Publication Date:
2017-04-04
Description:
During the early stages of the 2004–2008 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.
Description:
Published
Description:
B04305
Description:
JCR Journal
Description:
reserved
Keywords:
infrasound
;
events
;
04. Solid Earth::04.08. Volcanology::04.08.06. Volcano monitoring
;
04. Solid Earth::04.08. Volcanology::04.08.07. Instruments and techniques
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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