The Calaveras Fault in Santa Clara County
Philip Stoffer, US Geological Survey — August 30, 2008
Text and photos by John Karachewski, from the June 2009 newsletter
On Saturday August 23, 2008, 30 friends and colleagues joined Dr. Philip Stoffer (USGS) to examine the geology and landscape features of the central Calaveras Fault along the western flank of the Diablo Range in Santa Clara County. This field trip was also conducted as part of the NCGS commemoration of the 1868 Hayward earthquake. The field trip included morning stops at Coyote Lake and Harvey Bear Ranch County Parks (near Gilroy) and afternoon stops at Anderson County Park (near Morgan Hill).
The Calaveras Fault is part of the greater San Andreas Fault system and extends for nearly 100 miles from the San Ramon Valley near Danville in the north to about 20 miles south of Hollister. The Calaveras Fault was named for Calaveras Creek located east of San Jose where it was first identified. The Calaveras Fault was first mapped, but not named, by Andrew Lawson in 1908. This field trip examined the central section of the Calaveras Fault, which is characterized by historic creep of as much as 14 to 15 mm per year near its southern end, and has an earthquake recurrence interval estimate of about 530 years. The central section of the Calaveras Fault has experienced numerous moderate earthquakes during the past several decades, including the M5.9 Coyote Lake earthquake of 6 August 1979, M6.2 Morgan Hill earthquake of 24 April 1984, and M5.6 Alum Rock earthquake of 30 October 2007.
Bedrock exposures in the area include the Mesozoic Franciscan Complex, Coast Range Ophiolite, and Great Valley Sequence rocks that are juxtaposed with late Cenozoic volcanic rocks and terrestrial sedimentary rocks, including alluvial gravels which were folded and offset by later faulting. The rural setting of this trip also preserves many geomorphic features related to the progressive deformation and mass wasting of the landscape, such as landslides, stream terraces, tectonic modification of stream drainages and alluvial fans. In addition, the field trip examined the impacts of dam construction and hydrogeologic development on Coyote Creek (Anderson and Coyote dams) and also provided scenic vistas of the Santa Clara Valley region. Evidence of past and ongoing creep and earthquake activity along the Calaveras Fault was also discussed.
The field trip began in the fog at the Mendoza Ranch entrance of Coyote Lake–Harvey Bear County Park several miles northeast of Gilroy. The Calaveras Fault is located a short walk west of the parking area and is expressed as a series of linear troughs with vernal pools or sag ponds during the rainy season. Although poorly exposed, vesicular basalts and Quaternary stream gravels are locally present. Coyote Ridge is a great shutter ridge uplifted along the west side of the fault valley. Mass wasting including active landslides and erosion of adjacent Coyote Ridge results in the accumulation of sediments within the fault zone.
Field trip stops along the west side of Coyote Reservoir included Lookout Point, with late Miocene basalts and mantle-derived xenoliths; Calaveras Picnic Area, with serpentine slickensides and fault gouge; and Calaveras Dam constructed in the vicinity of the fault zone. The original dam design had to be changed when it was learned that it was located within the Calaveras Fault zone. An extra year was required to reinforce and complete construction of the dam in 1936. The Santa Clara County Water District owns and maintains the dam and reservoir, which has a capacity of approximately 23,000 acre feet of water. A Cretaceous conglomerate consisting of granitic and quartzite clasts as well as spectacular soft-sediment deformation is exposed along the east side of the dam.
With the morning’s fog burned off, the participants hiked from the park’s Harvey Bear entrance to a large active landslide just east of the mountain front. Perennial springs are associated with this landslide.
The group ate lunch at Woodchoppers Flat near East Dunne Road, which eventually accesses Henry Coe State Park. This picnic area lies almost directly on the Calaveras Fault, which is masked by a large, partially active landslide. Many different rock types excavated during the construction of Anderson Reservoir are on display around the parking area. The rocks include Mesozoic Franciscan greenstone, greywacke sandstone, serpentinite, and chert as well as late Cenozoic volcanic rocks (basalts and hydrothermally altered rocks) and travertine (spring deposits). Massive deposits of cobbles are also exposed along the lake and are derived from weathering of poorly consolidated Quaternary Packwood Gravels that underlie the eastern shore of the reservoir.
In the afternoon, the group drove to the top of Anderson Dam, where participants had a beautiful vista overlooking Morgan Hill, the southern Santa Cruz Mountains, Loma Prieta Peak, and the Santa Clara Valley region. Anderson Reservoir, completed in 1950, is the largest man-made lake in Santa Clara County and can store about 91,000 acre-feet of water. Surface water from the reservoir is also channeled into percolation ponds, where it recharges sand and gravel aquifers that underlie much of the valley. Due to the recent drought, the group was able to hike along an unmaintained trail near the spillway to examine faults, cinnabar-bearing serpentinite, silica-carbonate rocks, and volcanic rocks.
The final stop was at the Live Oak Group Area, where participants hiked to a large outcrop area below the dam’s spillway to observe complexly folded and faulted Franciscan chert and greenstone.
The NCGS wishes to thank Dr. Philip Stoffer (USGS) and Dr. Richard Sedlock (San Jose State University) for preparing the richly illustrated guidebook. Unfortunately, only Dr. Stoffer was able to participate as a field trip leader. The NCGS also expresses gratitude to Tridib Guha for planning and organizing this interesting field trip to the South Bay.