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PNSN > Volcanoes > Mount St. Helens > MSH Eruption, Ground Motions > Current Ground Motions |
This figure is automatically updated every 30 minutes. It shows the average level of ground motion recorded at seismographs near Mount St. Helens. Each point represents the average RMS (* see below) amplitude of ground motion averaged over a one minute period. Ground motion at 2 seismograph stations are shown for comparison. Dates on the horizontal axes are given in Universal Time (UT). Data and graphics displayed on this and other related pages are PRELIMINARY and may have errors or omissions. Check with the PNSN about the nature of any errors present in the raw data displayed here. See map for station locations. You may compare these plots with the 10 minute RMS values at SEP and VALT. or the current Webicorders at (VALT), (SEP).
High amplitude RMS values at the same time each day are caused by
daily calibration signals. Very high or very low values can also result
from data-transmission problems.
Usually such problems will show up one one station at a particular time, but
not at other stations at the same time.
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.
* 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 from seismometers with velocity transducers.
