Prior to 1938, the majority of residences were built to withstand one type of force or load as the engineers call it - the force of gravity. That's only an up-and-down force. Unfortunately the most damaging component of earthquake forces are from side to side, producing what engineers call a lateral load. Therefore, older buildings that were adequately built to resist up and down forces might collapse under the lateral stresses of some earthquakes.

When dealing with conventional wood framed residential structures, there are three basic types of lateral failure, and as a consequence there are three different needs in seismic reinforcement.

Bracing against one type of lateral failure does not protect you from the other two. That's why a retrofitting scheme should be planned by a knowledgeable professional -- an uncoordinated approach could leave your house almost as vulnerable after reinforcement as before.

-- Tips courtesy of HomeAdvisor.com