Heavy Rains Heighten This Berkeley Hills Hazard

A worsening landslide is prompting road closures and evacuations from an unstable Guerneville hillside. The slope failed after significant rainfall oversaturated the soil. In some places, Sonoma County has received over thirty inches of rain so far this year. Residents of mountainous regions are thoroughly aware that shifting earth follows heavy rain like a malevolent twin. From Crescent City to San Diego, California is no stranger to landslides. The same physics apply to the Berkeley Hills, whose landslide risk comes from above as well as below. 

The Berkeley Hills are like the crushed front end of a car that crashed into a brick wall head-on. Earthquakes are responsible for shaping every hill and mountain range you see in the Bay Area. East of the Hayward Fault, a shale and sandstone seafloor lies buried beneath younger volcanic and sedimentary rocks. Regional tectonic forces are compressing the crust, uplifting some here, downwarping some there. At the summit of Mount Diablo, primordial seashells receive snow in the winter. The pillowy hills of the inland East Bay owe their hummocky shape to wind, landslide, and complex faulting. The Central Valley’s primitive rivers carried silt to the prehistoric Bay Area, where fault activity lifted it aloft. Fast-forward to present-day, where a mountain chain spans the California coast—and they’re still growing. 

In California, the Earth is competing with the weather to build mountains faster than nature can wear them down. But don’t be quick to underestimate nature, particularly as the consequences of global warming become more extreme. Personally, I prefer the rain, especially as opposed to drought. However, too much of any good thing can have drastic consequences. Heavy rains are the primary cause of landsliding in the Berkeley Hills. 

A major landslide in the Berkeley Hills occurred in January 2023 at the onset of an exceptionally severe storm season. Shortly before 6:30 one Monday morning after back-to-back rainmakers, a landslide crashed into the house of Marjorie Cruz. Cruz was one of fourteen Park Hills residents whose homes were red-tagged. The slide represents the predominant slope type of failure in the Berkeley Hills, a shallow, rain-induced mud and debris flow. Meanwhile, another kind of landslide digs farther and deeper into the substrate, ongoing yet almost imperceptible. 

The Berkeley Hills are always on the move. The slopes are growing, shifting, rearranging, and not always to the delight of their residents. Parts of the Hills are actively sliding, some of them up to thirty-eight millimeters a year. Many “are in continual motion year-round in Berkeley; the largest two are just north of the Berkeley Rose Garden and John Hinkel Park.” (The Berkeleyside, 2023) It isn’t what most people picture when they think “landslide” (I jump right to Stevie Nicks myself). I find this type more tragic in that ultimately, nothing can protect the over 6,000 structures pinned to the Berkeley Hills. The creeping slopes are effectively “walking” the range along the length of their progenitor, the notorious Hayward Fault. 

It wouldn’t be California if earthquakes weren’t somehow involved. The Hayward Fault is why the Hills exist, why they’re still growing, and why portions may come crashing down. The next major quake, expected anytime now, might shake the vulnerable hillsides loose. Ongoing slides pick up speed while new ones take bites out of yards, canyons, cliffsides, propelling debris downslope. I would sell my vulnerable hillside home to somebody more comfortable with the risk. The whole state’s going to shake at some point, and when it does, landslides are inevitable. 

USGS research civil engineer Dr. Brian Collins says the “worst-case scenario” is for “an ill-timed magnitude 7.0 earthquake to rattle the Bay Area right now, after weeks of heavy rain.” If you’re a Berkeley Hill-dweller, put together a bug-out bag for rapid evacuation in case authorities tell you to leave. Establish a meeting point with loved ones on flatter ground. In the meantime, take any strong earthquake as a sign of imminent ground failure. If you hear or feel the ground rumbling, don’t ignore it. The way to survive is by paying attention, then getting out of the way.