I gave a talk to an Oakland group the other day that discussed three geology-related topics: quarries, landslides and earthquakes. Landslides were the hardest topic to condense. This post will serve as a home page for things I’ve posted a couple dozen times over the years on this blog.

^{Landslide at Sibley}

Landsliding is the ordinary, natural process that carries stuff down from the crumbling hills — gravel, sand, clay, and so on — to the streams and rivers that carry the stuff away. Rain and wind do a bit of that work, but landsliding does the overwhelming share. In the literature, geologists use the sanitary term mass wasting to cover everything from rainwash on exposed topsoil to rock avalanches that mobilize whole mountainsides. The rest of us think of landslides as “natural disasters,” the awkward, deprecated name for bad things we brought upon ourselves by standing in their way.

The first thing to know about our landslides is that rock doesn’t last. As erosion nibbles its way downward toward bedrock, solid rock starts to rot, setting off to become soil. Exposing rock to the weather, whether in a quarry or a roadcut or a building foundation, accelerates the rot. The second thing is that underground water lubricates, buoys and exerts pressure upon everything in the “critical zone” between stone and topsoil. Third, wherever there’s a slope, gravity adds its weight to this weakening situation. These factors add up as the years go by: rocks decay, rains come and go, streams dig their canyons deeper while tectonic forces raise the hills and shake them regularly. Human disturbances are often the last straw.

I’ve documented landslides all over Oakland, most often in the high bedrock hills (Vicente Canyon, Temescal Canyon, Sibley Volcanic Preserve, Thornhill Canyon, Shepherd Canyon, Redwood Heights, Crestmont) but also in the lower gravel hills (Bella Vista, Dimond District, Jungle Hill, Millsmont). Almost all of them stem from human actions, which makes sense because I found them where humans have been active. I’ve also found natural slides in off-trail places, so they exist too.

Let me say a little more about the role of water. A roadcut, unless it’s carefully made and maintained, concentrates rainfall runoff directly into the ground on one side and over the edge on the other. The extra water injected into the ground adds up over time; the extra water sent over the side adds to the surface wash below. Gutters and drains don’t help when they merely shunt the runoff straight downhill, where the rivulets dig ditches and gullies. Home foundations can be thought of as miniature roadcuts and have the same effects. During the rainy season, major storms put extra surges of rainwater into the ground and landslides follow, sometimes weeks later. Geotechnical engineering is the licensed professional practice that cares about the details of landslides and hillslopes.

At this moment in geologic time, all of our landslides, whether they reflect underlying human or natural causes, have been triggered by water. They happen in the rainy season. I say “at this moment” because the other major trigger of landslides is earthquakes, and we haven’t had one here on the Hayward fault since 1868, when there were no homes at all in the hills except a farmhouse or two. I wrote a post on that subject for the quake’s 150th anniversary. For a glimpse at what will happen when we get our next Big One, or just another big-enough one, see this photo of the mountains near Chengdu, China, after the great 2008 earthquake.

^{Photo courtesy Dave Petley (source)}

Notice that the slides are shallow and on the small side, but there are hundreds just in this one view. And it was an undisturbed natural setting, with no roads and houses making the slopes more vulnerable. In Oakland it will take years to fully repair the damage, or condemn whole streets and forbid new houses (as we did on Armour Drive and McKillop Road). It’s conceivable that in a few centuries the hills will be empty again.

Landslides in the high hills are a costly civic burden we brought upon ourselves — rather, one the Two Franks, Frank Havens and Frank “Borax” Smith of the Realty Syndicate, brought upon us a century ago by selling hillside bungalows on precarious land to white pioneers of the car-based lifestyle. And as I’ve noted, the wealthy, determined builders and defenders of today’s oversized, isolated houses continue to build exactly where the landslide hazard is greatest. The city pays to keep up the deteriorating roads and homeowners pay more for landscaping that’s hard to maintain against the heightened forces of erosion.

We will be living with landslides indefinitely.

This entry was posted on 11 May 2026 at 7:58 am and is filed under 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.