Earthquake swarm shakes ground near New Calaveras Dam

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

Written by Melissa L. Weber, science writer (@ Melwriter)

Quotation: Weber, M., 2020, Earthquake Swarm Shakes the Earth Near New Calaveras Dam, Temblor, http://doi.org/10.32858/temblor.124

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

The magnitude 1.9 earthquake on October 8 was the latest in a series of small earthquakes near the Calaveras Dam in northern California, 38 miles (61 kilometers) southeast of San Francisco. It is the latest earthquake in a continuous swarm that began to escalate on August 17, 2020. The United States Geological Survey has recorded more than 35 earthquakes of magnitude 3.5 since September 27.

More than 70 earthquakes have hit the area around the Calaveras Dam since mid-August.

Earthquake-prone area

The Calaveras Reservoir provides drinking water storage to residents across California, according to the San Francisco Public Utilities Commission (SFPUC), which manages the reservoir. It is the second largest reservoir in the San Francisco Bay Area 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 seismic activity over the past 100 years, ancient seismic studies at Leiden Creek about 1 mile (2 km) north of the dam indicate that this portion of the North Calaveras Fault has caused a number of earthquakes. That tear apart the Earth which is likely up to about 7.0 degrees over the past 2,500 years (Kelson et al. 1996).

These studies indicate that the timing of the most recent large earthquake is poorly restricted, but they indicate the average interval between surface rupture earthquakes from 250 to 850 years, highlighting the uncertainty in what seismologists call the “repeat period” of major earthquakes. In other words, we don’t know for sure if this northern part of the Calaveras Fault is caused by another major earthquake in the near future.

The SFPUC completion fact sheet for the recently completed Calaveras Dam Replacement Project (CDRP) states that the fault extends only about 1,000 feet (300 m) from the new dam (SFPUC, 2019).

Studies in the late 1990s indicated that the original dam, built in 1919, did not meet seismic safety standards due to its construction and proximity to the nearby Calaveras Fault. As a precaution, water levels in the reservoir have been reduced to around 40% of capacity in an effort to mitigate downstream hazards in the event the dam was exposed to a major earthquake risk. Construction of a new dam to fill the earth and rocks began in 2011 and was completed in May 2019, according to SFPUC. The new dam is designed to withstand a magnitude 7.25 earthquake. Engineers have moved 12 million cubic yards of earth and rock to make way for the construction of the new dam. That’s enough to fill Levi’s stadium from bottom to top four times, according to SFPUC.

The new Calaveras Dam was completed last year. Credit: SFPUC

New breakdowns and landslides

The Calaveras Fault heading from north to northwest makes a bend or tiered westward under the northern end of the reservoir, and passes from the eastern bank of the reservoir south of the dam to the western side of the observation mound that forms the left (west) pillar of the reservoir. The dam, according to a technical report on the Calaveras Fault. During excavations at Observation Hill, previously unmapped faults and numerous previously unrecognized landslides were discovered in the planned foundation area for the new dam.

“After the landslides were removed, we found additional, smaller faults that were not recognized during the original design studies for the new dam,” said Rick Harlan, an engineering geologist who worked with the SFPUC construction management team during the dam replacement project. The design of the dam was modified to ensure it was able to absorb small amounts of movement on these secondary faults, which may be caused by a large local earthquake on the nearby Calaveras main fault, according to the United States Dam Association Report. The excavated areas have been extensively mapped to maintain an accurate record of geological features in the subsurface that could affect the dam’s new earthquake resistance.

Construction on the new Calaveras Dam revealed faults that had never been previously painted. Credit: Rick Harlan (user with permission from SFPUC)

Seismic swarm near the new dam

More than 70 small earthquakes have struck the hill since mid-August. It is not clear which faults these earthquakes will occur, but this activity is a sign that the area is not calm at all.

There are several reasons for these earthquakes to occur here. Most likely, Harlan says, earthquakes simply release a build-up of pressure due to the continuous movement along the Calaveras fault. “I think the swarm is just a good reminder of the seismic hazards that exist along the fault.”

“We don’t really know what causes an earthquake,” said Roland Burgmann, a geologist at the University of California, Berkeley. He pointed to the occurrence of several large earthquakes associated with the construction of dams and reservoirs in India and Egypt. But historical storage records indicate that a recent refilling of the Calaveras reservoir is not the likely cause, given that similar reservoir fluctuations occurred in the past without emitting seismic pulses in that area.

“The swarms may indicate other activity in the area,” Burgmann said. “There could be fluids in the rift zone, for example.” He suggested that there may be regions of geothermal activity in the rift zone as well.

Earthquake swarms usually end without a major event and generally die within days to months. “They always spark debate and debate,” Burgmann said.

SFPUC began refilling the Calaveras Reservoir last year, after construction of the dam was completed, and now it has a capacity of 62%, according to SFPUC. When completed, it could store 96,850 acres a foot, or 31 billion gallons of water.

Regarding the safety of the dam, SFPUC confirmed that “there were no impacts whatsoever to the dam or its surrounding facilities” after any earthquakes in this swarm. This is a relief for the 27 million residents who depend 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, KI, Simpson, GD, Lettis, WR, & Haraden, CC (1996). Holocene slip rate and earthquake frequency of the North Calaveras fault in Leiden Creek, Northern California. Journal of Geophysical Research: Solid Earth, 101 (B3), 5961-5975.

Kelson, KI, & Sundermann, ST (2007). Digital Aggregation of Northern Calaveras Fault Data for Northern California Map Database: Collaborative Research with William Lettis & Associates, Inc. And the US Geological Survey.

Witter, RC, Kelson, KI, Barron, AD, & Sundermann, ST (2003). Map to trace the active fault, geomorphic features, and quadrupedal surface sediments along the Calaveras Central Fault, Santa Clara County, California. US Geological Survey Final Technical Report, National Earthquake Risk Reduction Program.

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