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PNSN > Volcanoes > Mount St. Helens > 2004 MSH Eruption, Ground Motions > Station JUN - Overview |
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 southeast of the crater.
The average amplitude rose dramatically on Sept 29 when we recorded frequent moderate sized earthquakes up to magnitude M=3.5. Vertical green lines on plots show times of observed ash and steam eruptions. Vertical gray lines show times of continuous vibration called "tremor". After October 3 the earthquakes decreased in intensity; they shut down to very low levels on the morning of October 5th.
As shown in the plots, the ground shaking goes up and down in response to the frequency and sizes of earthquakes that occur. However, even during apparently quiet periods the recorded background noise level is greater than normal.
Time on the plots is given as date in Universal time (UT = PST + 8 hours).
The bottom graph above shows the number of low, medium, and high frequency
events versus time. For each earthquake detected, a spectrum was computed and the
frequency at which the peak in the spectrum occurred was use to categorize the
event as low frequency (1-3Hz - red line), medium frequency (4.5-6Hz - blue line) or high
frequency (11-13 Hz - black line).
More detailed ground motion plots for station JUN
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.
Webicorder example for this period on Oct. 2, 2004 12:00-24:00 UTC
Webicorder examples for this period
Webicorder examples
