A swarm of small earthquakes is still shaking Mount Rainier, marking the most significant seismic activity at the Cascade volcano in decades.
The U.S. Geological Survey’s Cascades Volcano Observatory, which first noticed the earthquake swarm on Tuesday, said it had located 334 temblors as of 9 a.m. Thursday, with the largest reaching a magnitude of 2.3. Many more quakes have occurred but have not been located, the agency said.
This is now the largest recorded earthquake swarm at Mount Rainier since seismic monitoring at the volcano began in 1982, the Cascades Volcano Observatory said. With quakes ongoing, the swarm has already surpassed a major event in 2009 “in terms of magnitude, total events, event rate, and energy release.”
However, the earthquakes are not currently a cause for concern, and do not indicate an impending eruption, the observatory said.
The earthquakes reached a peak of 26 per hour Tuesday morning, then decreased to a few per hour by Wednesday morning, researchers said. Small quakes have continued throughout Thursday morning, according to The University of Washington’s Pacific Northwest Seismic Network.
The quakes are likely being caused by the circulation of fluids along preexisting faults beneath the volcano, the observatory said, which is considered background activity at Mount Rainier.
Earthquakes are common at the Cascade volcano, happening about nine times every month. Swarms of earthquakes happen once or twice per year. Larger swarms are more rare, the last one being the “vigorous swarm” in 2009, when more than 1,000 earthquakes, reaching up to magnitude 2.3, shook the mountain for three days.
While considered an active volcano, Mount Rainier hasn’t erupted in at least a century, possibly much longer. The U.S Geological Survey said there was no physical evidence to confirm a reported 1894 eruption, nor eruptions earlier in the 18th and 19th centuries. The last eruption with strong geologic evidence happened about 1,000 years ago.
“We have only been monitoring seismicity at Mount Rainier since 1982 and the volcano has existed for over 500,000 years. So, there is a lot we don’t know,” Holly Weiss-Racine, a spokesperson a geologist for the Cascades Volcano Observatory, said.
Officials will continue to monitor the earthquakes and will offer updates at least daily.
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