The Sierra Madre Fault stretches along the southern front of the San Gabriel Mountains above the San Gabriel Valley and cities such as Pasadena and Sierra Madre in southern California. It is a reverse (thrust) fault that thrusts the mountains southward over the valley in response to crustal compression bending around the San Andreas Fault.
Together with the adjoining Cucamonga Fault, it forms a thrust system responsible for the ongoing rise of the San Gabriel Mountains and their steep, canyon-cut face. Its slow but persistent uplift has helped make these one of the fastest-growing ranges in the region.
The 1971 San Fernando (Sylmar) earthquake (magnitude ~6.6) ruptured a western portion of this thrust system, killing dozens and causing severe damage to hospitals and freeways. Paleoseismic evidence indicates the fault can generate even larger ruptures beneath the Los Angeles metropolitan area.