Archive for the ‘oakland hazards’ Category

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.

Vantage Point Park

15 February 2009

vantage point park

This tiny park is at East 12th Street and 13th Avenue, on a low rise next to a lot of activity. Between here and the water—Brooklyn Basin, the innermost part of Oakland Inner Harbor—run East 8th Street, I-880, Embarcadero Street, BART and the Southern Pacific rail line, plus I’m sure a number of underground power and water lines and what not. It’s a highly concentrated lifeline corridor. Across the way is the Coast Guard base on Government Island. In the distant left corner in the 800×600 version (click the image) you can just see Alameda Island. Except for the hummock in the foreground, everything visible is made land, artificial fill, with a high water table. It was created more or less haphazardly starting a century ago, and under strong shaking a lot of old fill of this type is prone to liquefaction.

Past the left edge of this photo, a little buried creek valley running down 14th Avenue reaches the bay. It’s all filled in, too. All the crowded life lines I mentioned cross that creek bed. In the next big Hayward fault earthquake, this is a highly vulnerable spot.

I took this shot on 19 November 2008 during a walk I took the length of Oakland, from the San Leandro to the Ashby BART stations. Lots more photos from that day on my Fotothing site.


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