Small magnitude earthquake swarms detected beneath Mount St. Helens.
The current pattern of seismicity is similar to swarms most recently seen at Mount St. Helens in March-May 2016, and in 2014 and 2013.
Four swarms of small magnitude earthquakes were detected beneath Mount St. Helens beginning November 21, 2016. No anomalous gases or increases in ground inflation have been detected and there are no signs of an imminent eruption.
During a week's time, there have been over 120 tiny earthquakes, most too small to be formally located by the Pacific Northwest Seismic Network. The earthquakes are magnitudes 0.3 or less; the largest has been a magnitude 0.5. Most of the earthquakes are occurring in the shallow volcano plumbing system about 1-2 miles below sea level. These earthquakes are too small to be felt at the surface.
The current pattern of seismicity is similar to swarms most recently seen at Mount St. Helens in March-May 2016, and in 2014 and 2013. The magmatic system is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stresses drive fluids through cracks, producing the small quakes. Subtle evidence of recharge has been observed since 2008 and can continue for many years. It is a sign that Mount St. Helens remains an active volcano.
For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.
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