In the past eight weeks, more than 130 small earthquakes have trembled beneath the surface of Mount St. Helens.
The most active volcano in the Pacific Northwest has started to stir again. Since March of this year, scientists have noticed more than 130 small magnitude earthquakes beneath Mount St. Helens, with up to 40 happening per week. This behavior signifies that the volcano is steadily recharging – but while it may sound alarming, the experts point out they haven’t yet spotted any signs of an imminent eruption.
Washington’s well known volcano is best known for its ‘cataclysmic’ 1980 eruption, which delivered devastation and destruction that stretched on for miles, and visible ash fall nearly 1,000 miles away. The event has come to be recognized as one of the ‘deadliest eruptions in US history’
Beginning March 14, 2016, numerous small magnitude earthquakes have taken place beneath Mount St. Helens, at a depth between 2 and 7 km (1.2 to 4 miles). Over the last 8 weeks, there have already 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 escalating since March, reaching nearly 40 located earthquakes per week.
These earthquakes are too small to be felt at the surface.
The earthquakes are volcano-tectonic in nature, a sign of a slip on a small fault. Such events are frequently seen in active hydro-thermal and magmatic systems. The magma chamber is probably 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 comparable to swarms seen at Mount St. Helens in 2013 and 2014; recharge swarms in the 1990s had much higher earthquake rates and energy release.