Archive for the ‘The Hayward fault’ Category

South Dunsmuir Ridge

29 May 2017

I finally got to a sweet corner of town last week, the sunny side of Dunsmuir Ridge, this lovely hill in the Google Maps 3D view.

The view is to the north-northwest, such that the Hayward fault runs straight up about a thumb’s width from the left edge. The maps below start with the 1915 topo map, in which the ridge’s top is the lobed outline of the 625-foot contour.

That straight creek valley along the hill’s south side — the gorge in the foreground of the top image — keeps catching my eye, but it seems to be inaccessible, which might make it Oakland’s wildest piece of land. The watershed map below may help in visualizing the hill and its surroundings. The two black dots are where the fire trail I took starts and ends.

Dunsmuir Ridge is city land, rescued from development after several aborted attempts to put high-end estates on this broad hilltop overlooking (in both senses) the deadly Hayward fault. The fire trail starts at the end of Cranford Way and winds up the ridge to join the fire road from the other side, which I’ve featured here before.

The walk is very scenic. To the north, downtown rises against Mount Tam.

Or if you prefer, there’s the new profile of San Francisco.

Higher up, the view opens out. Here San Leandro Creek is made visible as a line of trees coming out of the canyon toward its mouth near the airport.

But the main attraction is to the south. This is the best place to take portraits of Fairmont Ridge and its quarry scar. Unlike most places, this trail sets off the hill with a foreground of wild, forested land.

The prominent cleared space midway up the trail — a staging pad for firefighters — has regular visitors who find the spot special.

Interestingly, this spot is mapped as a patch of the peculiar Irvington-aged gravel that first brought me to Dunsmuir Ridge in 2009. However, I didn’t notice much of it, if any. See it on the geologic map — the white dots mark the ends of the fire trail.

There are rocks to be seen too. The soil is thin in most places. This little cut displays a profile of the soil and the decaying bedrock — saprolite — just beneath it.

The bedrock varies, and it doesn’t match the geologic map very closely. I would say nearly all of the lower part is not Leona volcanics (Jsv) but San Leandro Gabbro (gb). It has the gabbro’s pepper-and-salt appearance but is stained orange instead of the pristine rock’s bluish gray (as I saw earlier that day in San Leandro). You’ll see it well exposed in the trail itself, where this winter’s heavy rains carved fresh runnels.

If the city fills them before you get there (which it should before they become gullies), there are still roadside exposures that display the rock well, and it’s unmistakably gabbro where the map says volcanics. The top of the hill, though, is unquestionably Leona volcanics.

My long-term plan is to revisit every bit of bedrock in Oakland and log it. Besides sheer nerdery and the chance to improve the map, my motive is to come back to views like this one over and over again.

The old quarry is still for sale. Developers have tried to put houses there, but they keep getting shot down. Better, I say, for the Regional Parks District to acquire the land and develop it for quiet recreation.

The East Bay Seismic Investigation

12 September 2016

On Friday I got my first look at a seismic survey that will be visualizing the Hayward fault (and the Chabot fault for good measure) this fall. It involves a 15-kilometer line of several hundred seismometers stretching from the San Leandro shoreline to Cull Canyon. A team of geologists from the US Geological Survey and Cal State East Bay will set off 19 small explosions along the line to serve as energy sources, and the seismometers will record the sound waves as they arrive, after traveling through the various layers of the ground. Here’s the line of charges (as usual, click it for the full-size version); the seismic stations are too numerous to bother showing.

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The Hayward fault is the red line, the Chabot fault is the blue line, and I’ve added the lidar swath along the fault between the light-blue lines. The purple line is the little-known Miller Creek fault.

The occasion on Friday was a photo op for the press. The team drilled a 30-foot hole in which charge number 9 will be placed, and a bunch of reporters took a bunch of footage and asked a bunch of questions.

It was a pretty spartan setup. That’s USGS press officers Leslie Gordon waving on the left and Susan Garcia standing on the right. I see them all the time at science meetings.

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It can be very helpful to have press officers around to answer basic questions in press-friendly ways, follow up with extra pictures and so forth.

Here’s the press gathering footage as Joanne Chan of the USGS works the Bobcat.

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Off to the side, the radio and TV people were talking to the chief scientists, Rufus Catchings of the USGS and Luther Strayer of CSUEB. The photogs put their bodies on the line seeking that grab-you image.

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On the table was one of the seismometers — it’s the orange dealie with the prongs. The can holds the power supply and electronics. You stick it firmly into the ground, hook everything up, and stand very still while the shots are fired.

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They’ll do the shots late at night to avoid vehicle vibrations. then they’ll put all the instruments away and fill in the holes. Also, thank all the property owners whose permission was required for this important research. As you can imagine, a boatload of planning had to happen before reaching this stage. The USGS’s National Earthquake Hazards Reduction Program is paying for the project.

I hope to see more of the project as it proceeds and will report back. Meanwhile, see how the press did:

Mercury News

SFGate

KPIX

Marks of the Oakland fault

11 July 2016

Two weeks ago I told you how the city of Hayward inadvertently destroyed a special street corner. It was an informal shrine among geologists because it so clearly displayed the creeping motion of the Hayward fault. I mentioned that Hayward still has plenty of other bent curbs.

So does Oakland. Almost nine years ago in this space, in my very second post, I said that Oakland should take over the Hayward fault and make it our own. With that in mind, here are eight places on the fault that are pretty iconic if you ask me. I present them from south to north.

1. Revere Avenue above Marlow Drive, in Sheffield Village.

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2. Encina Avenue below Castlewood Street, in Oak Knoll.

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3. Ney Avenue near Astor Avenue, just above Fontaine Street.

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4. 39th Avenue at Victor Avenue, in Redwood Heights.

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5. Medau Place near Moraga Avenue, in Montclair Village.

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6. The old firehouse on Moraga Avenue, in Montclair.

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7. Broadway Terrace just west of Route 13, in Upper Rockridge.

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8. In Lake Temescal Regional Park by the park office.

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Here are their locations, shown on the geologic map.

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There are more such places between and beyond these ones, and others in Berkeley and San Leandro too.