Archive for the ‘the hayward fault’ Category

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.

The Oakland Conglomerate

7 February 2009

oakland conglomerate

The Oakland Conglomerate extends from Montclair, a little north of Snake Road, south along the whole southern end of Skyline Boulevard and beyond Lake Chabot to the northern outskirts of Castro Valley. It’s part of the Great Valley Complex, a huge sedimentary pile spilled off the ancient Sierra/Klamath ranges in Late Cretaceous time, specifically the Turonian and/or Cenomanian ages (about 90 to 100 million years ago). This may be its northernmost exposure, on a fire road across from the Shepherd Canyon fire station where a basketball court has been cut out of the hillside. I brought home some samples and hope to have fun with them soon.

oakland conglomerate

What’s cool about this rock unit is that the big clasts are almost all cracked or shattered or dinged up. That doesn’t happen to these potato-sized chunks of quartzite and granite in riverbeds or the seashore, where the stones were originally shaped. Researchers at Cal State East Bay have argued that they were damaged by thousands of earthquakes on the Hayward fault as they lay buried some 5 kilometers down.

This rock unit is actually mostly sandstone, especially the farther south you go. It crops out all the way down to the Alum Rock area.

Earthquake day II

20 October 2008

hayward fault earthquake

October 21 is the date of the 1868 Hayward earthquake. It was on the order of a magnitude 7 and caused widespread destruction plus a couple dozen deaths. Over the last 2000 years, the Hayward fault has had large earthquakes at an average of every 140 years, and this year marks 140 years since 1868. There will be a public gathering on the 21st, at the Mission San Jose, at 7:55 a.m., the time of the quake. (At least it wasn’t at 5:13 a.m. like the 1906 quake.)

Unfortunately the officials are making the same mistake the San Franciscans do, which is to ignore daylight saving time and time zones generally. In 1868, cities determined their time locally from astronomical noon (or used the time of a larger regional city), so the contemporary time must be adjusted for us to experience the setting of that earthquake at the correct time of day. I don’t happen to know if Hayward used San Francisco time in that year, or if both cities used Sacramento time. In 1906, California was on Pacific standard time year-round, and 5:13 a.m. on April 18 was nearly sunrise, but nowadays they observe the moment, in a ceremony at Lotta’s Fountain, in the dark of night an hour earlier.

Oh, the photo? It’s the little valley across the freeway north of the zoo, where Arroyo Viejo makes a right-hand jog as it crosses the Hayward fault. We’re looking across the fault from Calandria Avenue in early 2005. The hill on the far side is a shutter ridge, cruising north at a long-term rate of about half an inch a year, which it does in meter-sized jumps every couple centuries. (It moved in 1868.) The hill has a large covered reservoir on top of it, to the left of this photo; you could easily imagine it rupturing in a large quake. That doesn’t mean it will rupture, because it’s well engineered, but it’s easy to imagine it failing. In the middle is Holy Redeemer College.


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