The earthquake jolted this house off its foundation. If someone had retrofitted this house to keep it on its foundation before the earthquake it would have remained more or less intact. The whole purpose of a retrofit is to keep the house on its foundation.
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 and the rooms are all cubes and cubes are a very strong geometric shape. The resistance of cubes to collapse prevent significant damage to the main house.
While working for FEMA the author inspected this house and it held together so well that the windows did not even crack.
Same House: earthquake damage on the Inside
Even if the house holds together well, if it falls off the foundation, it can still sustain damage to interior walls, plumbing, and electrical systems which can be catastrophic.
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.
Cripple Wall Collapse Photos
Below are some Examples of Homes that fell off their Foundation either because the cripple walls were not braced with plywood, were not bolted, of the floors were not attached to the cripple walls.
In the photo above, look at the top of the concrete steps, then see the porch which is a few feet below it. This is the first sign that the cripple wall has collapsed.
Prepare for aftershocks. The earthquake you just felt is probably only an appetizer