Many west coast homes built before 1950 do not survive earthquakes very well. Sadly, houses that are much newer than this can also have significant vulnerabilities to earthquakes. Houses built over a crawlspace beneath the first floor framing are particularly vulnerable to earthquake damage. The main floor of the house is supported on short "cripple-walls" that typically are not strong enough to brace against the shaking forces generated by an earthquake.

The most devastating earthquake damage occurs to wood-framed homes when the cripple walls collapse and the house falls several feet to the ground. Figure 1 shows a house that fell about four feet (note the stairs leading up to the porch, and how the top of the front door is about three feet above the porch).

Figure 2 shows a house where the floor framing sat atop concrete footings; in this case the framing was not connected to the footings adequately, and the entire house slid off of them.

Other common weaknesses are the connection between the house's foundation to the cripple wall or between the cripple wall and the main floor framing.

Further problems include the strength of the foundation itself and other factors that usually have to do with the floor plan or shape of the house.

Beware of Ineffective Retrofits

California has only recently adopted statewide standards for residential earthquake retrofits as The California Existing Building Code (Title 24, Part 10). This code section shows only a small fraction of the conditions that occur in the field; contractors or engineers must invent their own details for other framing situations. Sometimes the invented connections do not consider all of forces and movement that must be resisted by an effective retrofit. There are no special training or licensing requirements for contractors who engage in retrofit work. Many home owners spent thousands of dollars on retrofit work that will not protect their homes at all, or certainly not as well expected.

The length of “angle iron” with pairs of bolts at top and bottom (see Figure 3) was intended to prevent earthquake damage. This sort of connection is untested and most structural engineers believe that it is very poor. The shiny silver connectors at the left of the image were installed to "retrofit the retrofit." These new connectors have been tested extensively and are accepted by almost all building departments; angle irons are not. There are many other sorts of connections that will not perform as hoped in an earthquake..

An Earthquake Retrofit Success Story

Just before I graduated from college, I bought a home in a planned unit development in Paso Robles. The house has a concrete tile roof. Tile roofs add a lot of weight to a house, and increase the earthquake forces on them. About 60 other units in my development are virtually identical; the garages have narrow walls on either side of the garage door.

Eventually I changed jobs and moved three hours away, but kept the house as a rental. One time between tenants I took some initial steps to strengthen the walls that flank the garage door. I got about half-way through the process before my 'vacation' was over and I had to return to work. Soon the place was rented out to a professional roofer—I had installed about $25 worth of hardware, but I never completed the retrofit.

On December 22, 2003, the San Simeon earthquake hit. I called my tenant, the roofer, just after the earthquake to see how things were. He marveled, "It's really weird—all the other houses on the block had the ridge tiles on their garage roofs thrown about three feet to one side, but not a single roof tile moved on this house!" And that was with an incomplete retrofit.

Figure 4 shows narrow walls that did not have the strength needed to prevent severe damage to a garage. Current building codes require much greater strength than the tall narrow walls shown. A fairly inexpensive retrofit could greatly reduce the need for extensive repairs after a quake.

Why Condos are Vulnerable to Earthquakes

Many condos were built in the 1960's and 70's. Rows of identical units provided economical housing. However, even though the buildings may have met the building codes in effect at the time, experience has shown that some common types of condominium buildings from that era do not survive earthquakes very well. Enormous advances in earthquake-resistant construction methods have occurred in the last 20 years. Also a huge variety of construction hardware specifically intended to resist earthquakes is available today that did not exist 20 years ago.

Figure 5 shows a typical wood-framed condominium building. The front wall has many openings for garage and entry doors. The rear wall of the building is similar, with large openings for patio doors and windows. These openings leave little wall length to resist earthquake forces.

Figure 6 shows what used to be two 2-story buildings. The building on the left in the photo collapsed because the lower floor front wall did not have adequate strength to resist earthquake forces..