The Garlock Fault runs about 250 kilometres east to west across Southern California, forming the sharp southern boundary of the Sierra Nevada and the Tehachapi Mountains and separating them from the Mojave Desert. Unusually for California, it is a left-lateral strike-slip fault, meaning features on the opposite side appear shifted to the left, and it meets the San Andreas Fault at a prominent junction near Tejon Pass.
The fault is thought to accommodate the eastward stretching and rotation of the Mojave block as the crust deforms between the San Andreas and the Basin and Range province to the east. Geologists estimate its slip rate at several millimetres per year, and trenching studies reveal repeated large prehistoric earthquakes along its trace.
The Garlock has not produced a major earthquake in written history, but it is capable of magnitude 7-plus ruptures. Notably, the 2019 Ridgecrest earthquake sequence triggered slow creep along part of the Garlock, a rare observed interaction that reminded scientists this long, quiet fault remains a serious hazard.