Archive for the ‘Landslides’ Category

The Hayward earthquake: 1868-2018

15 October 2018

There’ll be lots of press this week about the anniversary of the “Great San Francisco Earthquake” — the original one, 150 years ago on 21 October 1868, caused by a big rip in the Hayward fault just before 8 in the morning. (Also the annual ShakeOut exercise, this year on 18 October at 10:18 am.) Behind the press stories there’s lots of sound info for you, and I’ll put a list of good links at the end of this post. Rather than write another standard thing about the earthquake, I’ll focus on what I’ve been studying lately, which is the problem of earthquake landslides.

Contemporary accounts of the 1868 quake tended to focus on homes wrecked and buildings ruined, but several geological manifestations were widely recorded: the ground cracked open from Oakland all the way to present-day Fremont, some of the cracks spewed muddy water, streams ran high afterward, and many new springs appeared near the crack.

Only in more distant places like the San Mateo Peninsula did people mention rockfalls and landslides — which tells me those were the most widespread forms of damage that day. Landslides were everywhere, even in places where buildings weren’t bothered. And though our buildings are even stronger today, the landscape is weaker than ever.

Landslides were everywhere in the East Bay hills in 1868 because that’s always what happens. No one made mention of them because they were unremarkable. The hills were empty countryside, a mix of spent rangeland and razed redwood groves. What matter was a landslide in that waste? But nowadays . . . when the next large Hayward fault earthquake comes, landslides will cause widespread and costly misery — in the high bedrock hills as well as the low gravel hills of the Fan and along the fault.

We don’t see the scars of the 1868 slides today because after a few decades, they fade out. Here’s a small slump in Sibley Regional Volcanic Preserve that I photographed in 2005, 2009 and 2017.

I think in another 12 years it will be pretty subtle.

The high hills owe their basic shape to earthquakes and landslides. Their sides are steep slopes, swept clean and straight as the Hayward fault raises the heights by about a millimeter a year, one earthquake at a time. So they’re always primed to slide.

USGS landslide researcher David Keefer estimated in a 1984 paper that earthquakes as small as magnitude 4 are large enough to cause rockfalls. A magnitude 6.8 event, the size of the 1868 quake, would cause up to 1000 rockfalls in the affected area.

And up to 1000 slumps like the one at Sibley.

And up to 1000 debris slides, what people usually call mudslides. The majority of these will start along roadcuts, which inherently destabilize hillslopes . . .

. . . and near hillside homes, which do the same.

The city will be overwhelmed. The roads will take months to clear, years to fix.

All these photos show rainfall landslides, or sites where one could happen. They’re the usual kind. They happen where the flow of water, both on the ground and below it, unbalances the slope and makes it fail. They especially tend to form on the sides and floors of valleys, where the slopes converge.

Earthquake landslides, though, tend to start on ridgetops and promontories, landforms that focus seismic energy toward their tops. They also aren’t confined to the wet season. Sites that aren’t prone to rainy-season landslides may instead be the preferred targets of earthquake slides. If we’re lucky during the next big quake, as we were in 1868, the ground will be dry. If we aren’t, and the ground is waterlogged, well, heaven help us because we’ll get both kinds of landslide at once.

This danger is built in with the tectonically active setting that makes Oakland so beautiful. We have to prevent what we can, and cope with what we can’t.

Here’s that list of Hayward fault resources. Actually, the whole list is on one page, from the U.S. Geological Survey:

In that list I want to single out “What to Expect in a Big Urban Earthquake,” a phone-friendly “geonarrative” aimed squarely at people who live in cities — that’s us.

Landslide update from the Sports Center fire road

20 August 2018

Ten years ago, I took my first walk on the fire road above the North Oakland Regional Sports Center (Caldecott Field), where I saw fit to document an incipient landslide there.

In June, standing on Skyline Boulevard, I noticed that the site was shrouded in black plastic, a surefire sign of a landslide. Passing by again last week, I noticed a change and made the time to visit. The change was that the center of the previous landslide had given way in a new landslide.

This new slide looks ugly, and of course the fire road will need fixing, but the slide material didn’t seem to go far downhill.

The picture seemed pretty simple to me, standing there, but then I came home and looked at the area, in Google Maps 3D, featuring imagery as of a couple years ago. The view is to the south.

Two things to see here. First, the fire road was resurfaced at the place where I shot the 2008 photo (just below the water tank). So the slide shrouded in plastic was likely the second one since 2008. Second, the hillside below the slide is stripped of trees — that is, it’s a landslide chute. Maybe the slide I shot in 2008 did that. Air photos from 1968 and 1939 show nothing distinctive at that location.

If only I’d been paying attention here over the last 10 years! But Oakland’s a big town with a lot to keep track of.

Landslides tend to be persistent: once one starts, others follow in the same place. This is especially true in the rock exposed here, which is mudstone of the Sobrante Formation. Here’s an exposure of it in a roadcut on Thorndale Drive. This stuff falls down real easy. Elsewhere in the hills I’ve called it “punk shale.”

A closeup of another roadcut shows wavy lines caused by shear within the rock.

The Sobrante was a big headache to the people excavating the Caldecott Tunnel bores. It caved in on the men digging the first bores, and the fourth bore required heroic engineering to keep it all shored up so the concrete could be poured. Likewise, houses built on this rock need strong foundations and designs that are sensitive to the site.

An Elverton update

23 July 2018

After a visit five years ago, I had high praise for Elverton Drive: “From end to end, it offers the best exposures anywhere of the Claremont chert.”

This stuff, as seen a few weeks ago during a return visit.

Those of you who’ve followed along know the amazing striped chert of the Claremont Shale, which crops out in a belt from Claremont Canyon along a couple miles of Skyline Ridge to Huckleberry Botanical Preserve and beyond in the hinterland. The fat pale stripes are layers of microcrystalline silica — chert — and the thin dark ones are layers of claystone — shale.

During this visit I walked from the south end of Elverton past the newest set of houses, near Huckleberry, and had a good stop in the old borrow pit. The wall has crumbled a bit since five years ago, opening this fine exposure.

I was hoping to find pieces of dolomite rock, which are present as an uncommon third ingredient, so I gave the rubble a good look. None of that there, but I was interested to see some extra-thick pieces of the chert and shale.

The chert, in fact, was very light. It was barely changed from its original state as diatom ooze on the seafloor, almost the balsa-wood lightness of the Pinole diatomite. I did not expect that.

At the other end of the pit is the same big-ol’ boulder that was lying there in 2013. This is not a decorative rock placed there to look good; no, it fell here from the beetling cliff above and stopped rolling just short of the roadway. I recalled writing in 2013, “if you feel an earthquake while you’re there, step the hell back.”

Every time I visit the high hills, the pleasure of geologizing gives way, sooner or later, to a sense of dread at the state of the roadcuts. The eucalyptus roots in this scene were exposed as the hillside crumbled away, and behind them is a concrete cast meant to slow down a landslide.

But thinking ahead I looked forward to admiring this again after five years away. Google Street View still shows it.

Instead, it’s being shored up and fitted with a shotcrete shroud.

And another splendid exposure farther along is being smothered too, with no finesse.

In fact, not long afterward I started to despair of Elverton Drive. Is this the point of occupying such a spectacular setting? To cover it with property? To look outward and not downward?

The Claremont chert isn’t as solid as it might seem. Given the tendency of these young rocks to crumble, there’s no guarantee a new house in the high hills will survive its first mortgage. Or that the road will last that long.

Look out. Don’t look down. Elverton Drive is falling apart while it’s still filling up.

I already miss the place.