The 1964 Alaska which lasted a record breaking 4 1/2 minutes jolted this house off its foundation and continued to shake it violently minutes afterwards. If someone had retrofitted this house to keep it on its foundation before the earthquake it would have remained more or less intact. This house was built before building codes protected houses against earthquakes from the first floor up. Even then, the walls did not collapse, the roof did not cave in; even the windows did not break. Many people worry about this even though there is no empirical evidence to support this fear.
When this old pre-building code house held up so well the windows didn’t even crack. The interior walls will have cracks, the plumbing and electrical systems will be torn out, and it will be a long time before it can be lived in. You can imagine how well a modern house built according to earthquake conscious building codes will hold up. In other words, unless you are on a steep hill or have living area above a garage, there is little reason to worry your house will be severely damaged in an earthquake so long as it can be kept on its foundation.
The whole purpose of a retrofit is to keep the house on its foundation. If this is properly done the potential for severe damage is practically zero.
While working for FEMA the author inspected this house and it held together so well that the windows did not even crack. The interior was a different matter as you will see in the photo below.
Same House: earthquake damage on the Inside
Even if the geometric shape holds together, if it falls from its foundation it can still sustain catastrophic damage to the interior walls, plumbing, and electrical systems. If the walls are plaster it is very common for the plaster to fall off the walls.
The author was the FEMA inspector who evaluated the beautiful house below after the 1989 San Francisco Earthquake. Two weeks after the evaluation I drove by and saw an empty lot. If it had remained on its foundation someone would still be living there.
The house shown below is a very typical tract house found in San Jose and the newer parts of the Bay Area. After being retrofitted these homes are extremely resistant to earthquake damage because a direct connection of the floor to the foundation can be make with steel and the rectangular shape means the house will react to an earthquake in a very predictable way. If you live in a house like this, and do a good retrofit, there is no reason your house should sustain serious damage even if you are quite close to an earthquake fault. If you are very close to a fault you simply need to make sure your house has an extra strong retrofit that will withstand any potential force it will need to withstand. House like this DO get cracks in the sheet rock and stucco, the chimney will fall down, if there is a brick facade if might fall off, and the doors may not fit perfectly in the door ways, but that should be about it and a good handyman should be able to fix everything.
Why so little earthquake damage?
The honeycomb of cross-walls in hallways, bedrooms, bathrooms, kitchens etc prevent collapse of the living area. The house itself has walls, a ceiling, and a floor which are all attached to each other. The house with its ceiling, floor, and walls is a cube and the rooms inside this large cube are also cubes. Cubes are a very strong geometric shape which is why houses from the first floor up suffer so little structural damage if they remain on their foundations. significant damage to the main house.