Images

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

This oblique Google Earth map shows the location of the 2026 San Ramon earthquake swarm, along with notable faults in the San Francisco Bay area and the nearest volcanic areas to the swarm. Google Earth image modified by J. Ball, USGS.

The location of GPS station KRAC and its predecessor KRAK, slightly north of the center of deflation in Long Valley. USGS figure by Josh Crozier and Alicia Hotovec-Ellis.

The average of individual measurements from GPS station KRAC/KRAK shows about 6 cm of ground subsidence since 2021. The multi-year deflation stands out above smaller fluctuations caused by noise in the data and seasonal water infiltration (the peaks and troughs in the dots for individual measurements). USGS figure by Josh Crozier and Alicia Hotovec-Ellis.

Locations of earthquakes of at least magnitude 1.0 in 2025 and the outlines of the boxes CalVO uses to count earthquakes around Long Valley. Only 6 earthquakes were located inside the Long Valley Caldera box. (Earthquake location data from Northern California Seismic Network.) Figure by Alicia Hotovec-Ellis, USGS. 

Graphs of the number of earthquakes located each year since 1980 in the Long Valley Caldera and nearby Sierra Block monitoring boxes. Note that the number of earthquakes on the left shown on a logarithmic scale, which means they increase an order of magnitude with each "step." Figure by Alicia Hotovec-Ellis, USGS.

This figure shows an approximation of the tectonic plate geometry of western California as it existed roughly 20 million years ago. A piece of the North American tectonic plate has been cut out to show how a slab window is formed when a spreading ridge is subducted.

This seismic station in the Lassen Volcanic Center was damaged during the Dixie Fire of 2021, which burned huge swathes of forest in the Lassen Volcanic National Park. Above-ground equipment, including the tree that held the antenna and solar panel above winter snows, was destroyed; belowground seismic sensors were recoverable. USGS photo by Ryan Presser, NCSN.

The California Volcano Observatory was created in 2012. Its logo shows Mount Shasta with a California grizzly bear (extinct) in the foreground.

The cryogenic magnetometer within the shielded room in the U.S. Geological Survey Paleomagnetics Laboratory. Major features of the cryogenic magnetometer are labeled. Picture by A. Pivarunas (U.S. Geological Survey)

Roughly a third of the total erupted volume of the Clear Lake volcanic field is represented by the ~ 35 km³ of rocks comprising Mt. Konocti and nearby hills. The mountain itself is over 1200 m (~4000 ft) high and is comprised primarily of a series of dacitic lava domes – Buckingham Peak, Wright Peak, and South Peak, and Howard Peak are all dacites.

Just as the Bartlett Springs and Maacama faults are the major bounding faults of the Clear Lake volcanic field, the Collayami (purple) and Maacama (green) faults bound the Geysers Geothermal Field.

The Clear Lake volcanic field erupted in association with and within the San Andreas Fault Zone. Although the San Andreas fault is the hallmark fault associated with the transform tectonic boundary between the Pacific and North American plates, the “boundary” between plates is more accurately described by a zone of faulting than by a single fault.

At over 4700ft (1440 m), Cobb Mountain is the tallest peak in the Mayacamas Mountains. Cobb Mountain is comprised entirely of ~ 1 million-year-old silica-rich volcanic rocks related to the Clear Lake volcanic field.

CalVO geologist Seth Burgess looking across Clear Lake at Mount Konocti, a prominent volcanic dome complex within the Clear Lake volcanic field. Photo courtesy of Alexander Rubin.

Block diagram of a normal fault, where the footwall drops below the level of the headwall

Block diagram of a reverse fault, where the headwall is being pushed above the level of the footwall

Block diagram of a lateral (strike-slip) fault, where two blocks are moving sideways past each other without vertical motion. 

Back-scattered electron images of olivine and chromium spinel crystals from primitive Clear Lake Volcanic Field samples. The labeled colored dots on the crystals show the locations where they were analyzed with an electron microprobe for geochemical information. Figure by Dawnika Blatter.

USGS CalVO Research Geologist Seth Burgess collecting a sample of old, primitive lava in the Clear Lake Volcanic Field for geochemical analysis. USGS photo by Dawnika Blatter.

This map, taken from a 2024 journal article by Furlong et al. 2024, shows the location and magnitude of seismic events (earthquakes) and rock type on a basemap of a portion of the west coast of North America.