Matoza et al St. Helens Infrasound JGR 09

B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS
Figure 11. Source time functions used in this study. (top) Arbitrary pulse-like source time function used
with an isotropic moment tensor. (middle) Crack source time function from Waite et al. [2008] and
corresponding moment tensor components rotated into our coordinate system. (bottom) Vertical singleforce
component from Waite et al. [2008].
limitations of this assumption by considering the seismoacoustic
wavefield resulting from an extended fluid-filled
crack source.
4.2. Results
4.2.1. Wavefield Structure From 2-D Simulation
[41] The acoustic wavefield structure resulting from an
impulsive shallow buried source is best illustrated in the 2-
D simulation results of Figures 10, 12, and 13 (see also
Movie S1). Two distinct acoustic arrivals result from this
source configuration, in general agreement with the observations
of Le Pichon et al. [2002, 2003], Mutschlecner and
Whitaker [2005], and Sylvander et al. [2007]. The first
corresponds to locally converted P/SV and Rayleigh wave
energy (identified by particle motion analysis, see Figure
S4), and travels along the ground surface at seismic velocity,
arriving at the infrasonic sensor coincident with the
seismic energy. Note in Figure 10 how the wavefronts in the
atmosphere form at a shallow angle (dependent on elastic
wave speed) to the topography surface and that amplitude
increases along the wavefront in the direction away from the
solid-fluid boundary, as this energy has left the solid at a
later time. These properties are the same as those of ‘‘leaky
waves’’ [Brekhovskikh, 1980; Viktorov, 1967] that form at a
solid-fluid boundary when the elastic wave speed is higher
than the sound speed in the fluid.
[42] The second arrival corresponds to energy converted
in the vicinity of the source epicenter, and travels along the
Figure 12. (left) r v and (right) (r v) y of the vector velocity field v for the simulation shown in
Figure 10. From top to bottom, snapshots times are 0.1 s, 1 s, and 3 s. r v illustrates P and Rayleigh
wave propagation, while (r v) y illustrates S and Rayleigh waves. Conversion of upward P to
downward SV is seen at the free surface, while local seismic-acoustic conversion is seen for both P and
Rayleigh waves leaving the source. Apparent rotational components traveling at acoustic velocity in the
atmosphere and P velocity in the elastic solid for (r v) y (right) are artifacts of high-frequency
numerical noise (curl does not precisely vanish).
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