MOUNT ST. HELENS, Wash. -- Multiple small earthquakes beneath the surface of Mount St. Helens the past two months suggest it may be recharging magma.
These tiny quakes which started March 14 have been happening at a depth of two and seven kilometers -- or 1.2 to four miles beneath the surface. Over the last eight weeks, there have been over 130 earthquakes formally located by the Pacific Northwest Seismic Network and many more too small to be located, says USGS.
The small earthquakes beneath the surface may suggest Mount St. Helens is recharging magma. The magma chamber is likely imparting its own stresses on the crust around and above it as the system slowly recharges. The pressure drives fluids through cracks, producing the small quakes, per USGS.
The USGS also says that the recorded earthquakes have been at low magnitudes of 0.5 or less, with the largest being 1.3. The quakes have steadily been increasing since March, reaching nearly 40 per week. However, they're too small to be felt at the surface.
Have no fear, though. The current pattern is similar to the swarms seen at St. Helens in both 2013 and 2014, and the recharge swarms in the 90s had much higher earthquake rates and energy release.
The USGS and Pacific Northwest Seismic Network have plenty of seismometers on the Cascade volcanoes to watch these earthquakes, so the hope is that we'll know well in advance if an eruption were to occur. As we saw between 1987 and 2004, recharge can continue for many years beneath a volcano without an eruption.
For more information, you can go to the Activity Updates for Volcanoes in the CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.