Archive for the ‘the hayward fault’ Category

Rettig canyon

5 May 2011

I paid a visit yesterday to part of the Hayward fault in Oakland, but while there I felt the pull of a neighborhood treasure I call Rettig canyon. The name is from Rettig Avenue, which traverses it. Here’s the topography, from Google Maps. This is just south of the LDS temple.

rettig map

The fault runs from top middle down Jordan Road and exits where Victor Avenue leaves the map. The hills to the west are the southernmost part of the Piedmont block of Franciscan rocks. Rettig canyon cuts right through the hills thanks to Peralta Creek, which comes here from Butters Canyon on its way to the bay through Peralta Hacienda and Foothill Meadows Park.

Normally when you see a stream cutting through a bedrock ridge, you explain it as either stream capture or a water gap. That is, either the stream eroded its way headward through the ridge or was running that way already when the ridge rose underneath it. Given the intense tectonic activity here, I’m inclined to call it a water gap, as I do Dimond Canyon (with the addition of tectonic stream capture).

I saw some possible evidence of this in the streambed. But first, a look at the scene.

rettig road

Rettig Road is a single lane through the canyon and is coned off as a landslide zone. It’s been that way for at least six years; I hope a local will say more in the comments. The canyon is steep, dark and thickly wooded. You can scarcely see the stream, but you can hear the water everywhere.

rettig canyon

But there is a place to scramble down to the streambed. It’s well populated with rocks that appear to be local Franciscan melange, pretty jagged and hence not transported far.

rettig streambed

I was looking for bedrock and found some candidates like this scaly schist. I didn’t have my hammer and was reluctant to disturb the scene anyway, so I can’t say much about it. It might be serpentinite.

rettig schist

This is the outcrop that excited me, showing what looks like a thrust contact.

rettig contact

Ignore the green patches; that’s just algae. The rock on the left is fairly soft and foliated parallel to the contact. I picked out a small piece and can’t say much about it, but in the hand lens it looks like a highly altered talcy kind of stone. At the base is a good centimeter-think layer of nice gray clay, then we hit clean tightly packed sediment with highly tilted bedding; indeed it’s tilted steeper than the contact above it. So my best guess is that it may be the contact between the Franciscan and much younger Pleistocene sediments. Due to squeezing along the Hayward fault, the older rock has been thrusted up and over the sediment. This isn’t unheard-of, but I haven’t seen it documented around here so I could easily be wrong. But that would explain the rising ridge, the topography of Jordan Road (which sits in a long trough here that may well be a sag basin) and the course of the stream.

I couldn’t resist bringing home a pocket-sized cobble of beautiful actinolite schist.

Fault monitor

6 December 2009

hayward fault

At San Leandro Chabot Park, which is actually located in Oakland at the very end of San Leandro’s Estudillo Avenue (I know this is confusing), there is a line of railroad spikes driven into the pavement of the road leading up to the Lake Chabot Dam. I counted 47 of them, but I could have missed some at either end. They appear to be 20 feet apart, so if there are 50, that would make 1000 feet. Anyway, they are perfectly aligned and extend across the Hayward fault trace. Someone periodically surveys them, I’m sure. But after the next big quake, someone will check them rather soon and then again, often, because movement on a fault doesn’t end with an earthquake. There is very often a postseismic creep that continues to displace the land along a fault, which is of interest to seismologists.

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.


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