Four swarms of more than 120 earthquakes shook Mt. St. Helens in late November in less than a week, The Seattle Times reports.
The quakes — so tiny that you wouldn’t have felt them if on the epicenter — hit one to two miles beneath the volcano’s surface.
Scientists say the quakes reveal the volcano is likely recharging.
A seismologist with the Cascades Volcano Observatory in Vancouver told The Seattle Times that each of these little quakes is a reminder of an eruption someday, but the recent swarms do not indicate when the next eruption may be.
They think that the volcano releases gas and fluids that travel into the cracks as magma is stored.
“The earthquakes are volcano-tectonic in nature, indicative of a slip on a small fault. Such events are commonly seen in active hydrothermal and magmatic systems,” the USGS wrote. “The magma chamber is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stress drives fluids through cracks, producing the small quakes. The current pattern of seismicity is similar to swarms seen at Mount St. Helens in 2013 and 2014; recharge swarms in the 1990s had much higher earthquake rates and energy release.”
As was seen at Mount St. Helens between 1987 and 2004, recharge can continue for many years beneath a volcano without an eruption.
Since the catastrophic eruption of May 18, 1980, scientists have been conducting research and collecting data on the volcano to learn more about its typical behavior.
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