Matoza et al St. Helens Infrasound JGR 09

B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS
vertical single-force component results in energy directed
vertically upward, while the volumetric moment tensor
components result in more hemispherical wavefronts with
a stronger horizontally traveling component.
4.2.4. Long-Duration Source Time Function
[47] Figures 16–18 demonstrate that using a long-duration
STF in the ground leads to a long-duration infrasonic
waveform in the atmosphere. The seismic-acoustic conversion
does not result in an impulsive infrasonic signal from
some component of the long-duration seismic source time
function. Furthermore, Figures 15–17 show that the amplitudes
of the locally radiating P and Rayleigh wave energy
should be comparable to that of the energy originating from
the source epicenter for this source depth, resulting in even
longer-duration and highly complex infrasonic signals. This
is at odds with our observations, as we have instead
recorded a simple impulsive infrasonic signal, and a more
complex, longer-duration seismic LP. This suggests that the
impulsive trigger and resonant crack components of the LP
event are separated into infrasonic and seismic components
at the source by a more complex mechanism not captured in
our numerical modeling using a single point source representation
(sections 5 and 7.1).
4.2.5. Effects of a Near-Surface Weathered Layer
[48] Here we briefly consider the effects of a near-surface
weathered layer on the seismic-acoustic conversion near the
source epicenter. A shallow lower velocity layer acts to
match the impedance between the subsurface and the
overlying atmosphere, having potentially significant effects
on the ground-atmosphere wave transmission. Given lack of
knowledge of the shallow subsurface materials at MSH, we
specified a conceptual 495 m thick weathered layer using
nominal values of Vp = 2000 m/s, Vs = 1155 m/s, and r =
2000 kg/m 3 [Virieux, 1986; Thelen et al., 2009; Scheu et al.,
2006]. Figure 19 shows the results of including the weathered
layer compared to the homogeneous solid used in
previous simulations. These simulations use the 2.5-D
geometry, but a smaller subset of the computational volume
extending to just 2 km from the source in the x direction.
The impulsive isotropic source is at 60 m depth below the
surface. The low-impedance layer enhances the amplitude
of the air pressure wave by a factor of 5 at 2 km but
increases the seismic amplitude in approximately the same
proportion, leading to the same P/Vz amplitude ratio. Also,
short-lived reverberation in this layer leads to more complex
seismic and acoustic signals. The locally converted seismic-
acoustic energy contributes more to the waveforms for the
weathered layer model.
5. Seismic-Acoustic Conversion From a Shallow
Buried, Fluid-Filled Crack
[49] So far our consideration of the airborne acoustic field
from a buried, fluid-filled crack has been restricted to
frequencies