Archive for the ‘The Hayward fault’ Category

Anomalies of Sausal Creek: Dimond Canyon

14 October 2019

This is the second of four posts about the peculiarities of Sausal Creek, going from its headwaters to the Bay. Here I’ll address Dimond Canyon, the 2-kilometer segment between the Warren Freeway and the flats of Dimond Park. The steep walls of the canyon, which is several hundred feet deep, are entirely hard sandstone of the Franciscan Complex, part of the Piedmont block.

This is the same stone quarried for decades in Rockridge (the Bilger quarry) and the land that would later become Piedmont (the Blair quarries and the Davie Stadium quarry). In fact the Diamond Cañon Quarry was one of two here in the canyon. It’s now occupied by the Zion Lutheran Church, as seen here from across the canyon.

The quarry scar appears on this terrain map as a big round nick in the canyon wall next to Park Boulevard.

A while ago in this space I described Dimond Canyon as a classic water gap — a stream-cut gorge crossing a bedrock ridge that otherwise seems impenetrable.

Geology textbooks will tell you there are two ways for streams to make a water gap. In the first way, the stream was there first (an antecedent stream) and a ridge of resistant rocks rose up around it. In dynamic California, this is a straightforward explanation of our water gaps. In the second, the ridge was there first, inherent in ancient deformed rocks buried under younger strata, and the stream (a superposed stream) cut down to, then into it while stripping off the overlying material. That’s how they explain the Delaware Water Gap and other examples in the gentle Appalachians.

Dimond Canyon is actually a semi-classic water gap. Yes, the ridge it crosses must have risen while the stream was cutting down, but the story is complicated by the fact that the watershed upstream lies across the Hayward fault, and is constantly being moved to the right. This means the canyon has hosted streams from several different watersheds over the past million years or so.

Therefore the streams feeding Sausal Creek today could not have dug the canyon; some predecessor watershed did it. There must have been gaps and surges in the water (and sediments) flowing through this canyon. If we ran things backward a million years, what would it show? The exercise would be blurred by serious uncertainties, but the matter is not beyond all conjecture.

I beg your indulgence as I present some slides from my talk to the Friends of Sausal Creek last month. They’re Google Earth views looking west across the fault. Here’s today, with the fault trace shown in red.

The view may be a bit confusing as I rewind the motion on the fault at about 10 millimeters per year. The far side looks the same because we’re focusing on it while it moves leftward, toward San Leandro. For a long time, Sausal Creek has been carried past small watersheds that, like today’s, could not possibly have carved Dimond Canyon. But about a million years ago, Dimond Canyon would have lined up with the watershed of Arroyo Viejo.

This looks promising because the watershed (the part above the fault) is about twice the size of Sausal Creek’s, giving it roughly twice as much water and cutting power to match.

But to make the canyon, you have to have something pushing up the ridge while the stream across it keeps cutting its way down. There’s nothing obvious that would have been pushing up the bedrock ridge at this time.

Going back a bit further, though, we line up with the great big watershed of San Leandro Creek, a dozen times larger. This stream has plenty of cutting power, evident in the canyon it’s dug where the dam and reservoir sit.

And finally, we have a mechanism here for uplifting the ridge that Dimond Canyon cuts across. The hills of San Leandro consist of a large slab of gabbro so big and strong that it deflects the Hayward fault slightly. Back when the sandstone of Dimond Canyon was grinding past the gabbro of San Leandro, the jostling between these two bodies of rock, caught in a vice by the geometry of the fault (a restraining bend), would have pushed both sides upward because that’s the only way out of the vice. And all the while San Leandro Creek would have been cutting a nice deep water gap as that hard rock rose.

Eventually, inevitably, the fault carried the water gap out of reach, and ever since then Dimond Canyon has housed lesser creeks for episodes of a few hundred thousand years. Sausal Creek trickles down the canyon today not doing much to it, the shrunken tenant of a structure built by a mightier maker.

This story (and that’s all it is really) appeals to me because it would also explain the presence of the Fan — the swath of gold on the geologic map representing Pleistocene sediment.

I’ve always regarded it as a fossil alluvial fan because of its shape on the map, but maybe that’s accidental. Maybe it’s just a chunk of old East Bay land that was lifted along with the Piedmont block, or washed off of it afterward.

