 The earthquake was caused by rupture along parts of the Imperial Fault , the Brawley fault zone and the Rico Fault , a previously unknown normal fault near Holtville , though slip was also observed on the Superstition Hills Fault and the San Andreas Fault . The maximum observed right lateral displacement on the Imperial fault — measured within the first day of the event to the northwest of the epicenter — was 55 – 60 cm ( 22 – 24 in ) , but measurements taken five months following the earthquake closer to the southeast end of the rupture showed there was an additional 29 cm ( 11 in ) of <unk> slip ( for a total slip of 78 cm ( 31 in ) . Several strands of the Brawley fault zone , to the east of the Imperial fault , ruptured intermittently along a length of 11 @.@ 1 km ( 6 @.@ 9 mi ) , and just one kilometer of the Rico fault slipped with a maximum vertical displacement of 20 cm ( 7 @.@ 9 in ) ( no horizontal slip was observed on that fault ) . 
