USGS Detects Swarm Of Earthquakes Under Mount St. Helens

The U.S. Geological Survey (USGS) has detected a growing swarm of small earthquakes underneath Washington's most famous volcano, Mount St. Helens. The tremors have shaken the region since March, leading scientists to believe the magma chamber deep underground is recharging.

Beginning about eight weeks ago, nearly 130 earthquakes, between one and four miles deep, have been recorded by seismologists at the Pacific Northwest Seismic Network. While some were barely measurable, most of the tremors registered as a magnitude 0.5 or less. The largest one came in at 1.3 magnitude.

USGS thinks the magma chamber under Mount St. Helens is recharging triggering a series of earthquakes.
A swarm of earthquakes under Mount St. Helens has been detected by the USGS. [Photo by Dimas Ardian/Getty Images]The USGS says the quakes are the type commonly seen in active magma systems. Measured by very sensitive equipment, underground stress forces fluid and gasses through crevices, which pushes and pulls the surrounding rock.
"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."
Even though this kind of seismic activity can occur over years, researchers say the tremors are not a signal that Mount St. Helens will erupt anytime soon.
"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."
However, data collected by the USGS indicate the volcano is still very active. Now numbering close to 40 per week, researchers have noticed the amount of quakes has been steadily rising over the past two months. While an eruption may be unlikely, scientists are unsure just how much magma recharging is needed before an explosion occurs.

The USGS, as well as the Pacific Northwest Seismic Network, have seismometers scattered throughout the Cascade Mountains to detect and record earthquake activity. Using these sophisticated sensors, scientists would ideally be able to tell well in advance of any possible volcanic eruptions.

"It's telling us [that] years to decades from now St Helens will erupt again," Seth Moran, scientist-in-chief at the Cascades Volcano Observatory, told KOIN News. "Our best long-term forecast is that the next eruption will be a lava dome building eruption, which will have some explosivity to it, but nothing as catastrophic as St Helens in 1980."

Moran has dedicated nearly 30 years to studying the pattern of Mount St. Helens. He says the mountain has been able to consistently build itself back up after every eruption.

Geosciences professor at Denison University Erik Klemetti said earthquake swarms are just a normal activity of volcanoes. He notes that underground magma has a long way to go before it can ever reach the surface and cause enough buildup for a catastrophic event.

Swarm of earthquakes detected under Mount St. Helens.
Smoke and ash is seen rising from the crater of Mount St. Helens in this handout photo provided by the U.S. Geological Survey as it erupts October 1, 2004. [Photo by John Pallister/USGS via Getty Images]Mount St. Helens is one of the most closely monitored volcanoes in the world. It last erupted in 2008 after several years of periodic activity. In 2004, the volcano awoke after several small earthquakes were detected with average magnitudes between two and four.

Mount St. Helens is part of the Cascade Mountain Range in southern Washington state. The volcano is about 95 miles from Seattle, Washington and 55 miles from Portland, Oregon.

On May 18, 1980, residents in the Pacific Northwest woke up to the biggest eruption of Mount St. Helens ever recorded. Reaching more than 1,300 feet into the atmosphere, the blast killed 57 people and blanketed the area with volcanic ash for miles. According to the USGS, the surprise eruption was preceded by a swarm of 10,000 earthquakes.

[Photo by David McNew/Getty Images]