New Earthquake Zoning Maps Should Not Rattle Bernal Heights

Image snapped and modified from California Geological Survey website

Uh oh?

Last week, the California Geological Survey released its latest set of fault zoning maps. These divide the state into a rectangular grid, with the parts of the grid containing active faults marked in red. Unsurprisingly, there’s a lot of red throughout the state, particularly around the Bay Area and greater Los Angeles. Bernalwood happens to fall along the edge of one of those squares.

So, what does that mean for us? Is the ground beneath Bernal more stable now, or less?

Luckily, the new maps mean very little for us everyday residents. Living within a red square of the grid doesn’t mean the entire square is riddled with active faults that nobody knew about until very recently. It just means that, somewhere within that square, there’s at least one active fault that can cause some mayhem. In the case of the square that contains Bernal Heights, that fault is the San Andreas. The shortest distance from the top of Bernal Hill itself to any part of the San Andreas is still a good 6.25 miles.

These new maps were created primarily for the sake of real estate developers. They’re part of the Alquist-Priolo Zoning Act, which was enacted in 1972 in the wake of the 1971 M6.6 San Fernando earthquake. Aside from some strong shaking that knocked down buildings and freeway overpasses, one of the major problems with that earthquake was that strands of surface faulting popped up in people’s houses unexpectedly.

The initial Alquist-Priolo criterion was that commercial structures or large-tract housing developments may not be built within 50 feet of an active fault, to avoid the possibility that half of a house may become offset from the other half by 20 feet or so. It’s difficult to avoid strong ground motions in a fault-ridden place as California, and a 50 foot distance from the fault isn’t really going to help much in terms of shaking, but avoiding structural surface ruptures is basically as simple as knowing where the faults are.

So, what these new maps (and the older ones) mean for developers? Anyone who wants to build within one of the red squares on the grid must now consult a more specific fault map to determine where they can actually build.

And that brings us back to the question of what all this means for those of us in Bernal Heights, the rest of San Francisco, and California in general. It means — as we already knew — we live in earthquake country. The map doesn’t say anything about shaking hazard, given that closer proximity to the fault generally means higher ground motion. For that there are separate maps for potential ground motion, and they all put San Francisco in a bad place.

But within that, Bernal’s solid foundation of chert means we’ll shake less than the unconsolidated fill in places like the Marina or SoMa — even though we’re in a red box and they are not. So we’ve got that going for us

6 thoughts on “New Earthquake Zoning Maps Should Not Rattle Bernal Heights

  1. It’s hard to tell how seriously to take those “red boxes.” For example, the map shows Santa Cruz in the clear – not even near a red box. And yet, buy all measures, Santa Cruz bore the brunt of Loma Prieta. It’s downtown was nearly destroyed.

    • The red boxes are all about surface faulting, and preventing things getting literally ripped in half by fault slip. The Alquist-Priolo Act has nothing to do with ground motion, though generally, the closer to the fault, the stronger the shaking. The damage in Santa Cruz was very severe, but it was due to intense shaking, not to surface faulting.

      The Loma Prieta rupture actually didn’t reach the surface at all. Any ground cracking was from spreading and settling and liquefaction, not the actual fault. The quadrangle that includes the fault responsible for Loma Prieta is marked as red on the map.

  2. It costs less than you would think to get your house bolted to the foundation and to put up a few shear walls so your garage/house won’t collapse when the big one hits. It’s nice that we are on rock but if you are smart you will take some reasonable precautions.

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  4. Thanks for this–I looked at that map but couldn’t quite figure it out.

    My husband and I moved into Bernal four days before Loma Prieta. We had just put our collection of ceramic pitchers up on the open shelving in the kitchen. At the time we were working in Sausalito, in a flimsy two story building on fill. When the earthquake stuck, the building swayed and bucked like a cardboard box man-handled by a giant; we could see waves ripple across the floor. Nobody was hurt, but as we drove home through the dark and saw the fires in the Marina, we were sure that our little house would be damaged. Amazingly, not a thing had moved! Every pitcher was in place. Hurray for Bernal bed-rock!

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