‘Larger swarm than anything we’ve seen’: Rainier quakes reach historic levels

The recent swarm of small earthquakes at Mount Rainier has not only made national headlines but also history: Researchers have never recorded seismic activity like this before at the active stratovolcano.

“It’s now a larger swarm than anything we’ve seen since we’ve been (seismic) monitoring anything at Mount Rainier,” Alex Iezzi, a research geophysicist, said in an interview Thursday.

Researchers started seismic monitoring at Mount Rainier in the 1970s, according to Iezzi, who works for the U.S. Geological Survey’s Cascades Volcano Observatory and said she conferred with other seismologists to identify the origin of observations at the mountain. Prior to then, it’s unknown whether the volcano had experienced a larger swarm — a term defined as a cluster of earthquakes occurring in rapid succession in the same area.

This swarm was first reported by the USGS on Tuesday morning. Since then, there have been hundreds of so-called volcano-tectonic earthquakes detected relatively deep below Mount Rainier’s summit. Researchers say there’s been no cause for concern or any indications that the volcano is on the verge of eruption — something that hasn’t occurred in about 1,000 years.

“Which I think is good,” Iezzi added.

Since Tuesday’s roaring start, the seismic activity has significantly declined and continued to dwindle into Friday. Still, the swarm has surpassed Mount Rainier’s last large one in 2009 “in terms of magnitude, total events, event rate, and energy release,” the USGS said in a status report Thursday.

Mount Rainier is viewed as “potentially the most dangerous volcano in the Cascade Range” due, in part, to its frequent earthquakes, according to the USGS. In ordinary times, Mount Rainier sees an average of nine quakes per month and swarms once or twice every year. Only Mount St. Helens has more temblors in the Cascade Range. Seismic activity is different in all volcanoes, and researchers are unsure why, Iezzi said.

The recent earthquakes have presented researchers with new opportunities to learn more about Mount Rainier, which Iezzi called “exciting,” even as their magnitudes have been small enough to go undetected by the public.

“Even if you were standing on Mount Rainier,” Iezzi said, “you wouldn’t be able to feel these.”

Researchers currently hypothesize that the swarm, as others have been in the past, can be attributed to hydrothermal fluids — very hot water and gases deep in the volcano — moving through preexisting faults beneath the mountain.

While earthquakes can cause volcanic eruptions, there has been no evidence of a correlation in the Cascades, and researchers have not seen any troubling signs during the swarm, according to Iezzi. Such warnings would include magma moving from the depth toward the surface, rockfalls and quakes increasing in size or becoming more shallow, she said.

On its webpage about general Mount Rainier earthquake hazards, the USGS noted that quakes could trigger landslides, leading to volcanic mudflows, known as lahars, which could be dangerous to surrounding valleys.

The agency said that the recent earthquakes are too small to pose a significant threat to hikers. A recent increase in rockfall and icefall at Mount Rainier has been attributed to warm temperatures unrelated to the swarm, it said.

More than 400 quakes since swarm began

As of Friday afternoon, the Cascades Volcano Observatory and Pacific Northwest Seismic Network have located 423 earthquakes since the swarm’s beginning and identified many others that it could not locate, according to the USGS. A quake can be detected by data, but it needs to be big enough and identified by enough seismometers to be pinpointed, Iezzi said.

The earthquakes were between 1.5 and 4 miles in depth, with the largest being a 2.4 magnitude on Friday afternoon, the USGS said Friday. After being detected at rates up to several per minute when the swarm began, quakes were occurring three days later at a much slower rate. Researchers officially located 30 earthquakes per hour at the swarm’s peak Tuesday but that figure had dwindled to a few per hour by Friday.

By comparison, researchers in 2009 located 120 earthquakes at Mount Rainier during a three-day swarm — the largest being a 2.5 magnitude originally identified as a 2.3 — although more than 1,000 quakes were detected, the USGS said.

A 3.9-magnitude earthquake documented beneath Mountain Rainier in 1973 is the largest on record at the volcano, according to the agency.

Iezzi said it was unclear when the current swarm would come to an end, a conclusion that researchers will reach when Mount Rainier returns to normal seismic activity. When the swarm began, the USGS said it likely would continue for several days and that most swarms at the volcano last less than a week.

“If the swarm lasts more than a week or so, that would be different than the last large swarm at Rainier in 2009,” the agency said in a statement Tuesday.

For Iezzi, the swarm has served as “a good reminder” that Mount Rainier is an active volcano, and she encouraged the public to keep aware of information distributed by the USGS.

“These are active systems,” she said, “so we’re constantly monitoring all the volcanoes in the Cascades.”