Greyhound Rock lies right next to one of the major faults within the San Gregorio Fault Zone. This fact might be no big surprise in earthquake-prone California, but what is startling is how ridiculously close we came to building a huge nuclear power plant right on top of this active fault zone.
In 1969, Pacific Gas and Electric (PG&E) announced it had leased 6,800 acres of Coast Dairies and Land property and intended to build a 6,000-megawatt-generating nuclear power plant at El Jarro Point, a wide marine terrace between Scott Creek Beach and Davenport Landing. The plant would have stood approximately where Swanton Berry Farm is now and required a major rerouting of Highway 1.
Creation of the plant was inadvertently derailed when a University of California, Santa Cruz (UCSC) geology graduate student named Jerry Weber took a walk on nearby Cove Beach in 1973. Weber noticed that a recent storm had eroded the sea cliff and revealed a large active fault in the cliff face. According to Weber:
“It is a classic exposure of fault movement. The fault had been completely covered by vegetation for probably hundreds of years. If not for the El Niño storms of 1972 and 1973, no one would have ever noticed it.”
In the 1970s, the San Francisco Bay Area was well-known for earthquakes along the San Andreas Fault, of course. But the San Gregorio Fault, which extends from San Francisco to south of Santa Cruz all along the coast, was just being discovered and mapped. Weber’s work was crucial in detailing the faulting around Año Nuevo State Park and Greyhound Rock.
By 1971, a passionate group of people was already on the move to stop the nuclear power plant at El Jarro Point. In September 1971, the Santa Cruz County Board of Supervisors voted 3 to 2 for a moratorium “in principle” on a permit for the plant. But it was really Weber’s discovery of the Frijoles Fault and other faults in the San Gregorio Fault Zone at Cove Beach that stopped the El Jarro Nuclear Power Plant dead in its tracks.
For a nuclear power plant to be approved, there can be no nearby active faults and, in geologic terms, “active” means a surface rupture within the last 10,000 years. By using radiocarbon dating of plant remains at the base of the cliffs at Cove Beach, Weber was able to prove clear movement within the San Gregorio Fault Zone at Año Nuevo/Cove Beach in the last 10,000 years.
Faults from the San Gregorio Fault Zone continue north and south and can be seen in the beach cliffs at Greyhound Rock. Greyhound Rock itself is not faulted.
The San Gregorio Fault is long and composed of many parallel faults. At Año Nuevo/Cove Beach, Weber has mapped at least six faults in a zone that is a mile wide. In fact, when you think of faults and earthquakes in general it’s best to think of fault zones or even a system of faults.
Indeed, the San Gregorio Fault Zone is one part of what is now referred to as the San Andreas Fault System because there are several faults involved, including the Hayward and Calaveras Fault Zones and the San Gregorio Fault Zone.
As you enjoy the California coast, keep in mind that it could start moving and shaking at any time. The San Gregorio Fault Zone has moved in the last 10,000 years and it will move again. We just don’t know when. Geologist Gerald Weber admits that he hopes the San Gregorio Fault Zone, which he spent much of his life laboriously mapping, might show some modest sign of movement while he is still alive.
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This piece is part of the Santa Cruz Marine Protected Areas Beaches Tour made possible by the Santa Cruz Collaborative with support from the California Marine Sanctuary Foundation and the Resources Legacy Fund. Download the free app with many tours of the Santa Cruz area and beyond.
- The Geology from Santa Cruz to Point Año Nuevo, with the San Gregorio Fault zone and Pleistocene Marine Terraces. Gerald E. Weber and Alan O. Allwardt. University of California, Santa Cruz
- Personal communication with Dr. Gerald (Jerry) Weber, Lecturer Emeritus University of California, Santa Cruz, Geologic Consultant. May and June 2016.