 The northernmost coastal area is one of California 's most seismically active regions and , in a 50 @-@ year period , the area including the Mendocino Fracture Zone at the southern flank of the Gorda Plate generated about 25 percent of all seismic energy unleashed in the state . The Mendocino Triple Junction ( strike @-@ slip / strike @-@ slip / trench ) formed 29 – 30 mya at 31 ° N ( west of present @-@ day Baja California ) when the Pacific @-@ Farallon spreading center initially approached the subduction zone off the coast of western North America . Simultaneously , the Rivera Triple Junction shifted to the southeast to its current position at 23 ° N. Once the Pacific Plate and North American Plate connected the boundary became that of a transform fault ( San Andreas ) due to the northwestward motion of the Pacific Plate relative to the North American Plate . The San Andreas Fault continues to lengthen to the northwest and the southeast as the two triple junctions continue their transient motion . 
