from the St. Helens volcano eruption, the shock wave energy and mean power have been estimated to be ~1016 J and ~2.3 TW, respectively. Based on the observations of the amplitude and duration of the trains of the infrasonic wave generated by the St. Helens volcano eruption, the infrasonic wave energy and mean power have been estimated to be ~1016 J and ~2 TW, respectively. Both estimates are in good agreement, but they are significantly different from those found in the literature; the latter seem to be overestimated. From the regression expression obtained, the penetration depth of the infrasonic wave is obtained to be about 33 Mm, whereas at other stations this scale length is estimated to be close to 24 Mm. Based on the theoretical dependence of the attenuation coefficient due to atmospheric turbulence, the attenuation length of the infrasound wave has been estimated for infrasound with 10–300-s periods. For 20–300-s periods, this value has been shown to be significantly larger than the values determined from the observations. Other mechanisms for attenuating the infrasonic signal are discussed (the partial radiation of the infrasonic energy through and losses due to the reflection from the waveguide walls). At the same time, the wave attenuation due to their scattering by turbulent fluctuations can be significant for the periods smaller than 20–50 s, depending on the turbulence intensity. Comparison of the regression functions obtained with the corresponding regression expressions for other sources of infrasound waves propagating in the atmosphere has been made. Keywords: volcano eruption, infrasonic wave, shock wave, signal amplitude, regression, signal attenuation