Where can you see earthquake faults?
Evidence of powerful fault movement is all around us (if you know where to look)
Reader Kathi C.'s submission to the Bay Area Bucket List project asks: Where are the earthquake faults? Any place we can see them?
The best way to see California's earthquake faults is to go to HBO Max and stream “San Andreas,” a 2015 action movie starring the Rock and Paul Giamatti that holds up surprisingly well.
No, seriously, evidence of fault lines is all around us — you just have to know what to look for. Here are some notable examples.
California Memorial Stadium
When football players take to the field at UC Berkeley's stadium, they're actually running right over the Hayward Fault. This is a significant fault that slices through the East Bay and is known as a “tectonic time bomb,” because some believe it's ready to explode with a large and damaging quake.
Head up the stairs from Piedmont Avenue, then circle right until you're at the stadium's southern perimeter behind seating section KK. Here you'll see a gap about the size of your fist running from the bottom of the stadium wall all the way to the top.
“Over time, this section of the Hayward (fault) creeps — it doesn't wait for a major earthquake to jump and move, it's slowly creeping, creeping, creeping. And you can notice the offset over time,” says Jennifer Strauss, external relations officer at the Berkeley Seismology Lab.
“After a while, you can't just let the stadium keep creeping like that,” Strauss continues. “So they had to do a rebuild (completed in 2012). This was a little controversial because people were like, `Hey, maybe we don't continue to rebuild the stadium on a fault that keeps moving.' It costs maybe half a billion dollars to reconstruct the stadium, and it's not like you reconstruct it one time and it's done. It's going to keep happening.”
The engineering solution to this endless creeping (at a rate of a few millimeters per year) was to install a host of dampening equipment and divide the stadium into halves, so the whole thing
isn't shattered in a quake but instead moves with it.
“Obviously the stadium is used for games, so it's not like there's a big crack in the middle of the field,” Strauss says. “But on the outside of the stadium, you can actually see it's in two pieces, so that the stadium stands can move with respect to each other.”
For more info about where to experience earthquake jollies around Berkeley's campus, the seismology lab offers a map and self-guided walking tour of the Hayward Fault at seismo.berkeley.edu/hayward/hayward_tours.html.
Point Reyes
The 1906 earthquake was so strong, it knocked a train heading from San Francisco toward Point Reyes right off the tracks. At the Point Reyes National Seashore, you can see another sign of its immense power — a fence that “jumped” 16 feet in a matter of seconds.
Head into the parking lot by the visitor center and walk to the left to start the Earthquake Trail, an easy 0.6-mile loop through fields and forest. Here you're right in the thick of the San Andreas
Fault system, the granddaddy to all the faults in the Bay Area; the Hayward is part of that system.
The San Andreas stretches roughly from the Salton Sea in Southern California up to Cape Mendocino, where it heads out into the ocean. It marks the location where the North American and Pacific tectonic plates grind inexorably past each other. About halfway into the Earthquake Trail, you'll come to a fence split into two pieces at a distance far enough to toss a baseball. This is a replica of a historic barrier,
probably demarcating a local ranch, torn asunder by the 1906 quake.
“Basically what that means is a lot of movement occurred on one strand of the fault system of the San Andreas right there on the Earthquake Trail. It was the major break point for the earthquake in that area,” says Will Elder, a visual-information specialist at the Golden Gate National Recreation Area.
How active is the San Andreas zone today? “There are sections where it's basically locked up, and that's the case around Point Reyes,” says Elder. “The sections that are locked up seem to have earthquakes of large magnitude — 8.0 or so — and they happen every 100 to 300 years. Then on the decade scale, you get smaller earthquakes in the sections where there's movement.”
The last earthquake of consequence in Point Reyes was the big one of 1906. But who knows? That fence could jump another 16 feet soon. “It's been more than 100 years for the San Andreas,” Elder says. “So it's due at some point — in the next 150 years probably.”
Sag ponds and lagoons
Many large bodies of water in the North Bay (where we get our delicious oysters from) are also shaped by fault activity.
“Tomales Bay is basically a block of rock that's been shattered by the faulting of the San Andreas, so it forms the bay,” Elder says. “Likewise, Bolinas Lagoon on the other side of Point Reyes reflects where the San Andreas goes through. The faults shattered the rock, making it weak there, and it easily eroded away.”
Crystal Springs Reservoir and San Andreas Lake on the San Francisco Peninsula are the same kind of animal. And if you hike at Los Trancos Open Space Preserve and the nearby Monte Bello Preserve, you'll encounter another unique water feature — “sag ponds,” depressions caused when two sides of a fault move.
“There's a trail there along Skyline Boulevard (near Page Mill Road) that goes along those sag ponds. It's pretty cool,” Elder says. “There's just a row of ponds on the top of the hill — very unusual to see that sort of feature on the top of a hill.”
Shifting curbs
Sometimes the terrible streets in the Bay Area aren't just the result of weather and traffic. Fault movement ever so slowly pulls roads apart, which is most evident in misaligned curbs that look as if they were kicked by a giant.
Andrew Alden at Oakland Geology has put together a pictorial guide for Oakland (find it at oaklandgeology.com/2016/07/11/ marks-of-the-oaklandfault), full of wonky curbs and cracked and stretched asphalt.
Some of them may not exist anymore — municipalities are always running out to fix damage caused by wandering faults. In fact, one curb fixed by the city of Hayward in 2016 garnered some negative media attention. It turns out geologists and geology fans had come to love the curb for its graphic demonstration of the Hayward Fault. But you won't find Strauss of the Berkeley Seismology Lab staff crying about it.
“It's really cool to see the fault move on the curb,” she says, “but we have many different ways to make those kind of measurements that don't require the curb to stay creeping forever. Eventually it's going to be in the road.”