I first posted about the problem of Dimond Canyon more than 10 years ago. Takes a while to figure out some things.

Anomalies of Sausal Creek: The Headwaters

30 September 2019

The Sausal Creek watershed is full of anomalies and questions from its headwaters to its San Francisco Bay outlet. Here I’ll look at the top end of Sausal Creek — the part that isn’t Sausal Creek. The creek originates where three different tributaries join: Shephard Creek, Cobbledick Creek and Palo Seco Creek.

The divide between Shephard and Cobbledick runs up Chelton Drive, then Darnby and Carisbrook Drives, up to Skyline. The divide between Cobbledick and Palo Seco runs mostly up Castle Drive to Skyline, so if you know the area these are easy to visualize.

That map, from the Alameda County Flood Control and Water Conservation District, has Sausal Creek proper beginning at the junction of Shephard and Cobbledick, which is also where the Hayward fault crosses the creek (more about that below). But because that’s in a culvert deep beneath the Warren Freeway, for purposes of this post I prefer to put the origin a little farther down, in an easy-to-miss opening east of the parking lot of the Montclair Golf Course driving range at the head of Dimond Canyon. That’s where Palo Seco Creek, the third tributary, comes in from the redwood-filled canyon of Joaquin Miller Park.

So with that settled, let’s look at the headwaters on the geologic map. This area includes a wide variety of rock units — the Sausal Creek watershed touches more different rock types than any other Oakland stream.

Don’t worry, I won’t go into the rocks, although there are a lot of them and they’re interesting . . .

I’ve added the Hayward fault to the map, as a thick red line, just to show how different the rocks upstream and downstream are. That’s because motion on the fault has been dragging the west side to the north for a few million years. That explains two major peculiarities of Sausal Creek, the first being its lumpy longitudinal profile.

I made this stream profile by walking down the creek from the top of Eastwood Court to the Bay, recording elevations with my smartphone altimeter.

A normal stream profile describes a nearly smooth listric curve — steep at the top and level at the bottom. The bottom of the curve represents what’s called the base level for the whole stream, sea level in this case. A stream with a nearly perfect curve is said to be at grade. Two basic things will put kinks in that curve: rocks that are especially hard or soft, and changes in base level. For instance, ice ages lower the sea level by hundreds of feet, and streams have to adjust during that time by digging down their beds toward the new base level. (I alluded to this in my last post with respect to Lake Merritt.)

There’s a big discontinuity in this curve right where the Hayward fault crosses, just above the 3 kilometer mark. The lower half, Sausal Creek, is at grade, even though it crosses hard sandstone in Dimond Canyon and young sediment farther down. But the fault has ripped its head off and put on another head — the Shephard-Cobbledick-Palo Seco system. It’s a Frankenstein creek. I think this head transplant has happened more than once.

Sausal Creek, over the last million years or so, has done fine even with its head ripped off and replaced. The worst that might have happened is that it had less water in it for a while. But in the upper creek system, these changes have drastically affected its base level.

Picture it: the two sides of the Hayward fault are moving past each other at about 10 millimeters a year, or a kilometer every 100,000 years. So for a good long time, Shephard Creek flowed down against the high rocky ridge of the Piedmont hills. Very likely it turned north for a long ways, the way Temescal Creek does today, before flowing around the north end of the ridge. As its route to the Bay got longer and longer, the slope of the stream grew gentler as it remained at grade. The effective base level, that is to say, was up around the elevation of the Thornhill district.

Then along came Dimond Canyon, moseying up on the far side of the fault, and at some point Shephard Creek switched over to that route. All of a sudden, it had a lower base level. It was not at grade. So it started eroding downward into its streambed and eroding uphill, trying to re-establish that nice listric profile.

When that happens to streams, what geologists call a knickpoint appears in the profile. The extreme case of a knickpoint is a waterfall, but more often they’re just rapids. In Shephard Creek, there appear to be two knickpoints.

Pay attention next time you ride down Shepherd Canyon Road. There’s a nearly level stretch in the road between Saroni and Escher Drives, where the railroad trail meets the road, then a steep “rapids” below. The other knickpoint is under the landfill of Shepherd Canyon Park, where the creek is buried in a culvert. A more carefully made profile would show it better, but that may not be possible.

