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Another swarm of earthquakes hit Mount Rainier
Hundreds of earthquakes have been rattling Mount Rainier over the last two days, marking the largest swarm since 2009. (This story aired in early July)
MOUNT RAINIER NATIONAL PARK, Wash. - A recent swarm of more than 1,350 small earthquakes near Mount Rainier raised alarms across the Pacific Northwest this summer, but according to new findings from the U.S. Geological Survey (USGS), there's no sign the volcano is waking up – at least not in the way people might fear.
Seismicity beneath Mount Rainier, highlighting July 8 - August 15, 2025, earthquake swarm (USGS)
Why you should care:
Researchers from the USGS Cascades Volcano Observatory say earthquakes near a volcano can be caused by several different factors, and identifying the source helps scientists prepare for potential hazards.
To find out what triggered this summer's swarm, a team of USGS gas specialists partnered with the National Park Service for a helicopter flight over the volcano's summit. Equipped with sensitive gas detection tools, they took measurements near small steam vents, called fumaroles, located in the east and west summit craters.
USGS gas geochemists Christoph Kern (left) and Laura Clor (right) during the airborne gas survey of Mount Rainier.
What they're saying:
"Volcanic gas sampling can help reveal the cause of earthquakes in a volcanic system, much like how blood tests help a doctor figure out what's causing a patient's symptoms," wrote the USGS.
Gas inlet window plate holding gas sampling equipment. View from the helicopter cabin showing the connections from the inlet window to the measurement equipment inside. (USGS)
What they found suggests the quakes were likely linked to hydrothermal activity, a process where underground water is heated by the volcano and begins to circulate. While that might sound alarming, scientists say this is a common feature of active volcanoes like Rainier.
Gas sensor packages mounted to the floor of the aircraft near the rear passenger seats, connected to the outside through black tubing that attaches to the window plate. Gas composition data is displayed in real time during gas flights. (USGS)
The gas sensors detected small amounts of water vapor and carbon dioxide, but no traces of sulfur-based gases such as sulfur dioxide or hydrogen sulfide. These sulfur gases tend to appear when fresh magma is rising toward the surface. The absence of those gases suggests no new magma is currently on the move.
So why were there so many earthquakes? The findings indicate the shaking was not caused by rising magma or an impending eruption, but likely by shifts in the volcano's shallow hydrothermal system.
A shaded relief map of Mount Rainier with GPS track from the gas observation flight. The colors correspond to CO2 levels in parts per million by volume (ppmv) that were measured during the flight. Yellow points indicate elevated CO2 levels, which wer … (USGS)
"The gas measurements indicate that despite over 1,350 small earthquakes happening at Rainier in July and August, the volcano's gas output has not changed much," wrote the USGS.
What does this mean for Mount Rainier's current activity?
Big picture view:
These results confirm that while Mount Rainier is still very much alive, its recent burst of seismic activity doesn't indicate anything out of the ordinary for an active volcano with an existing hydrothermal system.
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The Source: Information in this story comes from the United States Geological Survey (USGS).
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