Archive for the ‘Hayward fault’ Category

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Vicente Canyon

16 October 2023

Claremont Canyon and Temescal Canyon are big excavations into the rising Oakland Hills, and each is a traffic corridor. Between them is a smaller valley — a gulch, really — that I think is pretty distinctive, enough so to merit the title of canyon. I name it Vicente Canyon, after the stream running down it, Vicente Creek. All three canyons feed their runoff to Temescal Creek, as shown here in the county’s watershed map.

Take a close look at it (click it to see full size). The land surface is shown as a digital elevation model, and without all the distracting street names and such, some things are obvious. All three canyons are defined by the Hayward fault, which runs from top center to lower right. The hills on the east side are being pushed up, and the ridgetop is around 1300-1400 feet elevation. Vicente Canyon has only a slightly smaller elevation range, but it’s much shorter; that is, its slope is steeper. The two bigger canyons are long. Their streams, Claremont Creek and Temescal Creek, have stretches that are almost level, but not Vicente Creek. It’s working harder than its neighbors.

Here’s a thing people may not get about streams: they only erode and cut down their own narrow streambeds. Other than that, all they do is carry away what falls into them. As the Oakland Hills rise, the streams that drain them are compelled to keep cutting straight down to the level of the land at the fault, which is the local base level. This is a very effective process, geologically almost instantaneous — for example, you’ll notice that wherever tributaries join a larger stream, they always meet at the same level. The big stream is the base level for the little one.

Vicente Canyon has extra-steep walls that are ready to slide, and as I’ve explored this valley in recent weeks I’ve seen a lot of landsliding. Here’s a little example from high up in the catchment, on the unbuilt extension of Kenilworth Road.


A scoop of hillside has slumped and rotated on a curving underground surface. The tree died when its roots were torn off.

Down by the creek itself, the slopes are even steeper. This bit of Westview Drive suffered a fresh debris slide (what’s typically called a mudslide) just last winter, next to an older one.

The state’s official geologic hazard map corroborates me. Here’s the area that includes Vicente Canyon. The blue tint signifies areas at high risk of landslides.

The diagonal strip is the official “earthquake fault zone” defined by the state and the black line inside it is the trace of the Hayward fault. With that in mind, look at the digital elevation model of the same area.

See how the stream bends as it approaches the fault? It’s a great illustration of what the fault does to the landscape. The bottom side is moving leftward an average of about 10 millimeters a year. (The uplift part I referred to earlier is more like 1 mm/yr.) Humans can’t perceive movement that slow, but streams feel it as surely as we feel an escalator.

The rocks in Vicente Canyon are almost entirely mudstone of the same vintage as the rocks of Shepherd Canyon.

They’re rarely seen, though. They quickly turn back into the clayey sand they started from, as exposed in this excavation on Westview Place.

Once the creek crosses the fault, everything changes. The rock there is Franciscan sandstone, somewhat harder. The stream becomes a rocky gorge that winds through wooded yards for a few hundred meters to its junction with Temescal Creek, which is now culverted. The neighborhood is different, but not without its own charm.

All of Oakland’s high hill neighborhoods stack elaborate houses along narrow roads on steep, not-too-stable slopes, but because of its underlying configuration, the Vicente Canyon neighborhood is even more like that. At its best it presents a picture of architectural and engineering heroics in a lovely woodsy frame.

And especially along the canyon’s upper walls, the views across the fault to the San Francisco Bay basin are luxurious.

But the Vicente Canyon Hillside Open Space, a couple acres of oak woods between Vicente Road and Gravatt Drive, is a fine place for spry visitors. The locals manage a foundation that tends the land as a fire barrier.

Its benches have million-dollar views and none of the hassles of living up there.

Posted in Hayward fault, Landslides, Leona volcanics, Oakland geology views, Oakland streams and water | Leave a Comment »

Why you should see the Hayward fault in person

15 May 2023

Earthquakes have always shaken the Bay area, but for thousands of years residents have lived with them. Today we’re better off than any of our predecessors: we know just where the faults that cause earthquakes are located. We know where the ground is likely to rupture, and we even have some idea of when.

I submit that now is a good time to get to know the Hayward fault, and understand it a little better, before the next time it disrupts our lives. Though earthquake faults may be objects of dread, they’re safe to approach today, while they sleep (except for a little bit of creep).

The Hayward fault stretches from Point Pinole in Richmond to Alum Rock in San Jose, and its telltale signs aren’t hard to learn: lines of roadway cracks, skewed buildings, bent curbs, odd landforms. The fault may be hiding in plain sight, but it’s not a secret.

Since humans first arrived during the latest ice age, Bay area residents have known the land is prone to large, unpredictable earthquakes. The Indigenous tribes knew. The Spanish explorers and Mexican colonists who followed were not surprised, from long experience in their homelands. But starting in 1849, the Gold Rush brought a wave of naive outsiders to the Bay area. If the newcomers heard Mexican old-timers tell about the great shock of 1838 in Monterey, well, those were just stories, not experiences: there and then, not here and now.

Soon enough, California gave notice of its nature when the massive quake of 1857 shook the state from end to end. After that, major seismic events in the Bay area came hard and often. The newcomers wised up and adapted. Between 1858 and 1898 the Bay region experienced ten more earthquakes of at least magnitude 6 — the size of the Napa earthquake in August 2014. The largest of these, the “great San Francisco earthquake” of 1868, was the last big rupture on the Hayward fault.

