Archive for December, 2018

The seven stations of the Hayward fault

24 December 2018

Of all the East Bay cities, Oakland owns the longest stretch of the Hayward fault. In my very second post, back in 2007, I suggested that we take over the name, and a couple years back I pointed out eight iconic places to see the Oakland fault in action. To those who still can’t get enough of this amazing geologic feature, this post’s for you.

There are seven places in Oakland where alignments of markers are laid out across the fault trace. These are measured regularly by a team of scientists from the San Francisco State University Fault Creep Monitoring Project using a high-precision theodolite — an electonic gizmo mounted on a surveyor’s tripod. After my last post, I visited all seven places. Let me show them to you, north to south.

Lake Temescal

This line runs along Broadway as it passes Lake Temescal Regional Park. The signs of the fault here (unlike the beautifully cracked sidewalk next to the park staff building) are subtle, and I’ve never felt confident of the exact trace. Nor are there definitive markers. This nail in the concrete is the best candidate, across from the park entrance.

Each alignment station is supposed to have three markers, but I was happy to find even one. It’s probably just as well they aren’t obvious, or people might mess with them.

What they do with the marks is carefully measure the angles between them, then use the data to calculate how much creep movement has occurred along the fault since the crew’s last visit. At this station, creep has measured 4.2 millimeters per year since 1974.

LaSalle Avenue

The fault runs through the heart of Montclair Village, and a set of markers has been measured there since 1993. I don’t know exactly where they are. US Geological Survey Open-File Report 2009-1119 lists locations that are precise to a ten-thousandth of a degree, but they aren’t obvious at all in Google Maps because the precision of the maps is poor. Besides, the traffic on LaSalle was terrible when I visited. (Clearly the solution is to use my smartphone’s GPS capability, so I should get up to speed with that.)

The other thing is that there are lots of things in the street that could be used, like this longstanding fixture.

But even without the markers, the fault itself is evident where the sidewalk has been warped over the years. This view is looking up the north side of the street where the curb has been slowly distorted, the near side creeping leftward by 4.6 millimeters per year.

If there weren’t so many furschlugginer cars and stuff in the way, you could see these features more easily.

Lincoln Avenue

This site has been visited since 1970, the longest-running series of creep measurements in Oakland. It’s at the entrance to the LDS Temple complex, and the fault regularly cracks the pavement next to the Stake Center at the east edge of the property, on its way to the London Road landslide site. This little thing at the head of Maiden Lane might be one of the marks. It certainly looks old enough.

Other possibilities include this unobtrusive saw cut.

Or this more prominent mark.

But you know, all kinds of people have precision business on the ground — utilities, builders and so on. It really sinks in once you start closely inspecting the places you visit every day. And unlike the beautiful brass USGS benchmarks you may have seen, the markers used by the scientists who survey the fault don’t need to be fancy at all. Creep at this location averages 3.8 millimeters per year.

39th Avenue

I’ve featured this location before (twice, in fact), but this time I found the fine little marker shown at the top of this post.

Notice the circle of greenish spray paint around the marker. You’ll see it more in the following stations.

While I was there, I updated my shot of the sawcut in the curb. It’s continued to move, though not at the 4.1 millimeters-per-year pace of the fault as a whole. Creep displacement usually takes place across a wider zone measured in meters, not a single crack.

Maybe in years to come it will be as famous as the Rose/Prospect corner in Hayward once was.

73rd Avenue

This station is at the tight hook in the road where 73rd tops Millsmont ridge and becomes Sunkist Drive for one block. Like the LaSalle station, it was started in 1993.

This marker looked promising, but it’s stamped “EBMUD Survey Control.”

I think this is the real one; note the green paint.

This site has been off my radar as a creep locality, but it has possibilities. The cracks here in 73rd Avenue may resolve into a definite fault trace, if the city doesn’t pave it all over first.

Creep here has averaged 3.4 millimeters per year.

Encina Way

Measurements began on Encina Way, just north of I-580 off Golf Links Road, in 1989. I’ve taken groups here to show them the offset curbs, which are easy to see.

