The Hayward fault at Warm Springs

17 April 2017

Every extension of BART opens up a new region accessible to geologizers using public transit. So the other week I paid a visit to the far end of the Hayward fault, less than a mile from the new Warm Springs station in south Fremont. The station has nice views of the San Mateo Peninsula mountains to the west and Mission Peak to the east.

It appears, too, that the Irvington Gravels site to the north is accessible for determined walkers who bring provisions — that is, hikers.

To get to the fault just walk east on South Grimmer Boulevard toward the place marked “Weibel” on Google Maps.

Here’s the same area in Jim Lienkaemper’s detailed 1992 map of the fault. The map has a key to all the annotations. Note that both images are tilted to make the fault vertical; north is at about 1:30.

The fault runs through the “D” in “Blvd.”

Look back at the Google Maps image. See the line of green along the fault trace? That’s because of the 1972 Alquist-Priolo Act, which forbids new construction within 50 feet of an active fault. The area in the middle must have been built up before the act took effect. That’s where I went.

South Grimmer reveals the offset from fault creep well. This view is looking east toward Mission Peak. On the fault map, the locality (just below the horizontal dashed line) is circled and labeled “C1,rc,rf” signifying “strongly pronounced” evidence of creep in the form of right-offset curbs and a right-offset fence line.

And this is the other side of the road, looking west. Notice that the sidewalk is offset as well as the curb.

There’s another, much smaller offset higher up the slope that I didn’t get a good picture of. Repeated measurements show that together these offsets add up to about 6 millimeters per year. The slope itself is a sign of the fault, too.

To the north across little Arroyo Agua Caliente Park on Gardenia Way, this nice set of echelon cracks marks the fault trace. That’s what the “ec” in the circle labeled “C1,ec,rc,cc” stands for.

The fault nips the corner of Gardenia and Ivy Way, bending this curb (the “rc” in the label).

The city or the homeowner copes with the sidewalk by patching it as needed. You’ll see stuff like this everywhere on the Hayward fault.

Walking north through the park to Parkmeadow Drive on its north edge, you can look west down the street and see both an offset curb and the change in slope that marks the fault.

You can do this yourself all along the fault. The map has all the evidence (and the USGS has an updated version as of 2008).

A week later I hosted two French journalists — a writer and a photographer — for an afternoon, showing them fault offset features like these up in Hayward and Oakland. The writer went and spoke to a resident whose home was on the fault, and his fatalistic response took her aback a bit. She said “we don’t have attitudes like this in France.” I told her we Californians have been this way since the Gold Rush.

Shepherd Canyon landslides

10 April 2017

Last week I went to visit a landslide that had been in the news. As it happened, I saw three.

Shepherd Canyon always gets a lot of landslides, like its neighboring canyons in the high hills. The main reason is that Shephard Creek has a lot of cutting power, thanks to its relatively large watershed and the low base level provided by Dimond Canyon. That creates steep slopes and V-shaped valley profiles. A secondary reason is the relatively soft mudstone underlying those slopes.

My destination was the landslide that came down on the south side of Banning Drive. But along the way my path was blocked by two more mass movements. They’re marked by white asterisks on the geologic map below.

The Montclair Railroad Trail, my usual route, offers walkers good access to the canyon. On the inner side of the sharp curve and cut leading into the canyon, this slope failure exposed the rears of two houses. I classify it as a debris fall.

The majority of the material is broken rock, hence the term debris, and it tumbled in a heap rather than traveling any distance, hence the term fall. Only a little mud was present.

The area is mapped as the Oakland Conglomerate (Ko), although the debris appeared to consist mostly of fine sandstone and a little shale, like this. The rainwashed stone is well displayed.

Picking my way past that was no problem. Farther up the trail, though, was a complete blockage.

Like the lower slope failure, this one involved debris, but unlike it the material slid, so I classify it as a debris slide. Several large trees that came down with the rock didn’t appear to be to blame. However, this time of year is the most dangerous for trees because the ground is sopping wet and the limbs are heavy with young leaves, making them prone to catch the wind. Maybe they triggered the slide. Maybe the other way around.

