The Cucamonga Fault runs along the southern base of the eastern San Gabriel Mountains above the Inland Empire cities of Rancho Cucamonga and Upland in southern California. It is a reverse (thrust) fault, where the mountain block is pushed up and over the valley sediments as the crust is compressed north of the San Andreas Fault.
This fault forms part of the Sierra Madre–Cucamonga thrust system that has been building the San Gabriel Mountains and shaping the steep mountain front. Its dip-slip motion drives rapid uplift and produces large alluvial fans spilling into the valley.
No great earthquake has struck the Cucamonga Fault in historical record, but paleoseismic trenching reveals prehistoric ruptures capable of magnitude ~6.5 to 7. Given its location beneath a densely populated urban corridor, it is treated as a serious seismic hazard.