Archive for the ‘the hayward fault’ Category

Points south, north and meta

22 June 2008

hayward fault

The Hayward fault is not hard to see if you have practice seeing it and if you have a good map that you’ve studied well. But even so, in Oakland there are not many spots like this, where the evidence is unmistakable. This set of echelon cracks is in the Sheffield Village neighborhood on Revere Avenue, just above its intersection with Marlow Drive. Where my previous post showed Oakland’s northernmost point on the fault, this is the southernmost spot in Oakland where the fault is clear. A little farther along is Chabot Park, a corner of Oakland so remote that you have to get the triple-A map of San Leandro to see it and drive through San Leandro to reach it. But there the fault is apparent only as a break in slope.

Anyway, Oakland is as plain as a textbook compared to Berkeley. There the fault runs through rugged land covered with rocks, woods and homes. Near the University its location is well known and evidence is good, but to the north it wanders a bit and has vaguer signs. Keep that in mind when you visit the Walking the Fault blog, an occasional project by Berkeleyan Andy Datlen. Relying on the new USGS “helicopter tour”, he is quick to identify specific homes and other features as straddling the fault or otherwise direly threatened. I don’t blame him. I think that any citizen using the USGS tool is likely to reach the same conclusions. But I don’t, and I don’t point out specific homes as threatened, for several reasons.

First, I take a scientist’s more cautious approach to the maps. The red line is an inference, a hypothesis except in the specific points where trenches, measured offsets and cracks point precisely to a fault. I use the 1992 paper version of the online map, on which every piece of evidence is given specific degrees of certainty and quality. Scientists are in love with uncertainty as well as precision; where evidence presents a blurred picture they avoid oversharpening their vision, and so do I.

Second, the fault is not obliged to rupture exactly where it did the last time. Yes, deep underground it is safe to say that the fault is a clean surface, but our best evidence is that strike-slip faults like the Hayward are a tangle of cracks, a skein of fractures. If you were to cut across the fault and pull the cut apart to see a cross section, those fractures would gradually coalesce at depth. Looked at the other way, the deep fault flowers upward from a single crack into a fan of them, among which only one or two is currently active. This “flower structure” is seen commonly in geophysical studies of the San Andreas system, of which the Hayward fault is a part.

Third, whether someone’s home is in danger is not my place to say. Only a licensed professional geologist or geotechnical engineer can determine that responsibly.

Fourth, I hold that people should expect a degree of privacy, and identifying their homes on a website is not something I want to do.

The fault at Stonewall

19 June 2008

stonewall road

Stonewall Road is a cute little curving street, cozy at the bottom and overbuilt at the top, just across Claremont Boulevard from the Claremont Resort. The street sits at the break in slope where the Hayward fault passes through, its northernmost appearance in Oakland. The nice new red-painted curbs hide the evidence, as so often happens in this town, but I have a photo from 2001 (the second on this page) showing how it used to look. A stairway named Tanglewood Path runs west from the north end of the street, and it’s disrupted by aseismic creep near its top, although tree roots and landslip do their part as well. This view looks southward pretty straight down the fault, which runs along the hillside above the road and to the left of the hotel.

If you come out Stonewall onto Claremont Boulevard and look for fault evidence, you don’t see it amid the cracking and wear-and-tear that normally accompanies a heavily trafficked asphalt road in a stream valley near steep soil-covered slopes. But this wall, on the north side of the road right where the fault should be, makes me think that it must be some sort of homage to the fault. (click it large)

fault wall

There is some more-organized cracking in the pavement behind the hotel. The fault goes onward along Alvarado Road, ducking briefly into a salient of Berkeley then crossing Tunnel Road straight toward Lake Temescal.

Heights and flats

6 June 2008

oakland heights

In the East Bay, the Hayward fault separates high ground and low, with a few exceptions. Oakland is an exception (so is San Leandro, Berkeley and points north). From Oakland’s southeastern extreme at Lake Chabot up to the Panoramic neighborhood, the fault generally has a few hills on its Bay side. If you ride BART and look up at the hills, the fault is almost entirely hidden. The hill Piedmont sits on is the largest body of rock west of the fault. So Oakland is not like Hayward or Union City, where the fault is quite stark.

But here on upper Dwight Way, at Oakland’s far north end, is a spot where the height/flats dichotomy is laid right out plain. (Click the photo for a 900×750 version.) This little canyon is the one just north of Claremont Canyon, and I don’t know if it has a name. Behind me is little Dwight Canyon and just to its north is Strawberry Canyon, where the Cal stadium sits. High rock hills lie above the fault, and a plain of deep sediment lies below, an area where seismic shaking is liable to cause ground liquefaction. Of course landslides could happen where I stand; the brown patch below looks like a landslide scar . . . pick your poison.

The Big-Enough One

21 May 2008

The theme of this month’s Accretionary Wedge blog carnival is, “a geological event you consider most significant to you.” I know what that one is. It didn’t awaken my sense of awe and turn me toward science. It didn’t injure me or make me rich or poor. No famous historical figure was involved. But the very month my wife and I moved to Oakland, the Loma Prieta earthquake changed the city irreversibly.

