Earthquake swarm rattles ground near new Calaveras Dam

A series of small earthquakes near the recently built Calaveras Dam replacement is a reminder of the significant seismic hazard at the new dam site.

By Melissa L. Weber, science writer (@Melwriter)

Citation: Weber, M., 2020, Earthquake swarm rattles ground near new Calaveras Dam, Temblor,

Note: article updated on Oct. 13

Aerial view of the Calaveras Reservoir, looking to the east. Calaveras Dam is located on the left (north) edge of the reservoir in the photograph. Credit: Dicklyon (CC BY-SA 4.0)
Aerial view of the Calaveras Reservoir, looking to the east. Calaveras Dam is located on the left (north) edge of the reservoir in the photograph. Credit: Dicklyon (CC BY-SA 4.0)


A magnitude-1.9 tremor on October 8 was the latest in a series of small earthquakes near Northern California’s Calaveras Dam, located about 38 miles (61 kilometers) southeast of San Francisco. It’s the latest quake in an ongoing swarm that started ramping up on August 17, 2020. More than 35 quakes ranging as high as a magnitude-3.5 have been recorded by the U.S. Geological Survey since September 27.


Map of Calaveras Fault and 2020 swarm
Over 70 quakes have struck in the area around the Calaveras Dam since mid-August.


Earthquake prone area

The Calaveras reservoir provides drinking water storage for residents throughout California, according to the San Francisco Public Utilities Commission (SFPUC), who manages the reservoir. It is the San Francisco Bay Area’s second-largest reservoir and thus a vital resource for the community.

The reservoir sits atop a major branch of the San Andreas Fault system called the Calaveras Fault. Although the area has seen little significant seismic activity over the past 100 years, paleoseismic studies at Leyden Creek about one mile (2 kilometers) north of the dam indicate that this section of the northern Calaveras fault has generated a number of ground rupturing earthquakes of possibly up to around magnitude-7.0 over the last 2,500 years (Kelson et al. 1996).


Those studies indicate timing of the most recent large earthquake is poorly constrained but suggest an average interval between surface rupturing earthquakes of 250 to 850 years, highlighting the uncertainty in what seismologists call the “recurrence interval” of major earthquakes. In other words, we don’t know for certain if this northern section of the Calaveras Fault is due for another big quake in the near future.

The SFPUC’s Completion Fact Sheet for the recently completed Calaveras Dam Replacement Project (CDRP) indicates that the fault runs only about 1,000 feet (300 meters) from the new dam (SFPUC, 2019).

Studies conducted in the late 1990s indicated that the original dam, which was built in 1919, didn’t meet seismic safety standards given its construction and proximity to the nearby Calaveras Fault. As a precaution, the water levels in the reservoir were lowered to about 40% capacity in an attempt to mitigate downstream hazard should a major earthquake compromise the dam. Construction on a new earth-and-rock-fill dam began in 2011 and was completed in May 2019, according to the SFPUC. The new dam was designed to withstand a magnitude-7.25 earthquake. Engineers moved 12 million cubic yards of earth and rock to make room for and build the new dam. That is enough to fill Levi’s Stadium from bottom to top four times, according to SFPUC.


Construction on the new Calaveras Dam was completed last year. Credit: SFPUC


New faults and landslides

The north-northwest trending Calaveras Fault makes a westward bend or stepover under the northern end of the reservoir, crossing from the east bank of the reservoir south of the dam to the west side of Observation Hill which forms the left (west) abutment of the dam, according to a technical report on the Calaveras Fault. During excavation of Observation Hill, faults that had never before been mapped as well as several previously unrecognized landslides were discovered in the planned foundation area of the new dam.

“After the landslides were removed, we found additional, smaller faults that had not been recognized during the original design studies for the new dam” said Rick Harlan, an engineering geologist who worked for the SFPUC’s construction management team during the dam replacement project. The dam design was modified to ensure it could accommodate small amounts of movement on these secondary faults, which might be triggered by a large local earthquake on the main Calaveras fault nearby, according to a United States Society on Dams report. Excavated areas were extensively mapped to maintain a precise record of the geologic features in the subsurface that could affect the new dam’s seismic resilience.


Faults exposed during dam construction
Construction on the new Calaveras Dam exposed faults not previously mapped. Credit: Rick Harlan (used with permission of SFPUC)


Earthquake swarm near new dam

Over 70 small quakes have struck beneath Observation Hill since mid-August. It is unclear on which faults these quakes are occurring, but this activity is a sign that the area is anything but quiet.

There are several reasons why these earthquakes may be occurring here. Most likely, the earthquakes are simply releasing built-up stress due to the ongoing motion along the Calaveras Fault, Harlan says. “I think the swarm is just a good reminder of seismic hazard that exists along the fault.”

“We really don’t know what causes an earthquake swarm,” said Roland Burgmann, a geologist at the University of California-Berkeley. He noted that there have been several large earthquakes linked to construction of dams and reservoirs in India and Egypt. “As I understand it, the excavated material [at Calaveras] was mostly put back into the nearby new dam embankment,” he said. Because of this, “the static stress change at depth must have been minimal.” This suggests that excavation during dam construction is not likely to be the cause of this swarm. Historical storage records suggest the recent re-filling of Calaveras reservoir is also not a likely culprit, since similar reservoir fluctuations have occurred in the past without triggering pulses of seismicity in that area.

Neither is construction of the dam itself, because excavated material was not removed from the site, but instead only shifted around over a small area, Burgmann said.

“Swarms may indicate other activity in the area,” said Burgmann. “There could be fluids in the fault zone, for example.” He suggested there might be areas of geothermal activity in the fault zone, as well.

Earthquake swarms usually end without a major event and generally die out within days to months. But “they always cause discussion and debate,” said Burgmann.

The SFPUC began refilling the Calaveras Reservoir last year, after dam construction was completed, and as of now it is at 62% capacity, according to the SFPUC. When full, it can store 96,850 acre-feet, or 31 billion gallons of water.

As far as dam safety, the SFPUC confirmed that there were “no impacts at all to the dam or its surrounding facilities” following any earthquakes in this swarm. This is a relief to the 27 million residents who rely on the Calaveras Reservoir.


Further Reading

USSD Dams and Levees Bulletin of the United States Society on Dams Summer 2019 Issue No. 178, pg. 8, Calaveras Dam Replacement Project.

Kelson, K. I., Simpson, G. D., Lettis, W. R., & Haraden, C. C. (1996). Holocene slip rate and earthquake recurrence of the northern Calaveras fault at Leyden Creek, northern California. Journal of Geophysical Research: Solid Earth, 101(B3), 5961-5975.

Kelson, K. I., & Sundermann, S. T. (2007). Digital compilation of Northern Calaveras Fault data for the Northern California map database: collaborative research with William Lettis & Associates, Inc., and the US Geological Survey.

Witter, R. C., Kelson, K. I., Barron, A. D., & Sundermann, S. T. (2003). Map of active fault traces, geomorphic features and Quaternary surficial deposits along the Central Calaveras fault, Santa Clara County, California. Final Technical Report to US Geological Survey, National Earthquake Hazards Reduction Program.

This article has been updated to reflect recent earthquakes in the swarm.