This Washington volcano just recorded its 'largest' earthquake swarm since 2009

Scientists have recorded the "largest" earthquake swarm at Mount Rainier in Washington state since 2009, the U.S. Geological Survey reported.

The swarm at the second-most seismically active volcano in the northern Cascade Range started at 1:29 a.m. PT on July 8, the USGS reported in an X post. "Hundreds" of small quakes – the largest being a magnitude 1.7 — had been recorded as of 9 a.m. July 8, the USGS said, citing Pacific Northwest Seismic Network data.

An exact cause of the swarms wasn't immediately reported, but the USGS said past swarms like these have been linked to "circulation of hydrothermal fluids beneath the summit interacting with preexisting faults."

There were no reports that an eruption was imminent. The Pacific Northwest Seismic Network stated in an X post that there is "no indication that the level of activity is cause for concern, alert level and color code for Mount Rainier remain at GREEN/NORMAL."

"Mount Rainier is well monitored by a dense network of seismic, infrasound, GPS, and webcam stations," the USGS said. "No other changes are apparent at the volcano at this time. (Cascades Volcano Observatory) and @PNSN1 will continue to monitor activity and issue additional notices as needed."

Here's what to know about Mount Rainier.

A general view of Mt. Rainier in 2014. Kirby Lee-USA TODAY Sports Kirby Lee, Kirby Lee-USA TODAY Sports

When was the last time Mount Rainier erupted?

Mt. Rainier hasn't had a "significant eruption" in the past 500 years, according to the USGS's website. However, it is considered to be "potentially the most dangerous volcano in the Cascade Range because of its great height, frequent earthquakes, active hydrothermal system, and extensive glacier mantle."

How many earthquakes does Mount Rainier see in a year?

Scientists detect a "few hundred" earthquakes at or near Mount Rainier in a typical year, the USGS reported.

"Earthquakesassociated with volcanic activity at Mount Rainier will not directly cause major damage to areas surrounding the volcano, but they will give scientists important information aboutmagmamovement beneath the volcano," according to the USGS.

When "unusual earthquake activity" happens, scientists will use special instruments on and around the volcano to "monitorearthquakes,deformation, and other symptoms of volcanic unrest," the USGS said.

This information is used to conclude whether any emergency alerts need to be given to the public, the agency said.

What is an earthquake swarm?

Volcanically triggered earthquakes are usually smaller than earthquakes "caused by non-volcanic sources," according to the Pacific Northwest Seismic Network website. There are two kinds of volcanic earthquakes: volcano-tectonic earthquakes and long-period earthquakes.

"Volcanic-tectonic earthquakes (VTs) are, simply put, caused by slip on a fault near a volcano," the website states. "Volcanoes are often found in areas of crustal weakness, and the volcano's mass itself adds to the regional strain. Most VT earthquakes have nothing to do with the volcano's magmatic system but occur in response to regional strain exerted in an area of weak faults."

A swarm of these earthquakes does not indicate an "impending eruption," the website states. "However, if a significant introduction of new magma from depth occurs in the system, both VTs and long-period earthquakes are likely to be generated."

Volcanically caused long-period earthquakes are "produced by vibrations generated by the movement of magma or other fluids within the volcano," the website states. These earthquakes "indicate magmatic activity and may be a precursor to an eruption."

Long-period earthquakes were seen at Mount St. Helens "months in advance of the 1980 eruption" that killed 57 people, according to the website.

How many major volcanoes are there in Washington?

Washington has five major volcanoes in the Cascade Range: Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams, according to theWashington State Department of Natural Resources.

All five have a threat level of either "very high" or "high," according to a 2018 USGShazard assessment report.

How many active volcanoes are there in the US?

There are about 170 potentially active volcanoes in the United States, according tothe USGS.

This article originally appeared on Kitsap Sun: This WA volcano just recorded its 'largest' earthquake swarm since '09