Earthquake swarms in California: What’s the difference between magmatism and tectonics?

California Volcano Observatory is a weekly column written by scientists and collaborators at the California Volcano Observatory. This week’s contribution is from Jessica Ball, a volcanologist, and Alicia Hotovick Ellis, a geophysicist with the USGS.

Earthquake swarms are one of the first signs of volcanic activity. Lots of small earthquakes in the same place over a long period of time (hours to years!) under a volcano often mean that magma, gas, or hydrothermal fluids are moving through the ground. However, similar earthquake swarms can also occur on faults or in fault zones without a volcano present.

How do you know the difference? Think about these questions:

1. Is the swarm close to an active volcano or geothermal system?

It’s easy to check – use the map on the California Volcano Observatory’s webpage (or take a look at the map below). We identify the locations of the active volcanoes we monitor as well as the most recent earthquakes. If a group of earthquakes is not near a volcano or known geothermal area, it is likely to be tectonic (i.e. due to accumulated stresses from the extensive movement of tectonic plates). If it is near a volcano, it may be volcanic in origin, but tectonic earthquakes can occur near volcanoes as well. In the case of the San Ramon swarm, there is no active volcano nearby – the nearest volcanic rocks are 5-8 million years old and are very cold and dead.

An oblique Google Earth map shows the location of the 2026 San Ramon earthquake swarm, as well as notable faults in the San Francisco Bay Area and the closest volcanic areas to the swarm. Google Earth image modified by J. Ball, USGS.

Next, you need to consider the pattern of earthquakes in time:

2. How do the magnitude and frequency of earthquakes behave over time?

The size of volcanic earthquake swarms caused by magma or hydrothermal fluids often has no clear pattern in time: instead, earthquakes occur in bursts, waves, or even steady drumbeats. These “swarms” are very common in volcanic areas, even when the volcano is not erupting. Tectonic earthquake sequences, on the other hand, often follow a very distinct pattern: they typically begin with a large “mainshock” followed by many smaller earthquakes (“aftershocks”) that decrease in strength and frequency over time. However, there are cases (such as the San Ramon earthquakes) where tectonic earthquake swarms behave similarly to volcanic ones. This is why it’s important to take the site in context – if there wasn’t a volcanic system there to begin with, it’s unlikely to have been a volcanic source.

These simplified plots of earthquake magnitude versus time show the differences between tectonic earthquake sequences and magmatic earthquake swarms. Figure by Alicia Hotovick Ellis, USGS.

Since California is full of tectonic faults, it’s also worth asking:

3. Have there been swarms of earthquakes there before?

California is incredibly tectonically active, but swarms are relatively rare. The “right” conditions for a swarm to form are limited to a few special places, and these areas usually have a long history of swarms having occurred there in the past. Earthquake swarms are common in the San Ramon Valley, with similar swarms occurring in 2018, 2015, 2003, 2002, 1990, 1976, and 1970.

When the USGS volcanology team is reasonably certain that an earthquake swarm is tectonic, we leave the monitoring and reporting to our colleagues at the USGS Earthquake Science Center. Although we are happy to answer questions when we can, tectonic earthquakes are their specialty. We’ll be ready and waiting to talk about our volcanoes when they start shaking!

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