Officials at the U.S. Geological Survey (USGS) reported on May 5 that they have detected more than 130 small earthquakes beneath Mount St. Helens in the Pacific Northwest over the past few weeks.

With the volcano having been known to erupt violently in the past, agency scientists are looking at the possibility that these minor tremors could be indicators of an even bigger activity from the volcano.

In March 1980, a 4.2 magnitude earthquake shook the ground beneath Mount St. Helens, causing it to release massive amounts of steam a few days later. The north side of the volcano then began to bulge until another 5.1 magnitude quake in May caused its north face to collapse entirely. This sequence of events resulted in the largest avalanche of volcanic debris in history.

By the time the catastrophe ended, more than 230 square miles of vegetation and structures were flattened by the pyroclastic flow that spewed from Mount St. Helens. The volcanic eruption also released as much as 1.5 million metric tons toxic gas, including sulfur dioxide, into the atmosphere.

Following this major explosion, researchers have detected several eruptions from the volcano between 2004 and 2008, though they were much less violent than the one in 1980.

However, it appears that Mount St. Helens is experiencing an increase in its seismic activity in the past few weeks.

USGS scientists said as many as 40 small quakes have been detected every week since March but they don't consider them to be dangerous or signs that the volcano is set for another major eruption.

The recent earthquakes at Mount St. Helens occur about 1 to 4 miles beneath the ground, which researchers say are far too small to even be felt at the surface. A majority of these quakes have magnitudes of 0.5 or less, with the largest one recording a magnitude of only 1.3.

Other smaller tremors have also been recorded but they were so weak that researchers couldn't identify their exact locations.

Experts say the pattern of seismic activity the agency has detected shares similarities with those of earthquake swarms in the area of Mount St. Helens back in 2013 and 2014. They believe the recent quakes could be an indicator of a slip on a minor fault.

The USGS said such occurrences are often seen in active magmatic and hydrothermal systems, where stress from the magma chamber causes the ground around and above it to shake considerably as the system slowly recharges itself.

The agency explained that while the recent quakes have been numerous, they don't occur at such high rates as those observed during the 1990s. They don't also release as much energy as those detected in the past.

"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," the USGS said. "As was observed at Mount St. Helens between 1987 and 2004, recharge can continue for many years beneath a volcano without an eruption."

Photo: Ewen Roberts | Flickr 

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