Those first couple weeks of October 1989 were fun. We loved having a proper downtown with fine old buildings, great weather, a lively cultural scene and a city with real geography to it. We had moved our stuff from the house in Concord and were readying it for the next owner, so at 5:04 pm on 17 October we were out of town, cleaning the old house for the last time. It was totally empty. The shaking went on for a long time, but out there it wasn’t very strong. Driving back to Oakland a little later, we felt worse and worse as the news rolled in and the signs of damage appeared.

There was a pall over Oakland for a long time. I felt it for years, not just on the anniversaries but every time I went downtown; every time a new friend or neighbor told their earthquake stories and listened to ours; every time I saw the news from other places and people struck by earthquake. Even now the effects linger in the scars left on the map, buildings left empty and the new Bay Bridge yet unfinished. And while the downtown, the weather, the scene and the land have endured, I now have a deep-seated relationship with earthquakes, Oakland geology and the Hayward fault that gives me a pang every time I feel the little shakers from beneath our side of the bay and think of the Big One to come.

The US Geological Survey has unveiled three new publications on the Hayward fault, all of them well worthwhile. They are a four-page fact sheet on the current hazard, a 96-page field guide with tons of information and photos covering the whole length of the fault, and a Google-Earth virtual tour for deep background and visualization. Get learning!

A fault runs through it: Montclair

22 April 2008

montclair sag pond

The Hayward fault runs through the heart of Montclair, in the Oakland hills behind Piedmont. Montclair Park’s duck pond was constructed where the fault left a natural sag in the ground.

Only in seismologists’ equations, and perhaps deep down in the crust, are faults smooth, flat planes. In the world, on the surface, faults are as ragged and variable as any other geological feature. The Hayward fault is more of a zone, from a few to a hundred meters in width, with several fractures running through it. Where two strands overlap, a block of ground between them may slump in tension or rise in compression, depending on how the strands are oriented. At Montclair Park, two strands are mapped on either side of the sag basin. Lake Temescal is another example of a sag basin repurposed as a water feature. So, apparently, is Lake Aliso, the pond on the grounds of Mills College.

The first “great San Francisco earthquake” occurred 21 October 1868 on the Hayward fault. The epicenter appears to have been in southern San Leandro, and surface rupture extended from there all the way down to Fremont. In Montclair, the other direction along the fault, there was plenty of shaking of course, but no rupture of the ground from what we can tell. A trenching study, conducted along the third-base line of the little ballfield in Montclair Park, found no sign of recent movement along the fault there. But slow, silent motion does affect the fault in Montclair. The old fire station on Moraga Avenue has been rendered useless by aseismic creep, and some of the houses along the fault appear to show foundation disruption. But generally creep is invisible unless there is some structure that it affects, like the south curb of Medau Place, below. South of here, the fault crosses Route 13, reaching the other side at the head of Dimond Canyon.

hayward fault montclair

Sidestepping Piedmont

9 April 2008

glenwood glade

This morning I took a hike that circumambulated Piedmont without setting foot in it, while also incorporating pedestrian stairways and paths. There was something to see at every step; here are two highlights. The first is the charming hidden street of Glenwood Glade in the Fernwood district (what I would have called northern Montclair), where I snapped this verdant front yard with indigenous boulders. The whole street runs directly upon the Hayward fault. I would enjoy it now, because it isn’t likely to survive the next major rupture.

Then below is the Zion Lutheran Church on uppermost Park Boulevard, isolated on the very edge of Piedmont overlooking Dimond Canyon. Why would a church be dug into a bedrock hillside? The answer is, it wouldn’t; this is a former quarry. I stayed across the street, not wishing to touch Piedmont soil, but it looked like massive Franciscan sandstone, just as it’s mapped.

piedmont quarry

The roadway of Park Boulevard through Dimond Canyon is so massively reinforced that I suspect there was a railway there first. Does anyone know the history there?

The fault at Roble Road

11 February 2008

roblerock.jpg

From College Avenue you can take Chabot Road east, parallel to Route 24 toward the Hayward fault. It passes some exquisite and interesting houses, also the East Bay Municipal Utility District’s main water line, which cuts through the Oakland Hills in a tunnel that has recently been reinforced against fault movement. You also pass right underneath the BART line, on its way to Orinda. But when you get quite close to the fault, Roble Road exits to the north, a one-lane road with a few empty lots from the 1991 fire and a creek running alongside it. Just over the Berkeley line you start to see boulders. This one is at 57 Roble. It’s most likely a large specimen of Leona Rhyolite, but I’m not yet familiar enough with the two rhyolites, Leona and Northbrae, to tell them apart.

tunelroadchip.jpg

Roble Road debouches onto Tunnel Road very near the fault. If you head uphill on Tunnel, you cross the fault at the point in this photo. The house in the woods is at the corner of Vicente and Tunnel roads; its entrance has a stone block engraved “The Rocks.” Berkeley Rocks says that the house is perched on outcrops of the Leona Rhyolite. The wall in front divides the two opposite lanes of Tunnel Road. The chip on the right side, the only one on the whole wall, is probably due to movement on the Hayward fault.


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