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PNSN > Volcanoes > Mount St. Helens > 2004-2005 MSH Eruption, Ground Motions > Station JUN - Year Overview |
Earthquake magnitudes, RMS amplitude (proportional to average ground motion amplitude - see explanation below) and minimum and maximum values of RMS amplitude at JUN during the first year of the eruption. For more detailed plots of the first 50 days of the eruption, go here.
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). 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 and shut down to relatively low levels on the morning of October 5th. After October 5th the frequency character of the earthquakes changed as the eruptive style changed from explosive to extrusive. Beginning around October 15th, fresh lava could be seen on the surface. Extrusion of dacitic lava similar to that extruded during the 1980-86 dome building eruption continues through this plot to today. Explosive eruptions also occured on January 16th and March 8th 2005.
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 much greater than normal.
The plot above shows the computed magnitudes of located earthquakes during the first year of the eruption. This is not a complete sampling as less than 1% of the earthquakes have actually been located. Most magnitudes in this plot are determined using coda lengths as a proxy for earthquake magnitude. Efforts to calibrate the coda length magnitudes at Mount St. Helens with other earthquakes show that our method slightly over estimates magnitudes at Mount St. Helens compared with other crustal earthquakes.
High amplitude spikes at the same time each day are caused by daily calibration signals. Spikes can also result from data-transmission problems.
The ground motion displayed on the figure 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.
This plot shows the automatically determined, average level of ground motion recorded at Station JUN. Two graphs are shown. The top graph (red line) shows the maximum recorded RMS amplitude while the bottom graph (black line) shows the minimum RMS amplitude. Each half-hour period produces one point on the RMS amplitude graphs. The amplitude of the point is the maximum (or minimum) RMS amplitude computed for succesive 5-second windows in the half-hour period.
Physically, the red line tracks the average size of larger earthquakes and the black line tracks the amount of ambient noise (background vibrations) between earthquakes. Most "bad" data points have been removed automatically.
For a fairly complete record of webicorders of various stations spanning the time period above go here.
Coda Length Magnitudes of PNSN Selected Events (Sept 2004-Sept 2005)
RMS Amplitudes: Station JUN (Sept 2004-Sept 2005)
Webicorder Archives
