The U.S. Geological Survey reported on May 5, 2016, on the large number of small earthquakes occurring beneath Mount St. Helens, the most seismically active volcano in the Washington and Oregon Cascades, in the U.S. Pacific Northwest. This volcano is known for having erupted violently on May 18, 1980. It erupted again – less violently – in 2004-2008. Since March 14 of this year, scientists have been observing small-magnitude earthquakes at the volcano, but scientists do not believe another eruption is imminent. USGS said:

Over the last 8 weeks, there have been over 130 earthquakes formally located by the Pacific Northwest Seismic Network and many more earthquakes too small to be located. The earthquakes have low magnitudes of 0.5 or less; the largest a magnitude 1.3. Earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes per week. These earthquakes are too small to be felt at the surface.

USGS said these earthquakes – which are taking place below the volcano, at a depth between 1.2 to 4 miles (2 and 7 km) – are a normal part of what a volcano does when it’s not erupting:

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.

Erik Klemetti of Wired’s great earthquake blog explained it this way:

… new magma is rising up underneath St. Helens as it slumbers. As the magma intrudes, it imparts pressure on the rock around it and it heats up water/releases gases that can add to that pressure. This generates small earthquakes as the rocks shift in response to that stress.

USGS added:

No anomalous gases, increases in ground inflation or shallow seismicity have been detected with this swarm, and there are no signs of an imminent eruption. As was observed at Mount St. Helens between 1987-2004, recharge can continue for many years beneath a volcano without an eruption.

The small earthquakes in 2016 at Mount St. Helens aren’t nearly as dramatic as the observations prior to the volcano’s 1980 eruption. That year, magma – or molten material – pushed its way up from a reservoir deep inside the volcano, creating a bulge on the volcano’s north side as the magma drew closer to the volcano’s mouth. In 1980, scientists felt strongly that Mount St. Helens would soon erupt, although they weren’t entirely prepared for the violence of the eruption, which, according to Wikipedia:

…killed 57 people, nearly 7,000 big game animals (deer, elk, and bear), and an estimated 12 million fish from a hatchery … [and] destroyed or extensively damaged over 200 homes, 185 miles (298 km) of highway and 15 miles (24 km) of railways.

Mount St. Helens is 96 miles (155 km) south of Seattle, Washington, and 50 miles (80 km) northeast of Portland, Oregon.

The video below features scientists talking about their experiences during the 1980 eruption.

For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.

Bottom line: The U.S. Geological Survey reported on May 5, 2016, on the large number of small earthquakes occurring beneath Mount St. Helens, the most seismically active volcano in the Washington and Oregon Cascades. Earthquake rates have been steadily increasing since March. The cause is probably new magma, rising upward.