Archive for the ‘the hayward fault’ Category

Fault valley

21 October 2012

October 21, not October 17, is Oakland’s real Earthquake Day. While many of us remember the 17th vividly, the day of the Loma Prieta quake in 1989, the morning of October 21, 1868 was when the original Great San Francisco Earthquake struck. It was on this side of the bay, on our own earthquake fault, and the ground cracked from Fremont all the way to the edge of this view from the end of Pali Court, in the neighborhood just across route 24 from Lake Temescal.

hayward fault valley

The Hayward fault runs from the notch on the skyline, which is in Montclair, to the right edge behind the house in the middle distance. This part of the fault, between Mills College and Lake Temescal, is the only place that has large areas of bedrock on both sides. What that means for the purposes of today’s post is that it’s the most rugged and picturesque part of the fault and has probably the greatest concentration of million-dollar homes.

Today I invite you to review this blog’s category “the hayward fault.” I seem to have rattled on at great length on this subject. Some day the fault will rattle all of us at great length.

Today’s Oakland Tribune has an article about newly mapped faults in the Hayward fault zone; I won’t link to it because the Trib’s links die quickly. But the new Alquist-Priolo zone map can be accessed here.

Linear scarps

1 September 2012

Next Saturday, 8 September, I’ll be leading a short, rugged urban walk for Oakland Urban Paths that among other things will visit these faceted spurs along the Hayward fault. Seen from the north . . .

view south

and from the south:

view north

The downhill side is moving north with respect to the near side. The open land in the foreground is the King Estates Open Space.

This will not be a stairway walk. The off-street passages are steep, weedy dirt paths that have not been maintained. The land along the fault is steep, making for nice residential view lots. I haven’t finished the route yet but it will take no more than 90 minutes, 10 to 11:30—I have a lunch destination I’m anxious to make. So I would like to set a good geologist’s pace. Details and questions as they come to you over at Oakland Urban Paths.

Heritage in a dynamic place

10 February 2012

I gave my talk last night to the Oakland Heritage Alliance and I think it turned out well. I tried to talk about our geology not on its own terms, but weighted toward its relevance in Oakland’s general civic life. One of my slides was this lovely digital elevation model that makes the Hayward fault obvious. Click to see the whole thing. Sadly, I can’t recall exactly where I got it.

oakland DEM

Here are my concluding remarks from the talk.

Oakland is situated in an area that falls down every century or so. Today, a repeat of the 1868 earthquake would erase large areas of our city. I don’t see how we can prevent that. How can we hang on to our history in the face of nature? And how can we hang on to our history in the face of human nature? By that I refer to well-known tendencies in cities that are traumatized. Citizens have a strong, almost overwhelming urge to return to normal, and that drive is amplified in their leaders.

We have the example of San Francisco after 1906, which underwent a furious rebuilding and whose leaders had no tolerance for delay. San Francisco was the leading city of the West, and the stakes were very high. Democracy was short-circuited for a time. The development-driven city leaders, led by “mover and shaker” James Phelan, were prepared to obliterate Chinatown before the Asian-American community rallied to save it. There was no heritage community at the time, and who knows how it would have fared had there been one.

After our earthquake—and it’s coming—Oakland will still have all the geographical gifts that I showed you at the start of my talk. We can assume that the seaport and airport and rail lines and highways will be rebuilt as soon as possible. The stakes will again be very high. There will be little conflict with the heritage community over any of that. The hard part will begin once the emergency ends.

Earthquakes are part of our geography. These days we like to talk about sustainable living: in tornado country, for instance, that means everyone has a storm cellar. What does it mean in Oakland? Some of our most beautiful neighborhoods lie along the fault, and they probably will not be reoccupied once destroyed. The city that arises after the next Big One will be very different in some ways.

I think I can foresee a more sustainable Oakland in which more people will live in well designed multi-family buildings and recreate in a long greenbelt where route 13 runs today. On the other hand, much will endure. City Hall should survive thanks to its retrofit after 1989. Landmarks like Tech High, the Camron-Stanford House, Peralta Hacienda, and Dunsmuir will be restored if they survive the tumult. I know that the Oakland Heritage Alliance will be there, doing its utmost to save what can be saved. In the end, much depends on the citizenry at large: the values they hold most strongly, the values we are teaching them today, will be those that prevail.

The Oakland seismic landslide map

3 August 2011

A few years back, two guys at the U.S. Geological Survey did an exercise with a database that was subjected to a mathematical version of the Big One on the Hayward fault in the middle of the wet season. Their result was published as Miscellaneous Field Studies Map MF-2379, “Seismic Landslide Hazard for the Cities of Oakland and Piedmont, California.” Here’s a piece of it.

oakland landslide map

Dark-green areas are relatively fine, red areas are relatively awful, and the in-between colors are in between. Gray areas have slopes less than 5 degrees and don’t count. There’s a purple line running from top left to bottom right representing the Hayward fault; students of our street patterns may recognize the upper left corner as the intersection of 580 and 13. The lower right corner is 580 at the exit to the zoo. The left edge of the band of orange down the middle is Outlook Avenue. There are two bits of blue; the upper left one is the pond at Mills College and the lower one is the big hilltop reservoir near Toler Heights.

This map is of very little real use, because it’s just one worst-case scenario of one particular simulation, but it’s well worth studying anyway. (For real uses, like assessing your own property, you should hire a pro.) One big point is that bedrock matters. The big dark-green swath represents the solid metavolcanic rocks that are exposed in the Leona Quarry; they also underlie King Estates Open Space and part of the hill with the reservoir on top.

Fault gauge, 39th Avenue

27 May 2011

South of the LDS Temple, the Hayward fault runs down a well-defined gulch, past the stream offset at Rettig canyon, and becomes more subtle as it traverses Redwood Heights. It’s mapped going down Jordan Road and then Victor Avenue. Here’s where Victor butts against 39th Avenue.

hayward fault

It only takes a moment with a concrete saw to set an unobtrusive fiduciary mark like this. It will document offset along the fault, whether it’s aseismic creep or a more massive wrench when the fault lets loose.

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.


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