Recent large earthquakes near Lassen are just another day in the Basin and Range
Since May 11, 2023, a series of M5+ earthquakes and aftershocks have rattled the area around Lake Almanor, a reservoir about 20 miles (32 km) southeast of the Lassen Volcanic Center.
The initial M5.5 earthquake (https://earthquake.usgs.gov/earthquakes/eventpage/nc73886731/executive) was followed by a M5.2 quake (https://earthquake.usgs.gov/earthquakes/eventpage/nc73887046/executive) as well as dozens of smaller aftershocks. Because Lake Almanor is close to Lassen, CalVO received many questions about potential connections to the volcano.
To start, the distance and character of these earthquakes tell us that they're unlikely to have anything to do with Lassen. Focal mechanisms (a way of describing the type of slip) tell us that these earthquakes occurred on a normal fault. At a volcano, we would expect seismicity to have widely varying focal mechanisms, depending on earthquake location. Volcanic earthquakes also follow a more complicated pattern than mainshock-aftershock sequences (https://www.usgs.gov/observatories/hvo/news/volcano-watch-aftershock-forecasts-let-you-know-what-expect-after-large)
Our next clue is that Lake Almanor is not in the same physiographic province (defined by tectonics, landforms, and volcanism) as Lassen. The Lassen Volcanic Center is at the southern end of the Cascades Volcanic Arc, formed by a subduction zone. But only a few miles to the east are the Modoc Plateau and Basin and Range provinces. Lake Almanor falls roughly within the Basin and Range, and its tectonic history is very different from Lassen's. The area contains several N-S and NW-SE trending fault zones, including the Lake Almanor fault, which are active and prone to earthquakes. A similar earthquake sequence happened there in May of 2013 (https://earthquake.usgs.gov/earthquakes/eventpage/nc71996906/executive).
Finally, many of you asked if these earthquakes could affect Lassen. The short answer is no! While some eruptions might be triggered by large earthquakes, it only seems to happen when the volcano is already primed and ready to erupt - that is, there already needs to be gassy magma near the surface. We've seen no evidence of that at Lassen recently: no earthquake swarms, no ground swelling, and no change in the gases coming out of the volcano.
So there you have it - while these earthquakes were near Lassen, there's no danger of eruption. It's just part of living in a tectonically active state!
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