Archive for the ‘the hayward fault’ Category

Stonewall Road View

8 August 2009

stonewall view

If you go up Stonewall Road, pretty soon you’re high above the Claremont Resort and the rest of Oakland. The contrast in elevation across the Hayward fault is very great here; it may be the steepest scarp on the whole fault (although Revere Road, at the other end of Oakland, is a contender). Everything in this view is across the fault, except possibly the house below on the right. Click the photo for an 800-pixel version.

When a big earthquake strikes this stretch of the fault, shaking will be very intense, with seismic energy coming from north, south and below. Trees will snap off at their trunks. Boulders will come barrelling down from above. Every car and burglar alarm on the street will sound, during the mainshock and during aftershocks for weeks afterward. Some homes will fall down the hill. Water and sewer lines will break and begin leaking out of the ground. Natural springs will arise at the same time. And smoke from dozens of nearby fires will begin to fill the air, and the sea breeze will push flames toward the hills.

Tanglewood Path

5 August 2009

hayward fault

Tanglewood Path crosses the Hayward fault just on the Oakland city line (at the chainlink fence); you can see the rightward offset of the path just above a set of steps. (Remember that wherever you look across the Hayward fault, the other side is moving to the right.) I’m standing at the first bend in Stonewall Road, looking west into Berkeley. This area is already unstable being on a steep slope, and the disruption of the path is as much due to landsliding and soil creep as it is to the fault. But the slope of the hillside naturally tends to push everything to the left, whereas the bend in the path goes the opposite way.

Grandview

31 July 2009

grandview

Grandview is a small, isolated Oakland neighborhood on the ridge above the Claremont Resort. I’m being unfair, I know, but this shot of a house on the north side encapsulates the neighborhood for me: a setting of intimate and exclusive comfort with borrowed scenery. It’s like all of Oakland’s neighborhoods on the east side of the Hayward fault, but with that invisible drop of ever-so-much-more-so. The views are grand, absolutely; beyond this house is Claremont Canyon Regional Preserve and behind me the vast panorama centered around the Golden Gate. All the houses are new, even raw, and oversized; sidewalks are absent, and no one would walk them anyway being too steep. There are no services, no amenities, no history. Only four outlet roads, all vulnerable to fire, landslide and earthquake. But it’s quiet and peaceful up here. With good provisioning, you could hold out OK after the next big quake in an incomparable setting, although service wouldn’t be restored for quite a while.

In my walk here last spring I saw only hints of bedrock, all mudstone of the Great Valley Complex.

Looking at the fault, Sheffield Village

18 July 2009

hayward fault

Click the picture for a 1000-pixel version. I’m standing next to the Hayward fault on the access road to Dunsmuir Ridge, looking south at the southernmost part of Oakland. The hill on the horizon is in San Leandro, and the fault goes through the notch on its right. Downward and to the left is a bit of the pavement of Revere Avenue, where the fault has split the roadway (as shown in this post). The map below shows this stretch of the fault along with the evidence of movement on it—the dirt road I’m standing on is at the top.

fault map

“G” means geomorphic evidence, “1” is the most strongly pronounced while “3” is weak. The other codes are as follows: n, notch; rs, right-offset stream or gully; vl, line of vegetation; sl, linear scarp; hv, linear hillside valley; hb, linear hillside bench.

Dunsmuir Ridge and the Irvingtonian gravels

26 June 2009

Just northwest of Lake Chabot are some tiny areas mapped as “Irvington Gravels,” high above the Sheffield Village neighborhood in the Dunsmuir Ridge Open Space. They caught my eye because Irvington (part of present-day Fremont) is the site of a famous set of Ice Age fossils, from which the Irvingtonian age of North American land mammals was established. Yesterday I checked the area out, in case there were some sabertooth-cat fangs lying around. This entry has a lot of photos.

