Mount St Helen's in Washington may have lain dormant for years, but a recent flurry of earthquakes around the mountain serves as a poignant reminder that this is one very active volcano.

This underground grumbling is a sign that the volcano is likely recharging, said the United States Geological Survey.

The USGS measured a flurry of small earthquakes, one or two miles below the surface, in late November.

At a magnitude of mostly 0.3 less, they were almost too weak to be felt at surface level. But the flurry of quakes was strong, with four swarms of more than 120 within a week in late November.

"Each of these little earthquakes is a clue and a reminder we are marching toward an eruption someday," Weston Thelen, a USGS seismologist with the Cascades Volcano. Observatory in Vancouver, told the Seattle Times.

Another expert at the observatory, Liz Westby, said the volcano is "inflating subtly" even if it cannot be seen from the outside.

Recharging occurs when the magma that is stored underground the volcano releases gasses and fluids, which pressurises and lubricates cracks within the volcano and causes seismic activity.

Mount St Helen's last erupted in 2004, and it ended in 2008.

Swarms of earthquakes have been recorded since then, most notably between March and May this year.