VANCOUVER, Wash. (AP) — More than 400 earthquakes have been detected beneath Washington’s Mount St. Helens in recent months, though there are no signs of an imminent eruption, according to the U.S. Geological Survey.

Most of the quakes over a three-month span beginning in mid-July were less than magnitude 1.0 and too small to be felt at the surface, the agency reported last week. Small magnitude earthquakes detected with sensitive equipment signal a volcano is “recharging” as magma flows through chambers and cracks deep underground, Wes Thelen, a volcano seismologist with the agency’s Cascade Volcano Observatory told The Columbian newspaper.

From late August to early September, scientists observed about 40 to 50 earthquakes a week, a number that has fallen to around 30 a week. Since 2008, the volcano has averaged about 11 earthquakes per month.

While swarms of earthquakes occurred in the late 1980s and early 1990s, none directly led to volcanic eruptions.

In 1980, 57 people died when Mount St. Helens erupted, an event that permanently altered the area’s ecosystems. Before that event, only one seismometer was stationed at the volcano, the agency said. Currently, there are at least 20 monitoring stations.

The most recent eruption took place from 2004 to 2008, and allowed scientists to learn more about how the volcano works and to develop new monitoring tools.