The shock occurred on June 28 , 1991 , at 07:43:55 local time , and was the result of thrust faulting on the Clamshell – Sawpit Fault at the southern boundary of the San Gabriel Mountains . At less than , the total amount of uplift was very minimal , considering the of vertical displacement that was observed during the 1971 San Fernando earthquake . The uplift initiated about two million years ago , and is a result of the convergence of the Pacific and North American Plates near the restraining bend of the San Andreas Fault
1971 San Fernando earthquake
1971 San Fernando earthquake
1991 Sierra Madre earthquake