The sleeping giant rumbles again

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The sleeping giant is rumbling again, but this time it’s snoring rather than roaring. Thirty-six years after its cataclysmic eruption on May 18, 1980, Mount St. Helens is showing signs of life far below the mountain’s broad flanks.

Since March 14, an increased number of small magnitude earthquakes have occurred 1.2 to 4 miles beneath Mount St. Helens. In this eight-week period there have been more than 130 of these earthquakes located, and many more too small to locate. Most of the earthquakes measure less than 0.5 magnitude; the largest measured 1.3.

Earthquake rates have been steadily increasing since March, reaching nearly 40 per week. In an average month, 22 events are recorded, or about five per week.

At the Cascades Volcano Observatory (CVO) of the United States Geological Survey in Vancouver, Seth Moran is the Scientist in Charge for monitoring Mount St. Helens, along with the rest of the Cascade Mountain Range. 

The signs of volcanic activity are very subtle, said Moran, and couldn’t be detected without a really good network of instruments. A variety of tools are used to get a picture of what is happening deep below the mountain.

Keeping that information coming in can be a challenge. 

“The mountain is a hard place to keep things running over the winter,” Moran said. Snow buries the solar panels that power the instruments, and their batteries die.

Gasses emitted in the crater convey clues about the type of activity below. Interpreting the gaseous emissions can be complicated, and you don’t see them every time there is volcanic activity, but it can be very informative, said Moran.

Global Positioning Systems (GPS) are placed around the volcano and can detect small movements. These GPS have moved about .5 to 1 inch in about six to eight years. This is a result of magma seven to eight miles deep pushing outward on the walls of the mountain, like an inflating balloon might push your hands apart while you hold it, said Moran.

And seismic activity, the cause for recent attention, is monitored closely. Mount St. Helens has the best seismic monitoring network of all volcanoes in the Cascade Range, due to the eruptions of 1980-1986 and 2004-2008. It is also the most seismically active of the volcanoes in the range.

The current earthquakes are volcano-tectonic in nature, according to CVO, which indicates rock slipping along a small fault. This is believed to be caused by magma pressing through cracks into upper chambers of rock to “recharge” the system. This stress produces small earthquakes, similar to swarms seen previously in 2013 and 2014.

CVO is charged with being the “early warning system” for volcanic activity in the Cascade area. The mountain gives good warning signs if there is likely to be an eruption, said Moran. 



“What we are seeing now is not of that magnitude,” he said. 

Making sense of seismic charts is complicated by the tectonic quakes, caused by the shifting of the earth’s plates, which commonly occur across a broader part of the region. 

“One of the tricky things is to tell the difference,” Moran said. 

Measuring earthquake and volcanic activity on Mount St. Helens and throughout the Pacific Northwest is the job of the Pacific Northwest Seismic Network (PNSN). According to Bill Steele, public information director for PNSN, their function is the keep data flowing and instruments operating, and analysis of earthquakes. 

PNSN has more than 300 seismograph stations across the region and is the second largest seismic network in the United States. USGS and other organizations utilize the data collected by this network.

Mount St. Helens is likely to erupt again in this century, said Steele. Volcanoes follow a pattern, with quiet periods lasting 100-150 years, followed by unrest and an explosive eruption. The recharging, which occurs during quiet periods, is signaled by seismic activity.

Steele reads the pattern of seismic activity like bars of music, making sense of their tempo and cadence. Drumbeat earthquakes, which follow an eruption, repeat signals that look exactly alike. Patterns in the seismogram identify families of earthquakes, which indicates phases of activity. 

Low frequency seismic readings are likely to be rocks vibrating from fluids squirting through them; this could be magma, or even water. The snapping and breaking of rock produces a high frequency reading, and is often a tectonic movement. A harmonic tremor is a sustained stretch of seismic energy associated with magma moving underground, or venting volcanic gases.

The recent earthquakes are too small to be felt at the mountain’s surface. According to CVO, the magma reservoir beneath Mount St. Helens is slowly re-pressurizing following eruptions that occurred in 2004-2008. This is to be expected and does not indicate that the volcano is likely to erupt anytime soon. 

Repressurization of a volcano’s magma reservoir is commonly observed at volcanoes that erupted recently, and can continue for many years without an eruption.

More information and the latest updates can be found at the CVO website http://volcanoes.usgs.gov/observatories/cvo/.