PNSN Logo PNSN > Volcanoes > Mount St. Helens > 2004 MSH Eruption, Ground Motions > Station JUN - Overview > JUN Detail

Detailed Plot 2004 Eruption, Station JUN - Oct. 21 - Oct. 31

Earthquake counts, RMS amplitude (proportional to average ground motion amplitude - see explanation below) and dominant earthquake frequencies at JUN during the first 50 days of the 2004 eruption. Earthquakes counts are determined using an automatic algorithm that detects signal increase in continuously recorded digital data. RMS amplitude is average signal strength (see explanation below). Event frequencies determined by automatic processes. Automatic processes may misinterpret signals. Errors may be present in these plots. JUN is located 6 km southwest of the crater.

Overview RMS Plots of JUN
Detailed RMS Plots of JUN

  • Sept. 23 - Oct. 7 - Ash and steam eruptions, harmonic tremor, and most vigorous seismic activity
    Webicorder example - Oct. 2, 2004 12:00-24:00 UTC
    • JUN Saturated - signal amplitude frequently exceeds capacity to record.
    • FMW Lots of activity visible at station FMW located on Mt. Rainier.
    • JCWContinuous M 3+ seismicity, recorded on station JCW in nortwestern Washington. Steam explosion (no detectable signal) followed by harmonic tremor around 12:00-13:00 PDT.

  • Oct. 7 - Oct. 21 - End of large earthquakes
    Webicorder examples
    • Oct. 9, 2004 12:00-24:00 UTC JUN and FMW
    • Oct. 16, 2004 12:00-24:00 UTC JUN and FMW

  • Oct. 21 - Oct. 31 - Period of steady dome growth
    Webicorder examples
    • Oct. 25, 2004 12:00-24:00 UTC JUN and FMW

High amplitude spikes at the same time each day are caused by daily calibration signals. Spikes can also result from data-transmission problems. Spikes of these types will usually show up on a particular station at a particular time, but not at other stations at the same time.

* The ground motion displayed on the figures is the RMS amplitude in raw counts as recorded on the computer. That is, the square root of the average ground motion squared. The averages are computed from ground motion data sampled 100 times per second.

The amplitudes can be converted to ground velocity or ground displacement if one accounts for the sensitivity of the seismograph station (varies for each station and varies also with the frequency of ground motion in Hz). For example at station JUN the sensitivity to ground velocity is 4.9 millimicrons/sec/count at 3 Hz. The sensitivity to ground displacement is 0.26 millimicrons/count at 3 Hz. Note 1,000,000 millimicrons= 1 millimeter.

As an example, if the plot shows an RMS amplitude of 100 counts at station JUN and if we assume the vibrations have a frequency of about 3 Hz, then the true RMS amplitude of ground motion at JUN, 6 km south of the source of the vibrations, is (100 counts)x( 0.26 millimicrons/count) = 26 millimicrons.