Mount St. Helens Ground Motions

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Ground Motions at Mount St. Helens

Current time-averaged ground motions (RMS) updated every 30 minutes.
- 10-minute RMS - 2 stations: SEP and VALT
- 1-minute RMS - 2 stations: SEP and VALT
- Earthquake Overview for the Last Two Weeks. Earthquake counts, average ground motion, and earthquake spectral characteristics.
- Earthquake Overview: The first year Magnitude and rms plots of station JUN from the first year of the eruption.
- Earthquake Overview: Long term from Sept. 23 to Nov. 9, 2004 at station JUN - Earthquake counts, average ground motion, and earthquake spectral characteristics during the first 50 days. Overview and links to other detailed plots.
Peak Amplitudes of Earthquakes Detected in the Last 30 Minutes
The plot below displays the sizes of events detected at station SEP.EHZ in the last 30 minutes. Most triggered earthquakes are actually rockfalls off the newly formed dome.

The events are detected by monitoring the average RMS (root-mean-square) amplitude in 1-second intervals. An event is "detected" when this 1-second average exceeds a preassigned threshold value. The amplitude displayed is the maximum 1-second RMS amplitude during the event. The amplitude is adjusted for the sensitivity of the instrument, so the units of the amplitude are true ground velocity in millimeters per second. In practice, the true maximum ground velocity is about twice the maximum RMS velocity.