Archive for the ‘Hayward fault’ Category

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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.

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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.

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Lake Chabot Quarry, San Leandro

20 February 2023

You’ve all seen this quarry, looming over Lake Chabot Road and eating its way into Fairmont Ridge. It has a long history, and like all abandoned quarries it has a long afterlife ahead.


Seen from south Dunsmuir Ridge

The ridge was first opened up in 1886 by the Stone brothers, Egbert and Andrew, whose father Lysander made his fortune from the rich soil of the area now known as the Stonehurst neighborhood. Their construction firm, the E.B. and A.L. Stone Company, was a wide-ranging business that operated several quarries in the Leona Heights area.

By 1918 the quarry was owned by Joseph Costello, and in 1929 it came into the hands of the newly formed San Leandro Rock Company, which has owned it ever since although operations ceased before 2000.

Here’s the setting, from Google Earth. The quarry scar is in the upper center south of the dam at Lake Chabot.

The quarry exploited a body of rock mapped as basalt lava, of Jurassic age, that forms part of the Coast Range ophiolite. Here’s the same area of the geologic map. The Hayward fault is the solid black line just west of the quarry site.


Jurassic basalt (Jpb) with San Leandro gabbro to the west, Leona volcanics (pink) and Knoxville Formation mudstone (green) to the east.

And just for context, here’s the digital elevation model giving a closer view of the quarry site and the canyon of San Leandro Creek below the Lake Chabot dam, with the active strand of the fault shown in red and older strands in yellow.


Lidar data from Opentopography; fault traces from USGS. Illumination from the northwest.

The boulders that line the entryway are a good sample of what’s inside. They’re dominated by the dark, largely featureless basalt.

Here and there you can spot flow features and glassy regions that support the interpretation of this rock unit as pillow lava, the kind of blobby flows that form where red-hot lava meets freezing seawater. Hence the map symbol “Jpb” for Jurassic pillow basalt.

The quarry face itself isn’t half as well exposed, but beneath the rubble and grass the same material shows up in spots.

The thin white veins, when you can find a specimen that exposes them, appear to consist of hydrothermal quartz and olive-green chlorite. These rocks went through a few changes after the basalt first froze.

More entertaining is the view over the canyon and the Bay area beyond. You can see possibilities, whether you’re a would-be home developer or a would-be park planner.

The planner is the East Bay Municipal Utility District — the water company — which wants to buy the land and use the pit to dispose of the soil it digs up during trenching. It’s clean dirt, so no problem there; it’s useless land for anything else, so no problem there. And when they’re done, EBMUD wants to make it into a park. The problem emerges when they spell out the nitty-gritty in their project proposal: “The first stage includes using trench soils for fill operations for long-term phased placement and stabilization of approximately 3.4 million cubic yards of trench soil at the Quarry Site over approximately 40 to 80 years.” The problem is all those dump trucks over all those years on Lake Chabot Road.

The road that serves the quarry has always sparked contention. When it was the old back road between San Leandro and Castro Valley, the heavy-duty traffic from the quarry left the dirt-and-gravel road in rough shape. Eventually the route was paved, but the hillside is precarious and parts of it gave way during our latest wet season. All this before the next big earthquake.

The old road was bypassed by Fairmont Drive, a four-lane highway that was pushed over the ridge in the 1970s, but pleasure drivers, commuters and residents still use it. To preserve their quiet byway, the residents have opposed the heavy trucks of quarry traffic for fifty years, and they oppose it this time too. Today I’m fully on their side.

A possible route from the south would reach the quarry via Fairmont Drive. It would run up this valley, which is owned by the East Bay Regional Park District and is otherwise unusable because it runs right along the Hayward fault.

Considering that the park district and the water company are two trunks from the same root, maybe they could get along with a suitable easement through here.

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