Archive for the ‘the hayward fault’ Category

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.

Cal Stadium and the Hayward fault

10 September 2008

I mentioned UC Berkeley’s Andrew Lawson in connection with the Claremont Shale. Lawson also recognized, mapped and named the Hayward fault. It was very well known that the fault crossed the upper part of the Cal campus, and Lawson was not shy about it in 1921, when the University planned its big new stadium right atop the active trace. Like all geologists, Lawson knew that in a bet against nature, nature tends to win, but as a professor he also knew that in a bet against the university, professors tend to lose. So Cal Memorial Stadium sits where it sits, being slowly pulled apart by aseismic creep. (Lawson’s own home, at 1515 La Loma Avenue in Berkeley, was designed for earthquake resistance by Bernard Maybeck.)

Here’s the canonical view of the offset looking up from the parking lot at its south end.

cal stadium

Immediately east is this chunk of lumber amid the stadium’s reinforced concrete. It’s important to remember that good codes and good designs don’t ensure good construction.

cal stadium

Underneath the structure, there are cracked columns in many places. This one, cracked on its south side, shows that the ground is being carried north while the stadium, being a fairly rigid structure, is stationary.

cal stadium

The stadium is actually built in halves, with the idea that during an earthquake the two sides would gently slip past each other. No one knew about creep at the time. These two columns have been pulled out of parallel over the years.

cal stadium

Lawson may have thought that the stadium design was OK. He was not aware of much we have learned about faults and earthquakes since his time.

Shutter ridges

31 July 2008

A reader was unclear on the concept of shutter ridges, so I thought I’d try to show it as well as tell it. Look closely at this excerpt from the Oakland geologic map covering Lake Temescal.

shutter ridge map

The lake is the blue blob near the top. The Hayward fault slashes through it and across the map from top to bottom. The left (west) side moves north with every major earthquake on the fault. The blue area labeled KJfm (Cretaceous-Jurassic Franciscan mélange) is part of what I refer to as the Piedmont block; it makes up the ridge you see across the lake:

lake temescal

As that ridge moves north, it cuts off the course of Temescal Creek and forces it to flow north to get around it. That’s where the “shutter” term comes from—the ridge barrier moves like the shutter of an old-fashioned box camera. You can see on the map how Temescal Creek flows today, after tens of thousands of years of this process: it comes downhill on the right edge of the map, jogs to its right for almost a mile, goes through Lake Temescal, then turns left around the curve of Route 24 (the double purple line) in a culvert to resume its course to the bay. See the ridge from another perspective in this post.

Another excellent example is on the fault just north of the Oakland Zoo, where Arroyo Viejo comes down Golf Links Road and makes a similar jog around the hill of Toler Heights before resuming its bayward course under 82d Avenue:

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.


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