Earthquake Swarm Beneath Mount St. Helens

You likely did not feel the earth move under your feet but the United States Geological Society says there have been four swarms of small magnitude earthquakes below Mount St. Helens in the past month. The first was detected on November 21, 2016.

The most recent ones have been over the past week when more than 120 tiny earthquakes were detected. Most were a 0.3 magnitude or less and the largest was a 0.5. All of them were too small to be felt at the surface.

The USGS says that most occurred in the mountain's "shallow volcano plumbing system about 1-2 miles below sea level."

Scientists say that the current pattern of activity is similar to what was seen in 2013, 2014 and, most recently, between March and May of this year.

What they think is going on is the volcano's magmatic system is recharging, putting stress on the earth's crust above, causing the minor quakes.

"Subtle evidence of recharge has been observed since 2008 and can continue for many years," the USGS said. "It is a sign that Mount St. Helens remains an active volcano."

What all this activity IS NOT, they say, is sign of an imminent eruption.

Image via USGS