California Strong Motion Instrumentation Program Prior to the San Fernando earthquake , some structural engineers had already believed that the existing groundwork for seismic design required enhancement . Although instruments had recorded a force of .33 '' g '' during the 1940 El Centro earthquake , building codes only required structures to withstand a lateral force of .1 '' g '' as late as the 1960s . Even at that time , engineers were against the idea of constructing buildings to resist the high forces that were seen in the El Centro shock , but after a 1966 earthquake peaked at .5 '' g '' , and a maximum of 1.25 '' g '' was observed at the Pacoima Dam during the San Fernando event , debate began as to whether that low requirement was sufficient
1940 El Centro earthquake
1940 El Centro earthquake
1971 San Fernando earthquake