Scientists are trying to establish what caused an unusual spike in earthquakes at the Mount Adams volcano in Washington state by installing multiple temporary seismic monitoring stations at the site.
In September, six small earthquakes were recorded at the "high threat" volcano. Normally, it only experiences one earthquake every two to three years, according to the U.S. Geological Survey (USGS) Cascades Volcano Observatory (CVO).
"We are now working to gather more data to assess if this is really something unusual or just a volcano talking to us a bit more than it normally does," Jon Major, the CVO scientist-in-charge, told Live Science in an email.
"Our volcanoes chatter all the time," he said, adding that the nearby Mount St. Helens and Mount Rainier often experience 10 to 20 earthquakes per month. "Some volcanoes, like St. Helens, Rainier, and Mount Hood undergo episodes of increased earthquake frequency where we see bursts of many earthquakes, which may last for days to weeks," he said. "That is all part and parcel of background activity at our Cascades volcanoes. So what we are seeing at Mount Adams is very far from unusual for the Cascades — but different for Mount Adams."
At 12,277 feet (3,742 meters) high and 18 miles (29 kilometers) wide, Mount Adams is the largest active volcano in Washington, surpassing Mount Rainier — the state's highest peak — by volume.
The increase in earthquake activity, including another magnitude 0.9 quake on Sunday (Oct. 6), does not indicate an eruption will take place. The earthquakes were very small, ranging from magnitudes 0.9 to 2. "Had one been standing at or near Mount Adams one would not have felt these earthquakes," Major said. The volcano last erupted around 4,000 years ago, but this relatively long period doesn't mean that an eruption is due, he added. "Every volcano has its own personality and they do not erupt on any particular cycles," he said. "Some may erupt and then be active intermittently for decades or centuries, then go back to a state of dormancy for centuries to millennia."
Mount Adams generally produces effusive eruptions, characterized by slow-moving lava flows rather than destructive, explosive eruptions. The summit has a big section of unstable and weakened rock. If an eruption were to occur, the main danger would come from lahars, or muddy flows of rock, ash and ice "that surge downstream like rapidly flowing concrete," USGS representative wrote in a statement.
Major said that generally "volcanoes will broadcast pretty loudly" if they are building toward an eruption, triggering lots of earthquakes that increase in size and frequency. The recent mild earthquake flurry doesn't fit that pattern.
The ground around the volcano will also deform as magma pushes up — something that can be ascertained with GPS and satellite monitoring — and the volcano starts to release gases. "These are the main things we watch for to give us a sense of whether or not a volcano may be building toward an eruption," Major said. "We fully expect to see these types of changes at any of our volcanoes should they begin moving toward eruption."
At present, Mount Adams only has one permanent seismic monitoring station around 7 miles (11 km) southwest of the summit. There are plans to install more permanent stations at the site next summer, Major said.
"At this point, we simply do not have enough data to say much of anything," he added. "After the new [temporary] stations we install have had a chance to collect data for a while, we'll be in a much better position to assess the potential causes and significance of this seismic activity and then determine if any further action is needed."
David Pyle, a volcanologist at the University of Oxford in the U.K., said the recent earthquakes could be caused by a "whole range of processes," including the rocks creaking due to internal stresses and strains.
"At Mount Adams the small cluster of detected events is out of the ordinary, given how quiet the volcano has been for the past 40 years," he told Live Science in an email. "This is no cause for alarm, and it is too early to diagnose the cause, but the 'sentinel seismometer' [the permanent station] has done its job, and the team monitoring the volcano will now be able to keep a closer eye on the subsurface, using the array of ground- and satellite-based instruments to which they have access."
