Archive for the ‘Landslides’ Category

The landslides of London Road

29 October 2018

I spent some time last week in a concerted effort to examine the site of the 1970 London Road/Wilshire Heights landslide. (It took me five years to return to this haunted scene!) The effort was fun, but I learned more at home than I did on site. Gather round the campfire and I’ll tell you the whole tale.

To start with, here’s the most visible sign of this notorious event: the missing side of Kitchener Court, a hard-to-get-to cul-de-sac just south of the LDS Temple parking lot. I took this shot through a chainlink fence as a lone crow cawed.

Deep in the night, the land on the left side, along with its houses, fell into the narrow headwater valley of Peralta Creek, where it pushed a row of recently built homes off their foundations down below on London Road. To orient ourselves, here’s a time series of road maps with London Road in the center. Click it to see its full 1300-pixel width.

Today the whole south end of London Road is gone, except for a tiny stub at the top of Maple Avenue. The north end, of later vintage, extends south from Maiden Lane. The 2018 map, from Google Maps with terrain visible, shows the topographic setting well.

Once upon a time London Road ran up a steep, secluded little wooded valley. It was platted in 1923 as part of the Wilshire Heights development and named for Jack London. It was built out by the late 1950s, after surges of homebuilding in the 1920s and 1940s. Meanwhile, East Bay MUD put water and sewer lines through, and Shell Oil emplaced a fuel pipeline down the valley to carry gasoline from its Martinez refinery to the Oakland Airport.

What could go wrong? A lot. Every steep headwater stream in the Oakland hills is prone to landslides. And this particular stream has carved its charming valley into the pulverized rock along the Hayward fault. Here’s what that shattered, highly weathered material — fault gouge — looks like where it’s exposed in the headscarp.

It was mid-February in 1962, on a dark and stormy night, when the combination of heavy rainfall and leakage from a broken water main caused the hillside just south of Maiden Lane to fail. Three homes were destroyed, never to be rebuilt.

Who knows how long the main had been leaking, or why. Slow landslides could have broken the main, or steady creep along the fault. Either would suffice, but probably both took part. On top of that, literally, was the added load on the hillslope from the new homes and their construction. No ordinances were in place at the time to ensure good construction practices on unstable slopes. And of course, for their landscaping, homeowners just added water. The land was probably in motion long before it finally slid.

Then came more construction in the 1960s: the Warren Freeway on one side, the LDS Temple complex on the other. People in the neighborhood said they noticed new ground movements during those years, but no cause-and-effect relationship can be confirmed this many years later. It is true, though, that when the Mormons sculpted the hilltop for their temple they disturbed an underground realm of water and soil. This photo shows groundwater coming up through the parking lot pavement, a phenomenon people described as the emergence of “hidden springs.”

But for the catastrophe that followed, the unregulated 1920s-era homebuilding on the bluffs of Kitchener Court was cause enough. During heavy rains in early January 1970, the whole east side of Kitchener Court began to collapse onto the south end of London Road. Within two weeks, some 40 houses were affected as 15 acres of land turned to hummocky ruin. The pipeline full of gasoline held, but Shell shut the whole thing down shortly thereafter. (In March 1969 it had been blown up, down in Redwood Canyon, engulfing the village of Canyon in a horrendous fire. Then this. And opposition continued to grow until the bad idea was safely dead.)

This land at the very end of Jordan Road is still iffy after almost 50 years.

There’s also the rock — you knew I’d get around to that. I don’t quite trust the geologic map in this area, but I can attest that serpentinite occurs here as it does in many places along California’s faults. These two specimens are good examples of our typical mixed serpentine rock and blueschist, and large boulders or outcrops of the same are visible in several places. There’s more on the west side of the temple. Serpentinite makes slide-prone ground, too, even when it’s not steep or waterlogged as this place was.

But on the whole, my poking around was frustrating because the land is so heavily overgrown and fenced off. Fortunately the US Geological Survey got funding for a lidar survey of the whole San Andreas fault system, including the Hayward fault, and the processed data — a digital elevation model or DEM, available from OpenTopography — is a fabulous resource for this exact area. After subtracting the vegetation and the structures from the data, the DEM becomes a grayscale image representing the bare ground, with the ghosts of streets and houses, here computer-illuminated from due northwest.

What’s handier, here’s the same with the Hayward fault superimposed. I usually have to do a lot of talking and armwaving to explain things, but not this time. Click in and wander around. The blue outline signifies a sag basin.

My final piece is this closeup of the slide area along with the lot lines and rights of way — more ghosts, still there after all these years.

There’s just one little house left on the east side of Kitchener Court, right up against the temple property. Incredible as it may seem, the adjoining lot, half fallen downhill with cliffs all around it, has a For Sale sign posted. Talk about a haunted place!

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:

earthquake.usgs.gov/earthquakes/events/1868calif

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.