Quake busting

California is earthquake country. From the iconic 1906 earthquake, to the daily reminders that we live on top of 3 major tectonic faults, earthquakes are a big part of the local consciousness. You would imagine that, after San Francisco was almost leveled in 1906, the building codes would change to take into account such large tremors and build stronger houses. Unfortunately, that's not the case. It's true that a majority of buildings in San Francisco are wood-frame houses with at most 3 floors, which tend to be pretty flexible and withstand earthquakes reasonably well (although they can easily succumb to fire). Still, lots of older houses are vulnerable: they can shift and fall off the foundation, or the lowe stories can crumble under the weight above them. This is especially true of soft-story homes, which are prevalent in the Sunset and Richmond districts. To reduce earthquake risk, it is possible to retrofit an older home to better resist a large earthquake. There is a lot of good evidence that such retrofitting can make a big difference. So what goes into a retrofit?

1. Foundation work. Many older houses literally "sit" on the foundation with no additional reinforcement. When the ground shakes, it can "push" the house off the foundation. Even a small push, say a few inches, can have disastrous consequences, it can sever water, sewer, and gas pipes, start a fire or worse. To mitigate this, the house can be bolted to the foundation, so that the ground and the house move as one.
2. Cripple wall work. Most houses don't sit directly on the foundation, to prevent the wood from getting damaged or weakened by natural ground moisture and the like. Houses are either built on top of a narrow "crawl space", or, in the case of soft-story houses, the living quarters are built above the garage. In an earthquake, the heavy part of the house above carries a lot of inertia, and if the underside is bolted to the ground, the house above can shatter the walls underneath and fall. To mitigate this, the lower walls can be reinforced with plywood shear walls that strengthen them and essentially stiffen the house, preventing the upstairs from swinging wildly in a quake.
3. Garage opening reinforcement. Even with bolts and shear walls, the garage door opening remains a major weak point, as it weakens one of the key structural walls of the house. This can be reinforced with a steel frame or a steel beam to give it the same strength as the other 3 walls.

Here's what it looks like (courtesy of seismicsafety.com): For most houses, the retrofit work can be done just with the help of a skilled contractor, who's qualified to install bolts and shear walls. For some houses, notably those on a steep hill, or soft-story designs, the help of an engineer is needed to design the retrofit and compute the appropriate material strengths, after which the contractor can install it. ABAG has a wealth of information to help people decide the risk of an earthquake and the impact in their specific area. In particular, the maps of shake intensity and liquefaction risk are extremely useful. In San Francisco, having your house on top of a hill can dramatically reduce both shaking and liquefaction, though it does increase the changes of a landslide. Sadly, the Marina with its gorgeous houses and amazing views, is built on top of landfill from the 1906 earthquake, so it's very vulnerable to an earthquake; it's no accident that in the relatively minor 1989 Loma Prieta earthquake, the Marina suffered the most damage (map courtesy of thefrontsteps.com). Another way to mitigate against earthquake risk is to get earthquake insurance. In California, this is of questionable utility: if a major earthquake were to hit, the CEA would probably run out of money, at which point people say that FEMA would have to step in and help the reconstruction. During that time, people would probably have to live in temporary houses, like the ready-made earthquake shacks of yesteryear. Still, having some earthquake insurance can provide a good cushion in the case of a major natural disaster. I personally found that learning about earthquakes made me worry about them less and take the necessary steps to increase our safety. I realize there are no guarantees, but doing even little things can go a long way towards helping deal with this reality. And I certainly wouldn't trade living in San Francisco for anywhere else, earthquakes and all.