Archive for the ‘oakland hazards’ Category

Punk shale

23 February 2010

Up along Skyline Boulevard between Snake and Shepherd Canyon Roads is a long section of crumbling roadcut. The rock there is mapped as brown mudstone that has been questionably assigned to the Sobrante Formation. OK, enough of that. What struck me about it is how weak it is. This exposure is an excavation, probably for a garage, dug a good four meters deep into the hillside. And all the way in, it consists of this crappy stuff. Click the photo for an 800×800 closeup.

punk shale

The bedding slopes to the right; you can see three different units in this shot which is maybe two meters high. On top is a blocky layer richly stained with iron; the middle is lighter and crumblier, and on the bottom is a dark claystone. The big vertical streaks are backhoe marks, that’s how soft this material is. You can pluck it apart with your hands, scratch it with your fingernail. The dark layer is as creamy as chocolate between the teeth. As I stood there, the rattle of falling pebbles was nearly constant.

Covered with soil and shaded by trees, this rock will stay in place all right. But excavate into it and it turns to dry rubble. The roadcut is a steep slope of loose shale bits, topped with a meter or so of fresh strata and a big tangle of exposed tree roots dangling in the air. When the next big earthquake hits Oakland, expect this stretch of road to be buried and barred by fallen trees.

I think it’s earthquakes that have shattered this rock so pervasively over the years. It took thousands of them to lift these hills, and the process continues as surely as the continents move. Also, high, steep hills tend to focus seismic waves toward their peaks. Consider this account of the 1857 Fort Tejon earthquake in the Los Angeles Star (17 Jan 1857):

“We may here relate what has come to our knowledge through the Rev. Mr. Bateman, who was traveling to Fort Tejon at the time. Previous to feeling the earth’s vibration, his attention, and that of his party, was attracted by a tremendous noise issuing from a mountain in that neighborhood, south of the Fort. Immediately after, they felt the shock. In conversation with Mr. Botts, in charge of the mill at the Fort, he stated that his attention was also attracted by the same noise, and on looking towards the mountain, he saw issue from its topmost peak, a mass of rock and earth, which was forced high into the air—this was unaccompanied by smoke or fire. The shock immediately succeeded. Thereafter, a noise from that mountain was premonitory of every succeeding shock, no matter how slight.”

Tsunamis in Oakland

6 January 2010

The authorities have released new tsunami inundation maps, one of which includes Oakland:

tsunami map

Click it for the 2200×2200 version that I created just for Oakland.

There is no one tsunami that will wash over this much of the city; it’s a composite of a bunch of different possible tsunamis. In general, Oakland is like the rest of the Bay in being quite well sheltered from the kind of tidal waves that struck Sumatra five years ago. The Golden Gate keeps out the worst of the water. But it’s conceivable that some waves could wash up to about the 10-foot elevation level, especially in the parts of town nearest to the ocean. For residents, the worst threat would be in farthest West Oakland and the Jack London Square area. It also looks like the Webster Tube might briefly flood at the Alameda entrance, but probably not enough to make the tunnel totally impassable. The bridges would be fine, of course.

The time to worry about this is whenever there is a great earthquake in Cascadia or coastal Alaska. By “great” I mean an event of magnitude 8 or larger. But a lot depends on the details, like exactly where the quake occurs. If it moves a lot of seafloor, that would spawn the biggest waves. Even then, we would have several hours’ warning. In sum, tsunamis are a minor worry for Oaklanders—except for you living in boats.

UPDATE: By the way, TheOakBook.com spoke with me this afternoon about Oakland’s coming big earthquake and posted a pretty raw transcript here.

Sibley sights

13 September 2009

sibley

Sibley Regional Volcanic Preserve has some spectacular places like this spot at the north end of the park, where you can visualize when this was a working volcano. Click the view below for a bigger version.

sibley layers

This set of rock layers was laid down flat, but since then it has been tilted counterclockwise to nearly vertical. The red zone appears to be the baked top of a sediment bed that was buried in lava. The sediments were themselves deposited on an earlier lava flow, at right. I didn’t inspect this close up, though, for a couple of reasons. One, I was out for a walk and didn’t have my boots on. For another, this is easy to look at but extremely steep and hard to get close to. Finally, it’s hazardous, being prone to rockfalls:

rockfall

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.

