An earthquake is defined as any seismic activity that is caused by either humans or is natural. This activity then creates seismic waves. Earthquakes can be cause by many things, such as, instable faults, volcanic activity, mud slides, land slides and nuclear testing.
High ranked earthquakes are anything measuring above a 5 and have the potential to severely damage structures. Homes can slide off their foundations, or slide off slopes if built with stilts on a hillside an even suffer cracked slabs and destroyed cripple walls.
High-risk regions can be verified on the California Governors Office of Emergency Services. Click here to read more info and determine if your home is in an earthquake hazard area. http://myhazards.calema.ca.gov/
Retrofitting may be completed by a homeowner, but they are advised to take precautions while completing the project. Requirements and risks for homeowner retrofitting can be found through the California State License Board webpage.
The contractor should confirm the correct length of the wall. The may then indicate the specific sizes of plywood and sheathing needed and provide that information on a written report with the building permit. A building official must have final approval of these documents.