The quake was reported at 10:35 a.m. and was centered east of Alum Rock Park. Lian Xue, Roland Bürgmann , David R. Shelly, Christopher W. Johnson, Taka’aki Taira (2018), Kinematics of the 2015 San Ramon, California earthquake swarm: Implications for fault zone structure and driving mechanisms, in press, Earth and Planetary Science Letters. 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. The Calaveras Fault, like the San Andreas Fault to the west, is a right-lateral strike-slip fault that is part of the San Andreas Fault System in the San Francisco Bay region. The San Andreas fault system is to the west, the Garlock fault is to the south and the faults of the Sierra Nevada are to the east. “Swarms may indicate other activity in the area,” said Burgmann. Some scientists predict that the Bay Area's the next major quake will occur on this fault. It is also being subducted far to the northwest into the Aleutian Trench. The fault divides into three segments, each with different characteristics and a different degree of earthquake risk. At that time, most U.S. seismic observatories were run by scientist priests at Jesuit universities. Credit: Dicklyon (CC BY-SA 4.0). Construction on a new earth-and-rock-fill dam began in 2011 and was completed in May 2019, according to the SFPUC. They came one after the other along the Calaveras Fault in Santa Clara County. The Bay Area fault system and the spot (red star) where the Hayward Fault branches off from the Calaveras Fault. During the past week, 75 quakes have rattled the beautiful town of Danville, scene of the beloved Robin Williams movie, “Mrs. The Calaveras fault here is unusual because it creeps, moving a few millimeters per year, instead of storing up energy and releasing it all in a big earthquake. The first quake was reported at 6:58 a.m. USGS Swarms likely light up portions of faults that suddenly begin to creep—or slip at a much higher rate than usual. Kelson, K. I., Simpson, G. D., Lettis, W. R., & Haraden, C. C. (1996). There are several reasons why these earthquakes may be occurring here. This is a relief to the 27 million residents who rely on the Calaveras Reservoir. 's (1990) Zone V "stuck patch" on the central Calaveras fault, providing new support for their model of Calaveras fault earthquake activity. Nevertheless, should this swarm penetrate the Calaveras Fault, the chances of a larger shock will climb, and the monitoring vigilance will intensify. Most researchers believe that strain accumulated along the Calaveras south of Calaveras Reservoir is transferred to the Hayward fault on the Mission Hills fault (Oppenheimer and Macgregor, 1991). 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. are the most studied and understood fault systems in the world. 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. ... Like all of California, the Central Valley—which is ringed by faults—is earthquake country. Both occurred on Oppenheimer et al. “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. “We really don’t know what causes an earthquake swarm,” said Roland Burgmann, a geologist at the University of California-Berkeley. 8, Calaveras Dam Replacement Project. Construction on the new Calaveras Dam was completed last year. Along some active faults, deformation and offset occur continuously even in the absence of large earthquakes; this is referred to as fault creep which generally proceeds at rates of millimeters to tens of millimeters per year. It struck along the Calaveras Fault and had a depth of 7.4 kilometers, according to the USGS. The Hayward Fault runs from San Pablo Bay to Fremont. 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. Another swarm struck the central portion of the Calaveras Fault in April 2016 (http://temblor.net/earthquake-insights/calaveras-535/); neither triggered a larger shock. But by the time these professors began to die in mid-century, their lifelong seismogram archives were thrown out. The 2 to 8mm/yr range of creep rates along the HF is well documented by over 30 alignment arrays (AAs) and 5 creepmeters covering … The largest historical quake on the Calaveras Fault was a M=6.6 event in 1911 (Doser, 2009), but far too little is known about this large event. Earthquake Information for Calaveras County, California. Some 600 quakes struck San Ramon, also near the Calaveras Fault, in October 2015 (Xue et al, 2018). However, the rms residuals for P-wave arrivals at many of the network stations were as high as 0.25 sec due to the laterally heterogeneous structure of this area which deviates from the … UC Berkeley seismologists have proven that the Hayward Fault is essentially a branch of the Calaveras Fault that runs east of San Jose, which means that both could rupture together, resulting in a significantly more destructive earthquake than previously thought. The Calaveras fault system runs over 80 miles, from south of Hollister into the Danville-Walnut Creek area. Credit: Rick Harlan (used with permission of SFPUC). Doubtfire.” The quakes are occurring at a depth of 5-7 km (3-4 mi) on a heretofore unmapped fault or set of faults, and exhibit a diversity of mechanisms, some similar to the ‘right-lateral’ (whichever side you are on, the other side moves to the right) Calaveras Fault, and others perhaps related to the ‘blind thrust fault’ that has jacked up Mount Diablo. At the southern end of its central stretch it creeps at about 14 mm/yr dropping northward to about 6 mm/yr (0.24 inch/yr) of creep at its … Why faults start and stop creeping is a mystery, but most swarms and creep events do not cascade into larger earthquakes. Earthquake scientists are far more worried about the potential violence of a more northern stretch of the Calaveras Fault, between the towns of Danville and San Ramon.  The geometry of fault zones within the Earth is revealed primarily by seismicity. It is unclear on which faults these quakes are occurring, but this activity is a sign that the area is anything but quiet. 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. Within a minute, it was followed by a magnitude 3.6 quake along the Calaveras Fault in Santa Clara County. future moderate earthquakes on the Calaveras Fault. That is enough to fill Levi’s Stadium from bottom to top four times, according to SFPUC. The Hayward Fault splinters from the Calaveras Fault, which itself is an offshoot of the Fault connectivity defines the maximum earthquake rupture length, and the three-dimensional (3D) geometry of faults affects the dynamics of fault rupture, slip, and ground motion (11–15). The similarity of seismograms recorded by two seismic stations demonstrate that the 31 October 2007 moment magnitude M5.4 Alum Rock earthquake is a repeat of a 1955 M L 5.5 earthquake. 1996). It is the San Francisco Bay Area’s second-largest reservoir and thus a vital resource for the community. By Melissa L. Weber, science writer (@Melwriter), Citation: Weber, M., 2020, Earthquake swarm rattles ground near new Calaveras Dam, Temblor, http://doi.org/10.32858/temblor.124, Aerial view of the Calaveras Reservoir, looking to the east. 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