Slump

28 July 2009

slump

This slump sits at the edge of a level area in Sibley Regional Volcanic Preserve. Notice the well-formed curved headscarp on the right side and the flow of material exiting the slump at its toe on the left. A classic textbook slump has a rotational aspect, because it moves along a scallop-shaped underground surface. This is easier to show than to tell, using this USGS image:

slump diagram

So the Sibley slump has aspects of an earthflow—basically it lacks some of the features of a slump. In a classic slump, that middle part would be tilted slightly backward instead of slightly forward.

But why is it here? First, I’m not sure whether this area is bedrock or a big tailings pile; that would involve walking down there and poking around, or looking for old aerial photos (which I could do from my desk). If it’s bedrock, that would be terrestrial mudstone and conglomerate of the Orinda Formation, the stuff you see flanking Route 24 just east of the Caldecott Tunnel. This doesn’t resemble that at all, so my working hypothesis is that it’s tailings: waste rock and soil from the days when this was a quarry. It has little strength, and its slopes tend to give way.

And why is it a slump, and not just a washout or a slide? That involves water and the consequences of human acts. This tailings pile appears to have been graded without due regard for drainage. Rainwater and runoff would collect on and infiltrate into this flat top surface. Then the groundwater would trickle its way through the pile and exit as seeps and springs near the bottom, where Fish Ranch creek is happy to accept it. But in a wet year the water table would build up and exert pressure around its edges. Water would rise and buoy up the precarious sediments at the base of this slump, easing the gravitational force holding it against the ground beneath. The details are important, and that’s why landslide management requires the services of a geotechnical specialist.

That reminds me: the new California budget plan consolidates the state Board for Geologists and Geophysicists into the State Mining and Geology Board. In itself, this should not threaten the state’s licensing system for geoscience professionals, but it does give the governor an opportunity to eliminate board members whose scientific advice tends to contradict business as usual. I don’t have any scuttlebutt to pass on, but I do know that commerce does not always look kindly on the costs of living in the real world, on the real Earth.

Two dams

1 June 2009

I was asked the other day about the safety of living below Oakland’s dams. We have two of them, both of which I happened to photograph in March 2003 (back when I was still using film). Anthony Chabot built them both. This is the dam at Chabot Reservoir, at the very south end of Oakland. The Hayward fault is a couple hundred meters downstream.

chabot dam

This is the dam at Lake Temescal.

temescal dam

Both are earthen dams, basically massive piles of clay and boulders. The first was built in 1874-75 and the second in 1868. As the “great San Francisco earthquake” occurred on the Hayward fault in 1868, seismic safety was high in people’s minds. Here’s a page about their construction. These dams are generally considered sound and able to withstand another big one. Lake Temescal straddles the Hayward fault, but the dam is so massive and the water it holds so modest that even a 2-meter displacement on the fault will not lead to a dangerous failure, as I understand it.

The Calaveras Dam, farther south near Milpitas, is also of earthen construction. It crosses the Calaveras fault and is being replaced with a safer design; in the meantime the Calaveras Reservoir has been drained to half its volume.

Rock failure

27 April 2009

quarry failure

The Rockridge Shopping Center quarry is in pretty good shape . . . for a Bay area quarry. Most of the rocks in the Coast Ranges are battered and shattered to various degrees, what with being ripped apart and shaken and squeezed into hills by the activity along the San Andreas fault system. Most of this former quarry is holding up well. But the whole length of the rock face has a chainlink fence in front, with piles of rubble trapped behind it. This spot is near a zone of dark, serpentinite-looking rock that has been failing more than the sandstone and lava around it. If these stout timbers can’t deal with it, I doubt the fence will help much when the rest of the slope starts to give.

Rock naturally deteriorates when you strip the soil off it. Actually the soil is made of deteriorated rock itself. Solid rock may be overrated.


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