Archive for the ‘the hayward fault’ Category

The Hayward fault in Redwood Heights

3 May 2014

It happens that a commenter asked about the Hayward fault hazard in an area that I surveyed only yesterday, just south of the part of town I discussed back in January under “The 35th Avenue cut, Jordan swale and the Franciscan spike.” This post looks strictly at the Hayward fault between 35th and 39th Avenues and not, as I usually prefer, at the bedrock (there’s almost none to be seen here anyway). Here’s the fault trace, as mapped in 1992 by the US Geological Survey.


First, note that the map is tipped clockwise to make the fault run vertically. The fault is mapped here with an uncertainty of less than 40 meters; the ticks on the dashes indicate the downhill side. To help orient you, here’s much the same area in Google Maps, tilted to match.


The two arrowheads mark where the fault is mapped and the two numbers are the localities I’m showing below. The lower arrowhead coincides with the fiduciary mark on 39th Avenue.

The first locality is the lower end of Dunsmuir Avenue at Victor Avenue, where the street curbs are offset to the right.


The offset is more subtle on the south side (to the right in this view). The pavement on the corner is cracked, but that can happen when a garbage truck cuts across it, to name just one possibility. But there is an offset there, along with a recent sawcut to help gauge any motion there.


To the north, the next street to cross the fault trace is Atlas Avenue. The curb is offset there, too.


This is marked on the 1992 fault map with the notation “C2,rc,rs,ec.” That signifies “distinct creep evidence, right-laterally offset curb [and] sidewalk, en echelon left-stepping cracks in pavement.” The cracks are not evident now; presumably they were where the pavement has been patched. Offset concrete is harder to hide. (On 39th Avenue the code also includes “right-laterally offset fence line,” “surveyed offset feature” and the code L91 for a particular report documenting the survey.)

The features marked “G” are geomorphic ones, G2 for “distinct” and G3 for “weakly pronounced.” The codes are as follows: sl, linear scarp; lv, linear valley; ss, swale in saddle; df, fault-related depression.

The land is weird here. Drainage from the reservoir area appears to run north instead of west, perhaps feeding the head of Courtland Creek just south of 35th as I envisioned it happening from north of 35th too. If that’s correct, it would be another example of an offset streamcourse (and a corresponding shutter ridge). As earthquakes and creep affect the landscape here, water could shift from one drainage to another in this area where three different creeks run very close to one another.

The 35th Avenue cut, Jordan swale and the Franciscan spike

18 January 2014

If you’ve read this blog over the years you’ve seen me talk about the Piedmont block, a big hunk of Franciscan rocks riding north along the western side of the Hayward fault. Its easternment end tails off in a narrow wedge of undifferentiated rock, shown here in the geologic map.


The next three photos are taken from the locations marked with numbers. That’s 35th Avenue there, right at the curve in the road where it becomes Redwood Road. The curve is where the fault crosses the road, too, so it’s an apt place for the change of name. Just below the bend is this roadcut in hard bedrock. It’s mapped mostly as the material labeled KJf, undifferentiated Franciscan, on the geologic map plus some of the volcanic rocks (Jsv) exposed in the Leona quarry.


I don’t know how old the roadcut is. The road hasn’t changed course since the 1800s, but I guess it was widened in the 1960s or so, because the map base shows the split roadway in purple, meaning a recent change of the same vintage as I-580’s construction. Perhaps the road had a hump in it as it crossed the ridge. Above the bedrock ridge is a small valley with Jordan Road in it, shown below. The homes on Victor and Herrier Streets are visible on the Franciscan ridge beyond this swale (especially in the big version if you click on it).


A bit to the north, Peralta Creek runs into this swale (mapped as a sag basin related to the fault) and then cuts through the ridge in Rettig canyon. I can see the swale filling with water and emptying over the millennia, perhaps occasionally down Cortland Creek past the south tip of the Franciscan spike, as earthquakes and landslides rearranged the topography. The fault is mapped right at the intersection of Jordan and 35th on the west side, but I’ve never seen any evidence of creep there.

The roadcut, according to the geologic map, should expose two kinds of bedrock. It’s covered with boulders of basalt or greenstone, presumably quarried from the spot.


Bits of bedrock peek through, so it ought to be possible to trace the contact between the two rock types. That’s on my list of projects.

The crack by the temple

26 December 2013

The Oakland Urban Paths walk last month took us along a stretch of the Hayward fault that included the Oakland LDS temple. Here’s that segment of the fault on the map.


The temple is just below the dashed line across the map (which marks the 26th kilometer from Point Pinole). The codes refer to geomorphic (G) evidence of the fault, G1 being “strongly pronounced”: a linear valley (lv) and a notch (n). Less obvious are linear scarps (G2, sl) and a scissor point (G3, sc). The circle labeled C2 marks a measured feature that documents creep, a surveyed offset (so) documented in report number L91, which was a 1991 article in the Journal of Geophysical Research.

Knowing all that, I had my eyes peeled as we went through the temple grounds. This is right by the front gate between the grounds and Maiden Lane.


I can’t pronounce this a fault trace, but it’s consistent with one.


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