Category Archives: San Gabriel Mountains

Sightseeing Faults in the Los Angeles Area

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.

Near the San Andreas Fault crossing at Ave S.

San Andreas Fault Tour

Alquist-Priolo Zoning Act

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.

Additional Resources

Featured Image – 9/17/2020

Looking south toward the closed section of Highway 39 from Angeles Crest Highway (State 2) in 2009

A look back to happier times in the San Gabriel Mountains. Taken on August 22, 2009, just a few days before the Station Fire started. Over the past few days, this section of the mountains has burned in the Bobcat Fire, which is ongoing.

Featured Image – 4/17/2020

Step faulting, slickensides, and heavily fractured rock

The San Gabriel Mountains have been subject to quite a bit of deformation in their past. Some of which is still ongoing, such as the movements along the Sierra Madre and San Andreas Faults. Here, near Mt Wilson, the rocks exhibit multiple instances of deformation. The white vein is mostly composed of quartz, which has been offset by multiple faults. Slickensides can be seen as the reddish areas on the right side of the photo. The surrounding materials are complex metamorphic rocks.

Featured Image – 2/20/2020

Looking up the canyon from the West Fork San Gabriel River Bridge (53-2244). Taken in 2016.

Looking toward South Mount Hawkins in the West Fork San Gabriel Canyon from State 39. This road was planned to be improved but landslides prevented this work from being completed. In the case of this area, almost all of the low lying areas not covered in snow are landslides in one form or another. Even Crystal Lake, where this roadway is headed, exists because of landslides.

Mt Wilson Observatory Geology Talk

Lectures

On Saturday, June 16, at 5:30 pm Mount Wilson Observatory will be hosting a talk by Dr. Tanya Atwater, professor emeritus of geology at UC Santa Barbara. Dr. Atwater played a major role in piecing together the plate tectonic history of our part of the planet. She will talk about how the motions of the Earth’s crust created the current topography of southern California (including the perfect site for astronomers to explore the Universe). The talk will be followed by a rare chance to look look through Mount Wilson Observatory’s 60-inch Telescope, which revolutionized telescope design and allowed astronomer Harlow Shapley to find our place in the Milky Way Galaxy. Tickets are only $20 and proceeds will all go to the maintain the Observatory.