Archive for the ‘oakland hazards’ Category

Drainage

18 December 2010

Oakland is a fine town unless it rains too much. Then we have to worry about all the water.

drainage

All things considered, Oakland’s landscape would prefer to be steep, forested hills raised by tectonics along the Hayward fault and gentle coastal plains that absorb the sediment washed off those hills. It’s a rich recipe that produces redwoods in the heights and forage and fruit in the vales. But with impervious roads and homes carpeting the upper slopes, we increase the runoff and undermine our own infrastructure. People like the homeowners above Broadway Terrace run flexible plastic lines over the edge of their properties to put the problem out of mind. But if you walk the road, you’ll find fresh gullies that will work their way uphill to the source of the problem regardless. Landslides will probably follow.

Seismic engineering at Kaiser Hospital

10 December 2010

Kaiser Permanente is building a new hospital complex at Macarthur and Broadway, including this structure. The design is intended to keep the hospital fully functional after a major earthquake.

seismic engineering

That explains the sturdy steel, but also note the number of diagonal braces.

seismic braces

These braces are not rigid, the way they are in scaffolding. Instead, they are built like pistons, with the ends allowed to move inside the sleeve. They absorb energy and help damp the structure against rhythmic shaking that can destroy it. They deform to help save the rest of the framework. That way, after the quake they can be swapped out to make the building as good as new. Read more at this manufacturer’s site, for example. If you’re passing by the hospital (or any construction site, for that matter), take a look.

The big earthquake will probably cause cosmetic damage to the outside of the building, and some broken windows and so on. The hospital as a working institution, though, will not just endure but keep on saving lives without interruption. This is a big deal, and all of California’s hospitals are following suit to meet the state’s deadline of 2030 (see the pamphlet “California’s Hospital Seismic Safety Law” for details).

Ruins

7 July 2010

As you explore Oakland, you come upon places where something has been erased and not yet replaced.

oakland hills fire

The Oakland Hills fire of October 1991 left these, the first on Acacia Avenue and the other two on Roble Road.

oakland hills fire

There must be reasons for each of these remaining ruins after almost twenty years. But here they are, some quite public and others in quiet privacy. People pass and pay them no mind. Another few decades and the traces might be gone.

oakland hills fire

I got an odd request last year: Henry K. Lee, author of Presumed Dead, asked me to visit the spot in the Oakland Hills where a notorious murderer put the body of his victim. He wanted to know how a geologist would describe the ground there. It was shale, crumbling and easily dug. The place was shrouded in oak woods, but everyday life was within earshot: lawn equipment whining, bicyclists conversing, dogs. The site—which I will not call a grave—was still being visited. But its traces should be left to vanish.

Living on Bay mud

27 March 2010

The Tidewater district is the nose of land west of the freeway at the end of the channel between Oakland and Alameda:

tidewater liquefaction map

Right now it’s totally industrial, but landowners there want to open it up to residential uses, like the cozy parts of Alameda right across the channel. (Easy to catch up with the news by googling “oakland tidewater industrial“.)

I’m showing this image to point out a geological aspect to the zoning proposal. The map is a portion of the state’s official map of landslide and liquefaction potential, and it’s clear that this low-lying piece of mostly filled swampland is one of Oakland’s worst places in a large earthquake. Just saying.

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.”


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