The Hayward fault is a crack in the Earth's crust about 100 kilometers long in the San Francisco Bay area. Since its last rupture in 1868, during California's frontier days, nearly 3 million people have moved next to the Hayward fault with little regard for its earthquake potential. The next time it produces a large quake, the costs will be staggering. Yet until that day, the Hayward fault is an interesting geological feature that is readily accessible. Let me show you more about it.
Where It Is
The Hayward fault (black) and its neighbors (gray). Click for full size. US Geological Survey image.
The Hayward fault is part of the wide plate boundary between the Pacific plate on the west and the North American plate on the east. The west side moves north with each major earthquake on it. Motion over millions of years has brought different sets of rocks next to each other on the fault trace. At depth, the Hayward fault merges smoothly into the southern part of the Calaveras fault, and the two may rupture together in a larger earthquake than either could produce alone. The same may be true for the Rodgers Creek fault to the north.
The forces associated with the fault have pushed up the East Bay hills on the east and dropped down the San Francisco Bay block on the west. The California geologic map will show you more.
The forces associated with the fault have pushed up the East Bay hills on the east and dropped down the San Francisco Bay block on the west. The California geologic map will show you more.
The Hayward Fault in Hayward
Offset street curbs are common in downtown Hayward. Geology Guide photo
In 1868 the little settlement of Haywards was nearest to the epicenter of the 21 October earthquake. Today Hayward, as it is now spelled, has a new city hall building that is built to ride on a lubricated foundation during a major quake like a kid on a skateboard. Meanwhile, much of the fault moves slowly, without earthquakes, in the form of aseismic creep. Some textbook examples of fault-related features occur in Hayward, at the center of the fault, and they are easily seen within walking distance of the Bay area's light-rail line, BART.
The Hayward Fault in Oakland
Cracked sidewalks are just one sign of fault movement in Oakland. Geology Guide photo
North of Hayward, the city of Oakland is the largest city on the Hayward fault. A major seaport and rail terminal as well as a county seat, Oakland is aware of its vulnerability and is slowly getting better prepared for the inevitable large earthquake on the Hayward fault. Because I live in Oakland, I have helped raise awareness by leading groups on tours of the fault. Here are some photos from one of them.
North End of the Hayward Fault, Point Pinole
Looking north on the fault trace at Pinole Point. Geology Guide photo
At its northern end, the Hayward fault runs across undeveloped land in a regional shoreline park. This is a good place to see the fault in its natural setting, where a big quake will do little more than knock you on your butt.
How Faults Are Studied
A pit exposure of the Hayward fault was shown to the public. Geology Guide photo
Fault activity is monitored using seismographs, which is important for research into modern-day fault behavior. But the only way to learn the history of a fault before written records is to dig tranches across it and closely study the sediments. This research, carried out in hundreds of places, has documented approximately 2000 years of large earthquakes up and down the Hayward fault. It appears that sizable events on the fault occur roughly every 150 years, with a large range of uncertainty. I've been to several trenching studies on the Hayward fault, and you can see some photos of the techniques used and the results here.
Geologists Tour the Hayward Fault
When a large meeting of geologists took place in the East Bay, one of the field trips organized around the occasion was a daylong tour of the Hayward fault given by geologists for geologists. I made sure to be there for this rare chance to hear, in depth and with scientific rigor, from the experts on the Hayward fault as we stood where they had worked.Hazard Maps Along the Hayward fault
Landslides can carry or bury dwellings. Geology Guide photo
The California Geological Survey has been steadily mapping around all of the state's major faults, not just the Hayward fault. One of its most widely useful products is a series of maps showing where the ground is susceptible to two different hazards due to earthquakes: landslides and liquefaction. Steep slopes, of course, are vulnerable to landslides whether they are up in the hills or along riverbanks. Liquefaction is the ground's loss of strength due to shaking. It can cause buildings to fail just as if they had fallen through thin ice. But steps can be taken to mitigate such hazards.
Transform Plate Boundaries
The Hayward fault is a transform or strike-slip fault that moves sideways, rather than the more common faults that move up on one side and down on the other. Nearly all transform faults are in the deep sea, but the major ones on land are noteworthy and dangerous. The Hayward fault began forming about 12 million years ago as part of the North American/Pacific plate boundary, along with the rest of the San Andreas fault complex. As the complex evolved, the Hayward fault at times may have been the principal active trace, as the San Andreas fault is today—and may be again.Transform plate boundaries are an important element of plate tectonics, the theoretical framework that explains the motions and behavior of Earth's outermost shell.