You get there starting at the access at the end of Covington Road, a dirt fire road that goes straight up a steep hillside. The Hayward fault crosses the road partway up, at a little level spot at the edge of the woods. To the west of the fault, the rocks are mapped as San Leandro gabbro (Jurassic rocks of the Coast Range Ophiolite), but it’s really hard to tell:

dunsmuir ridge

Across the fault the rocks change to Late Jurassic volcanic rocks of the Great Valley Sequence, the same stuff exposed in the big Leona quarry:

dunsmuir ridge

Higher up are three small terraces where the gravel is mapped. This is looking south from the northernmost one:

dunsmuir ridge

It looks like a hopeless task to find rocks here. Luckily for me, the fire roads have recently been graded, so there was a window into the substrate. As I approached the terraces, the roadbed started to display river cobbles, quite unexpected in this setting:

dunsmuir ridge

I made a point of crossing the grassy slope to the other two terraces, looking for stones the whole way. Nada. From the southernmost terrace, here’s the view north. Click on the picture for a stereopair:

stereopair

There’s a house on a knoll at the same height as the terraces. The upper part of the Knowland Park Zoo land also lines up with the terraces. No gravel is mapped at either place, but there might be some.

Now the cobbles in the roadbed start to look interesting:

dunsmuir ridge

Above is another, higher terrace. It’s over 500 feet above the starting point and a bit of a trudge.

dunsmuir ridge

Just below it are scattered outcrops of the volcanic bedrock:

outcrop

The roadbeds on the upper terrace also have interesting cobbles. I took a few home to clean up and photograph. Remind me to bring them back on my next visit.

rocks

Russ Graymer, who prepared the Oakland geologic map, describes the suite of cobbles thus: “Cobbles . . . consist of about 60 percent micaceous sandstone, 35 percent metamorphic and volcanic rocks and chert probably derived from the Franciscan complex, and 5 percent black laminated chert and cherty shale derived from the Claremont Formation.” He holds that these little terraces started out near Fremont and were carried here by the Hayward fault. They started out at a much lower elevation too, I would think; just a sign that fault movements are not straightforward.

Chabot Road and the fault

4 June 2009

It was a good day yesterday to visit the Hayward fault at the top of Chabot Road. The previous night’s rain softened the ground and left the cut weeds smelling like fresh hay. This is the view north from the ground above the end of the road.

chabot road fault

All of this land is suspect today, and the rocks cannot be trusted. The high ground I was standing on is the rubble pile built to support Route 24. The high ground on the right is an old excavation or rubble pile, I’m not sure which, supporting the loop linking Route 24 west and Route 13 south. The flat ground is the former roadbed of the Oakland & Antioch Railway. The trees in the distance are on a rocky slope that roughly marks the Hayward fault, but it may well have been quarried in the past. The nice thing about a fault, for producers of crushed rock aggregate, is that it pre-crushes the rocks. But the fault is somewhere in this view, although it’s poorly mapped between the Claremont Resort and Montclair.

buckling

With that preamble, I feel free to speculate that the fault trace could possibly nip Chabot Road at its farthest end. We see displacement of the curbs, and at the farther joint we see evidence of compression. In both photos the near side would be west of the fault, moving leftward.

cracking

But just as likely, this trodden, retreaded land is shifting and settling all by itself. There may be slow landsliding involved. Also, heavy trucks and other vehicles could well have done this damage. The truth may come out after the next big earthquake ruptures the fault here. It’s one place I want to check out in the aftermath, if I’m lucky enough.

Oakland Conglomerate II

12 March 2009

Here are some of the samples I collected from the Oakland Conglomerate last month.

oakland conglomerate

This rock is utterly and thoroughly brown, deeply brown with iron oxides. You can’t scrub it off—it’s brown all the way down. That reflects two facts: the matrix is iron-rich volcanic material and it’s been pervasively shattered and flushed with fluids under oxidizing conditions to let the iron out. The clasts—the bigger stones in the conglomerate—are beautifully rounded, presumably in a vigorous river or beach. But the setting of Oakland at this time, in the Cretaceous, was way offshore in a shallow sea. It appears that all this coarse material was carried offshore in underwater landslides and laid down in a deep series of sandstones and conglomerate beds.

oakland conglomerate

Long after the conglomerate was laid down and lithified, the plate boundaries changed and the San Andreas fault system (including the Hayward fault) splintered coastal California and wrenched it northward, one earthquake at a time, for tens of millions of years. The forces of that time have crunched nearly every stone in the Oakland Conglomerate. Yet at the time these rocks were still deeply buried, and chemical action and pressure cemented the stones back together. The clasts are delicate, but intact. It’s impressive to imagine the force that shattered these hard stones like so many soda crackers. The next thing I want to do is open some of them with my hammer and see what the original lithologies are.


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