USGS: Quake swarm at Mount St. Helens signals recharge, not imminent eruption

It’s been nearly 36 years since Mount St. Helens last erupted, and the U.S. Geological Survey said the mountain appears to be recharging.

They have detected a swarm of more than 130 small earthquakes beneath the mountain over the past two months.

Despite all the activity, the USGS says there’s no sign of an impending eruption. The recharge earthquake swarms are normal behavior for a volcano like Mount St. Helens.

The shaking is between 1 and 5 miles beneath the earth and can’t be felt from the earth’s surface. The largest recorded earthquake was magnitude 1.3, but most of the quakes have low magnitudes of 0.5 or less.

Scientists say the recent earthquakes are a sign of new magma is rising under the active volcano.

Earthquake rates have been steadily increasing since March, reaching nearly 40 per week, but are not considered dangerous and do not mean the volcano is about to erupt again.

Scientists say the pattern is similar to swarms seen at Mount St. Helens in 2013 and 2014 and indicate a slip on a small fault.

“Such events are commonly seen in active hydrothermal and magmatic systems,” the USGS said in a prepared statement issued Thursday. “The magma chamber is likely imparting its own stresses on the crust around and above it, as the system slowly recharges.”

Although the quakes are numerous, swarms in the 1990s had much higher earthquake rates and energy release., USGS officials said.

“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,” according to the USGS statement. “As was observed at Mount St. Helens between 1987-2004, recharge can continue for many years beneath a volcano without an eruption.”

Source: USGS: Quake swarm at Mount St. Helens signals recharge, not imminent eruption | KATU