Geology
The San Gabriel Fault is a right-lateral strike-slip that was last active 1,000s of years ago — Late Quaternary west of intersection with the Sierra Madre fault, and Quaternary east of that intersection, and Holocene only further west between Saugus and Castaic-Gorman.
The San Gabriel Fault was once part of the San Andreas Fault, and is believed to be the former path of it. It is also believed that the northwestern end of this fault zone meets the current San Andreas Fault zone beneath the Frazier Mountain Thrust, near the juncture of the San Andreas with the Garlock Fault and Big Pine Fault.
Read more about this topic: San Gabriel Fault