Forming the boundary of the southern portion of the San Gabriel Mountains and running about 140 km, the San Gabriel Fault is a right-lateral strike-slip that was last active 1,000 of years ago (Quaternary: Eastern Region; Holocene: Western Region) Despite its inactivity, the fault will move if there is other seismic activity in the area (Then again, what doesn't?).

Today there is much evidence that it was once actually a part (i.e. The former path) of the San Andreas Fault. It is also assumed that the northwestern end of the fault zone meets the current San Andreas Fault beneath the Frazier Mountain Thrust, near the juncture of the San Andreas with the Garlock and Big Pine faults. Its opposite end is located in a complex geological region that has yet to be fully mapped.

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