Fortunately no houses appeared to be threatened above the headscarp, but now the slope is highly vulnerable.

A sewer line runs beneath the trail, so the city may have to clear the slide once the ground is no longer saturated. Meanwhile this is too dangerous to approach. It could fall with no warning.

The debris is made of fine-grained sandstone of the Redwood Canyon Formation (Kr).

Finally I got up to Banning Drive. It’s situated in one of the major side valleys in Shepherd Canyon, and the walls are exceptionally steep.

I classify this slope failure as a debris flow, what the news media often calls a mudslide. It traveled downhill a good hundred meters in a thick semifluid mass. The mud content was greater than the other two slides, and muck spilled into and around several homes on Banning. There’s plenty of footage of the scene online, so I don’t need to show you that. It was hard to watch the residents clearing out their red-tagged homes while the news vans gathered round.

I didn’t need to be there once I’d seen it. Presently I went uphill to Aitken Drive, where the slide originated.

Note a couple of things. Right beyond the gap in the road, a telephone pole was snapped off and the wires were hanging low. (The power was off.) The extra load caused the pole at the left edge of the photo to lean inward. The scar in the road reveals a wall of sandbags (I assume they were filled with concrete) that must have been put there after a previous slide.

Landslides occur where previous slides did. And sure enough, looking uphill I could see the young scar of a small rockslide, nestled in turn within a concavity in the hillside that looked like the scar of a much older slide.

There is another street higher up, Chelton Drive, but no houses up there appeared to be endangered. Meanwhile East Bay MUD had the road blocked while they were making sure the water lines underneath wouldn’t break and make more trouble.

Who’s responsible? Perhaps no one. The problem is above my pay grade, as I’m not a licensed geologist. But I can see the signs and warnings of landslides, and so can you if you pay attention to the landscape. The U.S. Geological Survey has resources, and so does the California Geological Survey.

East Bay diatomite

3 April 2017

The geologic map I rely on for this blog — U.S. Geological Survey map MF-2342 — extends north to Pinole, where it shows this little pod of rocks labeled “Tsa” and “Tdi” between Pinole, El Sobrante and Richmond.

Both units are of early Miocene age: Tsa stands for sandstone and Tdi stands for diatomite. The T stands for Tertiary, the catch-all term for Cenozoic rocks older than Quaternary, which — OK, you don’t need the whole lecture just now. The point is, I had to go see this diatomite because I didn’t know it existed in the East Bay. I’ve seen it in the Central Valley, but never around here.

Going north on I-80 you take the Appian Way exit right and immediately turn left on Sarah Drive. Down at the bottom of a valley is Pinole’s little, undeveloped Sarah Drive Park.

On the way to the hilltop, you start seeing this odd rock in the road. Pick up a piece and you’ll find it’s very light. That’s the diatomite.

The trail becomes very steep, exposing the bedrock. The hilltop affords nice views. I was especially taken with the view north.

And the view east looks up Pinole Valley toward Mount Diablo on the horizon. If you’re riding toward Sacramento on the Capitol Corridor Amtrak train, there’s a moment just east of Point Pinole where you can catch this same view of the mountain.

And there were butterflies.

So that’s all great. But here’s what’s cool about the diatomite.

Diatomite is composed of diatoms, the microscopic algae that make shells of silica. As an industrial commodity it’s also called diatomaceous earth, or DE, or kieselguhr if you’re feeling smart. As the stabilizing agent for nitroglycerin in dynamite, it made Alfred Nobel’s fortune, and that’s why we have the Nobel Prize.

As its silica content slowly turns into the crystalline mineral quartz, diatomite becomes the rock called chert. As it happens, the Pinole diatomite is about the same age as the chert in Oakland’s Claremont Shale. By some tectonic accident, it avoided being converted, and you can enjoy its lightweight charm without a trip to Los Banos.