By that time, builders in the young cities of the Bay area had learned to keep structures strong and low, no higher than three or four stories, to resist the effects of shaking. During the 1868 earthquake, the brand-new Wilcox Building, the tallest in Oakland at three stories, survived undamaged thanks to heavy iron bracing and still stands today at 9th Street and Broadway.

San Jose’s county courthouse also weathered it well. Architects had learned firsthand how to meet the demands of earthquake country.

The 1906 San Francisco quake, a great rupture of the San Andreas fault, gave birth to modern earthquake science. In the century that followed, California geologists learned to read the landscape in terms of tectonic movements along specific faults, active cracks in the earth’s crust. Today seismologists monitor active faults around the clock. Our phones can alert us to significant Bay area earthquakes before the shaking even arrives. Architects design quake-resistant structures, and building codes mandate them. We have good tools for living with earthquakes.

However, the Bay area faults have been much quieter than they used to be, producing only four magnitude-6 events in the last hundred years, none of them with epicenters in the central Bay area. During that century, the Bay area’s population has grown more than six times larger, and relatively few residents have ever experienced damaging shaking.

This matters especially for the East Bay, where cities grew up directly on the Hayward fault for decades before geologists mapped it. Modern practices have offset some of the resulting risk. But when the next big rupture happens on the fault — geologists put the odds during the next 20 years at about one in three — the region will be severely tested. As many as a million buildings in the greater Bay area will suffer damage, and tens of thousands of people will be displaced. It’s urgent to cope with this prospect in advance, but the task is not easy or simple.

We can’t depend on the Earth to keep reminding us about earthquake-resistant living. There’s a lot of slow, hard work yet to do — retrofitting or replacing vulnerable structures, teaching good practices to children and new residents, and maintaining disaster preparedness. The drive to sustain that work has to be embedded in our culture, generation after generation.

It’s easy to fear our active Hayward fault, and plenty of media stories reinforce the dread, but fear may not be the best long-term motivator. I believe that seeing this sleeping giant and taking its measure in person can help change attitudes. Two good places to see its marks on the landscape, with interpretive signs, are at Lake Temescal Regional Park in Oakland and in Fremont’s Central Park.

Geologists are wary of the Hayward fault, like the rest of us, but they also regard it with wonder and respect. They see the fault as an ancient channel of energy that has built our beautiful landscape over millions of years and maintains the landscape with each earthquake. As the Indigenous tribes might put it, Earthquake is our uncle, part of the country, who has a seat among us at the evening fire, and the Hayward fault is the place where it wakes and sleeps.

Posted in Hayward fault | 2 Comments »

Montclair spur

6 March 2023

One of my little geological fetishes is a geographic one: circumambulations. Thanks to the Hayward fault, Oakland has acquired several wineglass valleys, with very narrow mouths and wide headwaters. I’ve pioneered hikes that circle three of them: Claremont Canyon, Temescal Canyon and Shepherd Canyon.

These are strenuous outings, and I’m getting less and less young. This year I hope to resume them, but it will take some working up to. But I had a brainstorm: what about the faceted spurs between the wineglass valleys? I’ve written about faceted spurs before, but it felt kind of obscure so let’s try again.

Here on the 1897 topo map I’ve outlined the faceted spur that overlooks Montclair, between the narrow mouths of Thornhill and Shepherd Canyons. The fault runs from the upper left corner to the middle of the bottom edge. The spur is about 700 feet high and a loop on it is about 2.5 miles, as opposed to a further gain of 300-plus feet and 4 more miles for the circumambulation.

Visualizing this topography with the digital elevation map and picturing the hills covered with grassland, the way they used to be, I’m thinking this would’ve been a picturesque hike, on ridge routes the whole way.

You can see that the spur doesn’t have the ideal flat facet — it’s been dissected somewhat into small valleys — but the ridges that make up the rim are still nice and strong. And a lot of the streets run along the rim to offer a fair approximation of that 1897 hike. Here’s the route, starting from the end of the 33 bus line; I’ve taken it both ways and I feel fitter already. If you try this, be very cautious as you walk along Colton Boulevard.


Zoom in on the route at gmap-pedometer.com

The triangle of streets inside this loop isn’t part of either Thornhill Canyon on the north or Shepherd Canyon on the south; you might call it pure Montclair. It faces southwest, and as you climb you begin to peek over the Piedmont crustal block toward the Golden Gate. This view is from the north end, where the ridge runs east-west . . .

and this one is from the south end, where the ridge runs north-south.

On this ridge too, Asilomar and Drake Drives offer open views over the mouth of Shepherd Canyon toward the South Bay.

Note a couple of things in this view. The notch on the horizon behind the tree in the middle is where the San Andreas fault runs as well as Route 17 to Santa Cruz. The LDS temple to its right is where the Hayward fault runs, continuing right through Montclair along the freeway. The little valley that holds Montclair owes its existence to the fault, which grinds the rocks to an easily erodible state. Elsewhere along the route, you can look north along this valley, as here at the northernmost end of the Montclair Railroad Trail.

The two ridges meet just above the Forestland Reservoir, which is a nice quiet place to have a sit before starting down — or heading farther up the main ridge to the wonders of Skyline.

The rocks along the way are all pretty much the same: medium-grained sandstone of the Redwood Canyon Formation. The south end of the loop is mapped as the Shephard Creek Formation — sandstone plus shale — but you won’t see any of it.

I have no great insights or cool things to note about these rocks. Like I said, walks like this are a geographic fetish.

Posted in Hayward fault, Oakland geology walks | 2 Comments »

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