Oakland Geology

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.

This entry was posted on 15 October 2018 at 8:02 am and is filed under Earthquakes, Landslides. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.