I could conjecture a story that accounts for these features, but there’s a lot I don’t know so it would just be armwaving. For instance, the stream profile is based on the elevations of the roadway rather than the actual streambed except for the part between Shepherd Canyon Park and Mountain Boulevard (which is so thickly wooded I don’t recommend you visit, even though I did) and the one data point at the golf club. For another, the history of vertical movements along the Hayward fault is almost completely unknown, other than that the hills on the east side are rising today at about a millimeter per year. So enough about that.

I mentioned that the creek has two major peculiarities, and here’s the second one. The upper part of the Sausal Creek watershed is not a pretty, textbook stream network shaped like a nice tree — what geologists call a dendritic drainage pattern. It’s more like a bush in a gale, and all of the streams that cross the fault are warped. Here’s how it looks in the set of stream maps on the Oakland Museum website:

And for comparison here’s Temescal Creek:

And Arroyo Viejo, the weirdest of all.

There’s a struggle going on between the stream’s innate tendency, driven by gravity, to dig itself a home with an optimal shape and the motion of the ground beneath, driven by tectonics, that keeps messing it up. The shapes of the streams, and the landscape they live in, are a snapshot of that struggle. They remind me of the shapes of trees high on windy mountains — although one case involves organisms and the other purely physical systems, the similarities are tantalizing.

Geologists are starting to explore this topic with computer models. A recent paper in Geophysical Research Letters, with the dry title “The Role of Near‐Fault Relief Elements in Creating and Maintaining a Strike‐Slip Landscape,” has some state-of-the-art animations that address the exact situation of Oakland’s fault-crossing streams. You don’t need to understand all the modeling details — I certainly don’t — to enjoy the illustrations and the movies.

Oakland’s wild rail path

5 August 2019

The seasons are changing now, if you follow the pagan calendar. This weekend marks the turning point between astronomical pagan summer (6 May to 6 August) and pagan autumn (6 August to 6 November), or as I think of them, High Season and Waning. They are offset exactly half a season from the conventional astronomical seasons. High Season consists of long days, and Waning consists of shortening days. (Likewise, Low Season consists of short days, and Quickening consists of lengthening days.)

Nature is acutely aware of these seasons. The belladonna lily (Amaryllis belladonna) sends up its naked-lady flowers at this time. The strawberry tree (Arbutus unedo) ripens its rich little fruits (I can understand why Pliny the Elder named them “eat-only-one” because they’re so satisfying).

And of course the blackberries are in full swing.

I returned last week after eight years to the “secret street” at the south end of Florence Avenue, where it meets the old railbed of the Sacramento Northern Railway (also known as the Oakland, Antioch, and Eastern Railway). Unlike that first visit, when I was busy and could only gaze up the path, this time I had the leisure to walk its whole length, up to Broadway Terrace where it’s fenced off.

The path has geology up at its north end, but it’s worthy just as woods. Even right next to the Warren Freeway, it’s as secluded as any place in Oakland.

It’s shown as the dashed route on this map. You can see that Florence Avenue, heading over a saddle in the ridge above Piedmont, used to connect with Florence Terrace once upon a time. That’s the Lake Temescal park at the top.

There are lots of blackberries growing here, so don’t wait. The first ones are the best. Near the north end is a landslide scar that was repaired with much labor to protect some homes on Sheridan Road. The work was finished with dark shotcrete, but it doesn’t really blend in.

If you look close you’ll see little splotches of white. Those mark cracks where lime-bearing groundwater has seeped through and deposited calcite as it evaporates.

I can foresee these growing into falls of travertine in a few years. Beyond the landslide is a high cut into the hillside, made decades ago when the rail line was first pushed through. And the bedrock here is mapped as classic Franciscan melange, the big blue field on the geologic map — the edge of which happens to correspond to the Hayward fault.

I half expected the rock exposed here to be fault gouge, the fine-ground, mealy stuff that fills many of California’s active faults (for instance at the London Road slide). It’s real close to it: highly weathered mudstone that’s likely to come down hard in our next big quake. Whether the railbed will be cleared again afterward can only be conjectured. I’ll look at this cut again more thoroughly next time I’m here, whenever that might be.

On your way back, look again for blackberries. I know I didn’t get them all.

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.