Matoza et al St. Helens Infrasound JGR 09

B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS
Figure 8. Comparison of infrasonic waveform correlation with available wind data 10–12 November
(JD 315–318) 2004. (a) Infrasonic CC with master event (Figure 7). (b) Spectrogram of low-frequency
(0–1 Hz) pressure at CDWR central infrasonic sensor element. (c) Black, 1-min wind speed average at
CDWR; blue, hourly wind speed max at CDWR; red, hourly wind speed max at NWAC. (d) Black,
hourly wind direction average at CDWR; red, hourly wind direction average at NWAC. At least one
measured wind speed increase (JD 315.5–316) is associated with a loss in signal correlation. The diffuse
peak at 0.2 Hz in the spectrogram is the microbarom peak. Wind direction is defined as the direction
from which wind is blowing.
location or source time function. Acoustic propagation is
further subject to time-dependent variability in atmospheric
conditions, especially changes in temperature and wind. In
this case, a change in the waveform correlation with time
can imply a change in the source location, source time
function, or a change in the atmospheric conditions.
3.1. Waveform Changes: 1–16 November 2004
[25] We analyze CDWR data for 1–16 November, or
Julian days (JD) 306–322, 2004 (Figure 7). This corresponds
to the time period depicted by Matoza et al. [2007,
Figure 3], where Progressive MultiChannel Correlation
(PMCC) [Cansi, 1995] detection of infrasound from LPs
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was observed to switch on and off while the seismic LP
events were continuously observed (see Figure S3). We also
analyzed data from 1 to 19 March 2005 using the same
method and obtained similar results. All data were bandpass
filtered at 2–4 Hz and the infrasound data were beamformed
(azimuth 153°, speed 330 m/s). Events were picked
using the STA/LTA detector, then progressively selected for
correlation with the master event using 11 s windows (3 s
pretrigger, 8 s posttrigger). In Figure 7, the master event
(seismic, 1803:38 UTC; infrasonic, 1804:16 UTC, 11
November 2004) was an event arbitrarily chosen from a
time period of good seismic and infrasonic signal-to-noise