Matoza et al St. Helens Infrasound JGR 09

B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS
Figure 13. Synthetic record sections for (top) acoustic pressure and (bottom) seismic vertical velocity
for a 2-D simulation from an isotropic impulse point source (Figures 10 and 12). The synthetic
seismograms show a faint P arrival followed by the dominant Rayleigh wave train (R). Note the
backscattering from topography at 10 km (see Movie S1). The synthetic acoustic data show the arrival
of two distinct packets of energy. The first corresponds to locally converted P and Rayleigh wave energy
and travels in the atmosphere along the ground surface at seismic velocity. The second corresponds to
energy converted from strong ground shaking near the source epicenter and travels through the
atmosphere at acoustic velocity (A). Note the asymmetry in amplitudes of A with respect to range,
resulting from asymmetry in topography. Although useful for identifying the principal acoustic arrivals
from a buried source, 2-D simulations do not adequately predict the amplitude loss due to geometrical
spreading.
ground surface at the much slower acoustic velocity, arriving
time delayed from the seismic-acoustic coupled (first)
arrival (Figure 13). The wavefronts for this second arrival
are much steeper, and are hemispherical in the absence of
topography (confirmed by calculation not shown here). The
lateral extent of this infrasound source is restricted to a
radius at which the peak vertical seismic velocity reaches a
limiting value for effective infrasound generation, which in
turn depends on the amplitude and depth of the seismic
source [Mutschlecner and Whitaker, 2005]. We note that
there is an asymmetry in the amplitude of this second arrival
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observed in Figures 10 and 13. The acoustic signal propagating
to the SE is weaker than the signal propagating to the
NW. This appears to result from the asymmetry in topography.
The topography in the crater is dipping to the NW
(toward CDWR, left in Figure 10), while to the SE the
wavefront must diffract over the SE crater wall, which is
immediately adjacent to the epicenter of the source where
the energy conversion is taking place. This illustrates the
importance of topography in the immediate vicinity of the
source epicenter for the radiated far-field acoustic amplitude
from a buried source.