Now that I’ve backed myself into a metaphorical corner, let me put the situation in simpler terms. To make a wood-frame house more earthquake-resistant, you need to prevent it from turning into a parallelogram by tying together all the joints in the frame, and connecting the house to the foundation. Generally, the way to do this is with plywood and specialized metal fasteners. The specifics of how best to install these materials are an ongoing discussion between engineers, building officials, and contractors, and they do not always agree. Nevertheless, the following information, based upon the seismic codes adopted by Los Angeles and other California cites after the 1994 Northridge earthquake, seems to be fairly widely accepted. Be aware, too, that any strengthening will be for naught if the framing is damaged by wood-destroying insects or rot, so be sure to repair these areas before beginning any seismic work.
Retrofitting a house to resist earthquakes begins with having the best possible foundation. Ideally, the foundation is reinforced concrete and shaped like an inverted T with a wide base, called a footing, that spreads the load like snowshoes. Unfortunately, few old houses have such a foundation. A foundation of unreinforced concrete (poured without steel reinforcement bars) is okay, but unreinforced brick, stone, or concrete block foundations are not good because they will disintegrate in an earthquake like our proverbial croquembouche. Other types of foundations, such as those comprised of individual piers, built on hillsides, or constructed as garages under the house, are beyond the scope of this article and should be evaluated by an engineer. In general, consulting an engineer is a good idea for any sort of house, particularly those built with balloon framing and cripple walls (more on this later).