Michael F Ballard is the author and administrator of the Southern California Regional Rocks and Roads website, https://socalregion.com, since 1995. I have been studying the geology and highway history throughout southern California for most of my life. I am also the President of the Ridge Route Preservation Organization. Founder of the Historic Highway 99 Association of California, https://historic99.org .
1953 aerial view of the Camino Del Rio (later US 80) / US 395 interchange, looking south. Courtesy – Caltrans
San Diego’s Mission Valley has changed quite a bit over the last century. It has gone from farms to condos. Dairys to grocery stores. Dirt roads to major freeways. In 1947, the first iteration of the current State 163 / I-8 interchange was completed as shown here. Left on and offramps were the order of the day. Challenge Dairy’s San Diego headquarters can be seen in the upper left corner of the interchange. Additional changes to the junction took place in 1959, 1978, and 1991. All of these bridges have been replaced but their legacy remains today with the outer ramps on either side of the 8.
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.
1971 Earthquake Scarp Location
Deflected curb along the south side of Cometa Ave.
Abrupt rise from the curb to the yard is evidence of the fault scarp from 1971.
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
Western Imperial Valley – Imler Road site
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.
Imler Road Mole Track, looking south
Southern Imperial Valley
Southern Imperial Valley Fault trace
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.
McCabe Road canal shift
McCabe Road fractures
Chick Road western canal shift
Chick Road eastern canal shift
Roadway damage on Chick Road
I-8 between Meloland Road and Barbara Worth Road prior to pavement replacement. Offset can clearly be seen in the distance along with newer asphalt paving over the concrete.
Cracking as a result of the various earthquakes along the Imperial Fault at old US 80. This section has since been resurfaced but the telltale cracks are still visible.
En-Echelon cracks forming along Old State Highway 111 just north of Aten Road (near Imperial Valley College)
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.
Etiwanda Area Scarps
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.
Out in Rancho Cucamonga inspecting the Sierra Madre Fault.
Of course, no fault tour of Southern California is complete without the San Andreas Fault. The fault has numerous and visible features.
San Gabriel Fault Zone through central Santa Clarita
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.
Shown here, I-805 (upper) crosses the 52 freeway (lower) at San Clemente Canyon. Built in 1972, it formed the eastern terminus of the 52 for many years. In 1987, was extended to Convoy St. I-805 also follows the former boundary between the San Diego Mission Lands (east) and San Diego Pueblo Lands (west). This becomes quite evident in the North Park area where Boundary Street runs quite close to the freeway.
If you missed my presentation on September 13 on the history of the Ridge Route Alternate, you can see it on YouTube on the Historic Highway 99 Association of California’s channel. Come take a look!
Another wildfire has started at the south end of the Ridge Route in Marple Canyon along the 5. This fire, known as the Route Fire, has so far burned 250 acres and is not contained. The fire has burned across the old Ridge Route at the Callahan Line Change location, about 2 miles north of Castaic. Evacuations are underway as far north as Templin Highway. The fire has burned as far north as Martin’s, south of Templin Highway. It is not known if the structure survived.
At this time, I-5 is closed from State 138 to Castaic. The Ridge Route is also closed to through travel from Castaic to State 138 as the gates are closed. Do not use any portion of the Ridge Route as a bypass for I-5 at any time as it is not built for heavy traffic. Big rigs have attempted this in the past and got stuck, blocking others in.
For further information regarding road closures and detours, we strongly recommend Caltrans Quickmap at https://quickmap.dot.ca.gov/ which has detailed information and updated traffic conditions.
We will post updates, as needed, if the Ridge Route itself or historic structures are damaged or in danger. After the fire is out and things have settled down, we will assess any damage to the roadway and post a report.
