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Over the nearly four decades since the cataclysmic eruption, the USGS has noticed signs of recovery near Mount St. Helens.

These signs of regrowth are positive, but there are also signs of increased seismic activity under the mountain.

"Mount St. Helens is at normal background levels of activity," Liz Westby, a geologist with the U.S. Geological Survey–Cascades Volcano Observatory, told ABC News. "But a bit out of the ordinary are several small magnitude earthquake swarms in March to May 2016, November 2016 and April 16 to May 5, 2017. During the April 16 to May 5, 2017, swarm, we detected well over 100 earthquakes, all below a magnitude 1.3."

Those quakes originated between sea level and 3 miles below sea level and were too small for people to feel on the surface, even if they had been directly above them, she said.

Even though there has been a swarm of earthquakes, Westby said, that doesn't necessarily mean that an eruption of Mount St. Helens is coming soon. Volcanic forecasts can be tricky.

"There are several reasons why it is very unlikely that this swarm is a precursor to imminent eruptive activity at Mount St. Helens. It is similar to ones in the past that did not lead to surface activity. It consists of very small earthquakes occurring at relatively low rates. There are no other geophysical indicators (like surface deformation, tilting, increased volcanic gas emissions) of unrest," she told ABC News.

Westby said these swarms are extremely interesting and helpful to scientists, since each geophysical signal gives them a better understanding of how a volcano functions.

"This is why we maintain a close watch over these giants, so we can detect the earliest signs of reawakening," she said.

The agency sends out weekly updates on seismic activity around the volcano. Mount St. Helens' last eruption in 2008 was insignificant compared with the devastating eruption in 1980.

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