Matoza et al St. Helens Infrasound JGR 09

B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS B04305
Figure 4. A comparison of infrasonic and seismic amplitudes for 2963 LP events observed during 1–16
November 2004, 16 December 2004, 1–10 and 24–28 February 2005, and 1–19 March 2005. (a)
Scatterplot of infrasonic amplitude (Pa) versus vertical seismic amplitude (m/s). Dashed lines correspond
to constant values of P/Vz, clockwise from top, 10 6 ,10 5 ,10 4 ,10 3 Pa s/m. The vast majority of events
have P/Vz between 5 10 3 and 5 10 4 Pa s/m (solid lines). (b) Histogram of P/Vz values for data shown
in Figure 4a. Long tail of events with P/Vz >5 10 4 Pa s/m not shown. Note the unimodal distribution
with a peak at 1.3 10 4 Pa s/m. (c) (Gaussian) kernel density estimate of the probability density function
of P/Vz for the different time periods. (d) Kernel estimate and empirical cumulative distribution functions
for all data. The median is: 1.7 10 4 Pa s/m, and the interquartile range is 9.4 10 3 Pa s/m.
large number of events sampling a wide range of atmospheric
conditions. Figure 4a shows a scatterplot of peak
infrasonic pressure amplitude (P) versus peak vertical
seismic velocity amplitudes (Vz) for 2963 LP events from
1 to 16 November 2004, 16 December 2004, 1–10 and 24–
28 February 2005, and 1–19 March 2005. The unit-gain
beam-formed data were filtered at 2–4 Hz (found to be an
optimal band for picking), and events were picked using the
STA/LTA detector described in section 2.1.2. Subsequently,
the beamed data were refiltered at 1–5 Hz, and the
maximum absolute amplitude in a 20 s window following
the pick was assigned to each event. Since the maximum
seismic amplitude does not coincide with the time delayed
maximum infrasonic amplitude but appears later in the
waveform during the resonance coda (Figure 2), the P/Vz
we measure should be considered a lower bound. In order to
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exclude spurious picks not associated with LP events at
MSH, only events consisting of a seismic pick followed by
an infrasonic pick 38 ± 4 s later were included. However,
for the large quantity of data considered, picks related to
transient noise occasionally match the selection criteria.
This is responsible for the small number of detections with
P/Vz >10 5 Pa s/m. However, compared to the number of
detections associated with LP events, these occurrences are
insignificant.
[18] Figure 4a demonstrates that the vast majority of
events have P/Vz between 5 10 3 and 5 10 4 Pa s/m.
Figure 4b shows a histogram of P/Vz ratios for the events
shown in Figure 4a, better illustrating the distribution of
values. A smoother estimate of the underlying probability
density function (PDF) was obtained using a kernel density
estimator with a Gaussian kernel (Figure 4c). The PDF has a