Earthquake retrofits consist of strengthening houses built before seismic building codes were standard practice, to make them more resistant to earthquake damage. Many homes built before 1979 are not designed properly to resist lateral movement by earthquakes, which may cause the homeowner to consider a seismic retrofit.
A structural/civil engineer or certified inspector can determine if a structure needs a seismic retrofit. Seismic retrofits can be designed by a licensed structural/civil engineer and will strengthen the structure to resist earthquakes. The retrofit typically consists of installing anchors in the structure determined by the engineer to connect the structure to the foundation.
Owners of the structure would typically perform a structural retrofit to make their structure safer with a peace of mind and it can even reduce their earthquake insurance.
Overall it is recommended to do a seismic evaluation on a structure prior to 1979 to determine if the structure is safe or requires a earthquake retrofit.