Matoza et al St. Helens Infrasound JGR 09

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B04305 MATOZA ET AL.: INFRASOUND FROM LPS AT MOUNT ST. HELENS 6of38 B04305 Figure 3
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 7of38 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
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