Mount Adams experienced the most earthquakes in one month during September since monitoring at the volcano began in 1982.
Typically, one earthquake occurs every two to three years at Mount Adams. But six small-magnitude earthquakes were detected last month, although they weren’t felt at the surface, according to the U.S. Geological Survey.
The USGS Cascades Volcano Observatory and Pacific Northwest Seismic Network haven’t raised the alert level and color code for Mount Adams from normal and green, but rank the volcano among those in the region considered to be a threat.
“Other volcanoes in the Cascades tend to have higher rates of background seismicity, and Mount Adams has a very low rate of seismicity,” said Alex Iezzi, a research geophysicist with the Cascades Volcano Observatory. “It’s nothing that we’re worried about at this time.”
Since 2020, 14 small-magnitude earthquakes occurred at Mount Adams, with the most recent happening over this past weekend.
Located in south-central Washington, Mount Adams is the state’s largest active volcano and the second tallest. The Mount Adams-King Volcanic field includes 120 smaller volcanoes located near Skamania, Lewis, Klickitat and Yakima counties.
The six earthquakes last month ranged from magnitude 0.9 to 2.0 and were detected by a seismic station 7 miles southwest of the Mount Adams summit and other stations farther away.
Due to Mount Adams’ seismic network locations, smaller earthquakes have occurred and couldn’t be located, according to the USGS.
The Cascades Volcano Observatory is installing temporary seismic stations south and southwest of the volcano to improve monitoring and help understand the cause of the earthquakes. A temporary station was installed last week, and Iezzi said the USGS hopes to install two more this week. The USGS’s website states that data from the temporary stations and existing data will help seismologists determine if additional actions need to be taken.
With the seismometers being closer to the volcano, the observatory team can get a better idea of where the earthquakes are located and how deep they might be, Iezzi said.
The volcano last erupted between 3,800 and 7,600 years ago, according to the USGS. Those eruptions unleashed lahars, which are muddy flows of rock, ash and ice. If that happened today, the lahars would threaten thousands of people who live on the volcano’s flanks.
“Mount Adams has been known to have these in the past, but they’ve generally been associated with volcanic unrest,” Iezzi said. “Just having these six located earthquakes in September doesn’t raise our concern level for lahars.”
The level of earthquake activity is not a cause for concern at the moment, according to the USGS. The Cascades Volcano Observatory and Pacific Northwest Seismic Network are monitoring the activity and will release updates if necessary.
“Because we haven’t seen this before at Mount Adams, we’re basically on a fact-finding mission right now,” Iezzi said.
