Southern California has many faults, some hidden, some very obvious. One of the more fun aspects of geology is to see where the land moves. To witness the power of what Earth can do can be quite awe inspiring. Some features are easily recognizable, such as the San Gabriel Mountains which have somewhat rapidly risen due to faulting. Other features are less apparent in the modern urban environment, such as “setbacks” along major fault zones. This page will showcase some of the more visible fault features throughout Southern California.
NOTICE: Some of these locations are either along major roadways or in residential areas. Please use caution when visiting these sites and be respectful of those living there. Parking regulations vary at each site so be aware before you park.
Maps used are courtesy Google Earth and USGS Quaternary Faults layer – adapted by Michael F Ballard.
Recent faulting and Fault Scarps
San Fernando Valley
On February 9, 1971, an M6.6 earthquake struck early in the morning in the northwestern San Fernando Valley. This quake, known as the 1971 Sylmar / San Fernando Earthquake caused a great deal of damage from collapsed buildings, pipeline breaks, downed powerlines, and collapsed freeways. The damage was gradually repaired but some evidence remains today.
A fault scarp along with a deflected curb can be seen quite readily on Cometa Ave, just east of Fernmont St. The front yard at 12676 Cometa Ave abruptly rises from street level at an angle. Looking west from this location, you can see a slight bend in the curb on the south side, another reminder of the 1971 quake.
Imperial Valley Scarps and Offsets
Numerous earthquakes in the Imperial Valley have produced a significant amount of surface rupture. Evidence of these ruptures can still be seen today in many parts of the valley. Two areas of interest can be visited. One in the western end of the valley and another southeast of Brawley.
Western Imperial Valley
Earthquakes in 1968 and 1987 produced a small amount of surface rupture along the Superstition Hills Fault Zone, which is an extension of the Elsinore Fault Zone. This is visible today as a low “mole track” which follows the fault line and cracks in the roadway on Imler Road where it makes a big bend at Huff Road. This track can be followed for many miles across the desert but be cautious as the area surrounding the roadway is a military bombing area.
Southern Imperial Valley
On May 18, 1940, an Mw 6.9 earthquake struck in the Imperial Valley along the Imperial Fault. The quake caused a lot of damage to buildings and infrastructure. It produced a significant amount of surface rupture as well, which offset canals and roadways. Additional events in 1966 and 1979 increased the offset. The fault zone can be followed quite easily today by tracking the broken and offset pavement as well as shifted canals.
Pre-Holocene Events
Older seismic events in Southern California also leave their marks. A fault scarp along the Sierra Madre Fault is visible in the foothills above Etiwanda.
At the northern end of Etiwanda Avenue and a short hike later, you can walk along the scarp of the Sierra Madre Fault. This section hasn’t broken in a long time but is still quite active, as the freshness of the scarp suggests. This can be traced along the foothills of the San Gabriel Mountains for many miles where it hasn’t been erased by development or erosion.
Of course, no fault tour of Southern California is complete without the San Andreas Fault. The fault has numerous and visible features.
Alquist-Priolo Zoning Act
Not all faults need to produce recent surface rupture to be considered a problem. In 1972, the Alquist-Priolo Zoning Act was passed by the California legislature which required setbacks and additional building requirements surrounding fault zones. How wide a zone was dependent on how large an earthquake could be expected from that fault. Any fault that was capable of causing surface rupture got special attention. An example of these setbacks can be seen in Santa Clarita along Newhall Ranch Road between Hillsborough Pkwy and Dickason Dr / Ave Tibbets. A large shopping center called Bridgeport Marketplace was constructed atop the San Gabriel Fault Zone. A large setback was required for construction, which can be seen by the very wide separation of structures and the more unusual location of the parking lot. Normally such centers have the parking adjacent to the roadway. In this case, it is behind the structures.
