By Paul Belz
“This is something you’d see in a textbook, right here in Oakland,” Andrew Alden told a group of 50 explorers who stood near the entrance to the Oakland Zoo. He showed a creek that flowed from the hills and turned abruptly to the right as it hit the Hayward Fault and described the creek’s behavior as a sure sign of a right lateral strike-slip fault line. His presentation showed that Oakland’s wildness is always with us, even in a quiet neighborhood.
The Hayward Fault is made up of the Pacific plate on the west and the North American Plate to the east. The North American plate side of the Hayward Fault remains still while the Pacific plate moves horizontally to the north at a rate of about 2 millimeters per year; this behavior defines it as a strike-slip fault.
Like other faults, the Hayward Fault is a geological zone instead of a visible crack in the earth. The Hayward Fault is 100 yards wide, but “Faults are not easy to see,” Andrew said. “They’re easier to see if you’re up in an airplane. You’ll see the alignment of landforms and how streams change directions.”
Our Hayward Fault is part of the San Andreas Fault system that extends from Cape Mendocino in the north to the Gulf of California in the south. The Hayward Fault extends along the base of the East Bay Hills from Alum Rock in San Jose to Point Pinole, about 74 miles. It parallels parts of Interstate 13, cuts straight under Hayward City Hall and the University of California football stadium, and passes through a number of cities including Oakland, Berkeley, Richmond, El Cerrito, Hayward, and Fremont. The Hayward Fault could give us a minute long earthquake with a magnitude of 7.1-7.6, causing the moving block to quickly shift two meters to the north. Another fault lies across the Bay from Point Pinole; if quakes occurred on both regions at once, the earthquake could reach 7.5 magnitude, putting about 150,000 housing units out of commission throughout the Bay Area.
The group walked through quiet neighborhoods with neat lawns, coast live oaks, and Monterey pine trees. Andrew, a writer and editor who has a degree in geology, pointed out how the curb on Encina Avenue leans northward along the road. He mentioned that motion along the fault often results in cracks in sidewalks, roads, gas lines, and water mains.
Andrew pointed out a steep vacant lot along a hillside, and said that there may have been houses here that had fallen apart because of the fault’s motion. The homes in this neighborhood were largely built between the 1920s and the 1950s, before the 1971 San Fernando earthquake led to zoning restrictions. Now new houses must pass geological site studies to make sure fault motion won’t make them collapse. No one will be able to build on the steep, empty lot!
Earthquakes occur on the Hayward Fault every 150 years on the average. They do not happen regularly- realistically, they can occur every 60 – 300 years. The last quake happened in 1868, 140 years ago. “I like the idea of earthquakes,” Andrew said. “I like the little ones. They’re cool, they’re like the earth talking to me. “ He shook his head when he mentioned the major quakes.
“This is a beautiful valley, a real nice place to live,” he continued. “It’s like living on a volcano. You can spend your whole life there and be fine, or you can be there when it goes off. These are chances we all take. If you live here, you know that!”
The explorers left the neighborhood and walked into King Estates Open Space. A woman named Ms. Ivy once owned this rolling grassland dotted with coyote bush, poison oak, and the occasional coast live oak tree before the City of Oakland acquired it in 1956. Bald, unforested hills like these were once typical of those parts of Oakland that lie outside of the redwood belt.
Hills and other spectacular changes in elevation are signs of an unsteady earth. 90% 0f the Hayward Fault’s energy is produced by the northward drift of the Pacific plate. The other 10% comes from pressure that pushed these hills upward. Depressions like Lake Temescal result when the two sides of the Fault pull away from each other. Andrew said that the gravel here was an unusual collection of pebbles that had been deposited by an ancient stream. He wanted to study this area to determine their origin point- the fact that they were not completely rounded indicated that the stream was a short one.
Andrew pointed out other features as the group circled back to the hike’s beginning point. Arroyo Viejo and other streams had shifted as much as 60 – 80 miles as the fault block moved north like a conveyer belt. “Scientists at the University of California, Berkeley study places like the Himalayas,” he concluded. “They don’t consider their backyard very interesting, but I think it’s exciting!” Oakland’s geology will give him material to investigate for years!