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A short earthquake swarm in the southern part of the Lassen Volcanic Center reminds us that volcanoes can shake, rattle, and roll even when they're not erupting.

Shaded relief map showing Lassen Volcanic National Park, with Lassen Peak labeled in the northern portion of the map, Growler & Morgan hot springs to the southwest, and the June 24 2024 swarm to the southeast
This map shows the location of the June 24, 2024 earthquake swarm at the Lassen Volcanic Center relative to Lassen Peak and Growler & Morgan Hot Springs. Earthquakes are indicated by white, blue, and yellow circles, scaled to the earthquake size.
On the morning of June 24, 2024, two earthquakes were automatically located by the Northern California Seismic Network (NCSN) near Lassen Volcanic National Park. They occurred near Willow Lake on the southern boundary of the park, southeast of Sifford Mountain. These earthquakes had estimated magnitudes of M1.9 and M2.5, which is larger than usual for the area, but not unprecedented. On review, California Volcano Observatory (CalVO) seismologists found dozens of smaller earthquakes in the continuous recordings of seismometers in the area which had not been located by automated processing. NCSN analysts were eventually able to precisely locate 17 of these earthquakes.
This kind of activity with many small earthquakes in a short period of time (often called a "swarm") is common at volcanoes, even when they aren’t erupting or getting ready to erupt. Lassen has a history of swarms that are due either to (1) motion along mapped faults or (2) to the circulation of fluids in hydrothermally active areas like Bumpass Hell. While these earthquakes are occurring outside of that highly active hydrothermal area, there are smaller hydrothermal areas nearby, such as Terminal Geyser. These earthquakes also occurred near the northern end of the Butt Creek fault zone, so a purely tectonic origin is also possible.
Helicorder record showing dozens of tiny earthquake traces occurring over 12 hours, with each line comprising 15 minutes and earthquakes looking like drum cymbals turned on their sides.
This digital helicorder record from station LSIB on the Northern California Seismic Network emphasizes the dozens of tiny earthquakes in Lassen's June 24 swarm. Earch line of the helicorder shows 15 minutes of time, with the entire record covering 24 hours. Red colors indicate maxed-out amplitudes because of proximity or shallowness of earthquakes, while blue and black colors are only used to distinguish alternating lines. 
The last swarm of earthquakes in this vicinity occurred in late October 2022 and lasted approximately one day. In November of 2014, a more vigorous swarm approximately 5 miles to the west of this area lasted 12 days. The largest earthquake during that time measured M3.85. This was the largest earthquake in the vicinity of Lassen Peak in more than 60 years. A few hours later, Growler and Morgan hot springs began putting out twice as much water, and new areas of steaming ground were observed nearby. CalVO scientists determined that the earthquakes had increased the permeability of the rocks where hot water was stored in the local hydrothermal system, allowing more water and steam to reach the surface. (See the paper “Hydrothermal response to a volcano-tectonic earthquake swarm, Lassen, California” for more information about the 2014 swarm.)
Swarms can be one of the first signs of volcanic unrest, but the level and briefness of the most recent swarm did not lead CalVO scientists to think there is a magmatic origin for these earthquakes. Earthquakes from hydrothermal activity can occur from the heat of the deep magma that exists in this volcanic area and doesn't mean new magma is rising to the surface. As a result, the likelihood of an eruption at Lassen right now is low. CalVO will continue to monitor the activity closely, and more information will be reported if new swarms or other signs of unrest are observed.

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