But I had never sought out the creep stations. A splotch of green paint led me to this elegant little bronze dome nestled up at the curb, the size of a half-dollar, with a dent at its center.

Creep here has averaged 3.3 millimeters per year.

Chabot Park

Yes, Chabot Park is run by the City of San Leandro, but it sits inside the Oakland city boundary. Nine years ago I made note of a long row of spikes driven into the road up to the dam. I assume that was the original line established in 1993. It’s much more elaborate a setup than is needed for a simple creep measurement. Perhaps it was a master’s project aimed at measuring the details of the wider active trace of the fault; perhaps it was something else entirely. All I know is that earlier this month I revisited the park and saw the road had been repaved, erasing all sign of the spikes. I did see this nail, though, and there’s the telltale paint too.

Creep here has averaged 4.0 millimeters per year.

Finally, here’s a portion of a cool graphic in the USGS report (800 x 500 pixels) showing the motion measured at these seven stations.

It shows the variations that affect the data — some from the annual wet/dry climate cycle, some from the fault itself — and the effect of our largest local earthquake, the 4.2 shaker of 20 July 2007. The report gets updated, so check it once this post starts getting old.

The lavas of Easter Hill

10 December 2018

It all started in the Oakland History group, on Facebook, when someone posted an image from a glass-plate negative for sale on eBay: a road-building crew at work somewhere in the East Bay hills. Was it Oakland?

We quickly determined the view overlooked the area that would become Richmond, but what was that little round hill in the rear center? It was too large and rounded to be a shellmound. It must have been bedrock; however, modern maps show nothing like it. But Andrew Lawson’s geologic map of 1913 did — an outlier of Franciscan lava at a locality named Cerrito.

The 2000 geologic map shows it too. Incidentally, I love the old map because it shows that Potrero San Pablo, the rocky ridge on Richmond’s west side, was essentially an island a hundred years ago.

The handsome little hill of Cerrito was called Easter Hill, because it was popular for Easter sunrise services once upon a time. Photos in Calisphere’s Richmond Local History Photograph Collection show it in about 1910:

and in 1912, behind the Stauffer Chemical Company plant, which used to process sulfur-bearing ore from mines in the Oakland hills:

The hill was laid out with roads and called El Cerrito Terrace at the time, but apparently never got more than partially settled before the World War II years because the Kaiser industrial combine acquired it for a quarry, to help fill in the marsh and build the tremendous shipyards of Richmond.

And that was pretty much the end of Easter Hill the hill, but in the 1950s the site became a pathbreaking low-income housing development, Easter Hill Village. After a few decades the neighborhood had gone sour and the buildings had deteriorated, but in the early 2000s Richmond renovated part of it, added new small multifamily homes and renamed it Richmond Village.

I had to go see if the rocks were still there. They are! Take the 23rd Street exit from 580 west and go right on Cutting Boulevard for two blocks, then right on South 26th Street.

Remnants of the hill add topographic interest to the site, and large boulders from the old quarry are placed all around.

Those boulders display a variety of igneous textures and rock types that’s remarkable in an exposure so small. There’s fine-grained, vesiculated (bubbly) andesite.

There’s lava jammed with centimeter-sized feldspar crystals (phenocrysts).

There’s lava ground into fragments by movements of the hot lava around it (autobreccia) — actually two kinds of autobreccia, hot and cold.

I wasn’t sure that all of these came from the same body of rock until I saw them all in one place, packed cheek by jowl in the same outcrop.

But wait there’s more — a textbook-quality outcrop of pillow lava.

Close up, the pillows look almost as fresh as the day they squeezed their way red-hot onto the seafloor and froze in this distinctive form, the outcome of cold water playing whack-a-mole with rising lava.

If you make your way to the hilltop, the rock is kind of punky but the views are inspiring whether you’re looking west, southeast or southwest.

So Easter Hill is still an excellent place to geologize. The village seems like a good place to live, too.