Archive for the ‘the hayward fault’ Category

City Hall and the Loma Prieta Quake

11 October 2009

oakland city hall

Oakland’s City Hall was the tallest building west of the Mississippi in 1914, when it was completed. It’s still an impressive structure, 324 feet high, covered with intricate stonework and flooding the plaza with warm reflected light around midmorning.

City Hall weathered the 17 October 1989 earthquake without collapse, although there was serious damage and it is said we nearly lost the clocktower. After the quake the city was motivated to retrofit the structure. In evaluating the possibilities, Charles Rabamad and Donald Wells write, “To minimize the amount of new construction, the existing structure was given credit for the strength it exhibited during the Loma Prieta earthquake. This performance-based approach required less strengthening than conventional, code-based design, which ignores the existing capacity of the building.”

Today City Hall rides on a grid of 113 big, fat rubber-and-lead base isolators 19 inches high and either 29 or 39 inches wide. These soften the shaking and allow the building to be strengthened with the least impact on the historic building’s interior. The building will shift back and forth as much as 17 inches. It’s designed for a magnitude-7 earthquake on the Hayward fault, after which some cracking, fully repairable, is expected. Completed in 1995, the retrofit was the world’s first base-isolation project for a high-rise building, setting the precedent for many more retrofits including several at UC Berkeley. Now Oakland City Hall is in all the engineering textbooks.

The Earthquake Engineering Research Institute has a publication on the whole thing, and a 15-page paper by Mason Walters with the dirty details was presented to the Third Seminar on Utilization of Strong-Motion Data in 2003.

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.


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