Oakland Geology

Chapter 1 of Deep Oakland features the Hayward fault—Oakland’s prime mover. The energies that play out on the fault have built a distinctive East Bay landscape that has looked roughly the same as it does now for a least a million years, maybe two: a long ridge of mixed rock types no more than a kilometer high but never very low either; a middle ground along the fault where the hills, streams and valleys are strangely organized; and a wide alluvial flat and gentle coastal zone. A great wave from the ridgeline to the shoreline: Swell, surf and swash. The fault is the builder and maintainer of our special landscape. “The fault has made us beautiful,” I start off saying, “but it also keeps us on our toes.”

The Hayward fault has a central role in the book, and this chapter has a lot of ground to cover. I visit a few sites along the fault where its effects are easily seen; some (like this and this and this) are things I’ve posted about here.

In Deep Oakland I try, as I do on this blog, to approach the fault as a thing of interest rather than a source of dread. On the fault that maintains Oakland, construction is always in progress, destabilizing the land.

Some of the changes are slow and constant. Deep in the Earth at the base of the fault, the rocks warp like taffy in response to steady, relentless shear between the North America and Pacific plates. Higher within the Earth’s crust, where the rocks are hard and brittle, the same motion happens in bursts. Earthquakes! (Oops, there’s the dread.) And up near the Earth’s surface, in the top kilometer or two where conditions are gentler, the Hayward fault moves steadily, a few millimeters a year, in a counterpart of the deep shear known as creep.

Creep is the most disruptive thing the fault is doing right now. Over the years, creep damages whatever is on or under the ground.

The big work, meters at a time, is done in earthquake jolts. Big ones are occasional rather than regular, but they’re inevitable. The fault hasn’t ruptured the ground in a large earthquake since 1868, when Oakland was a small, ambitious town of about ten thousand people. The fault zone in the hills was a remote frontier, way out of town. In the years after the quake, as homes and roads and infrastructure moved in starting with Anthony Chabot’s dams, no one spoke of the fault (although Chabot built his dams extra strong).

So here we all are today (hi, me too), living on ground that’s bound to shake us down. How do we live with it? In the book I say it takes “the right mix of denial, preparation and equanimity.” How we should live with it is another question, with no easy answer.

Between quakes and creep, the fault keeps nudging Oakland’s landscape into unusual and precarious configurations. It gradually uplifts the hills; they respond by eroding and falling down. By pulling the land sideways, the fault disrupts all the streams that flow across it, adding kinks to their courses and swapping watersheds from creek to creek over the ages. And the rising hills on the one side urge the other side down, keeping it just below sea level and maintaining the beautiful San Francisco Bay. Without the Hayward fault and its siblings that together have built the Coast Range, Oakland would be just another Central Valley town. Oakland is earthquake country.

In the end, I come back to a long view of the fault: “Patiently over millions of years, it has arranged Oakland’s landscape, affecting all of its parts—and the lives of those in them—in ways it takes a geologist to appreciate.” The fault is a presence in each of the book’s eleven chapters.

PS: In the endnotes, Deep Oakland cites my blog posts as places to learn more about a topic. I figure this site will last about as long as the book does, and it’s archived too. It’s as good as paper for practical purposes.

This chapter refers to the following blog posts:

The fault at Stonewall
Shutter ridges
The Oakland fault
Fault gauge, 39th Avenue
Marks of the Oakland fault

_{Images: View from the self-storage building on San Pablo; Ney Avenue; Broadway Terrace at Lake Temescal}

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