Category Archives: Earthquake

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

Earthquake Information

Felt an earthquake? Want to know where it was and how big? Follow the links below to get up-to-date information from the USGS and the California Geological Survey.

If you felt the earthquake, let the USGS know by contacting them at: Did You Feel It? This data helps them determine magnitude of the earthquake, how the geology affects that, and how the type of structure reacts to the event. Most earthquakes larger than 4.0 are listed in the “Events”  section of the page.

Additional Earthquake Information:

Featured Image – 2/8/2021

1970 view of the I-5 / SR-14 Interchange construction. This “build” would partially collapse on February 9, 1971 while still incomplete. Courtesy – Caltrans

To commemorate the 50th anniversary of the February 9, 1971 M6.6 Sylmar / San Fernando Earthquake, our featured image shows the construction work in progress on the I-5 and State 14 interchange in 1970. This work would be halted abruptly in the early hours of February 9, 1971. After this, it would be reconstructed two more times. The earthquake changed the way freeways such as this were built and did further seismic codes throughout the state.

For further information about the interchange:

M 3.5 near Castaic, CA – Updated

This morning (2/21/2020) at 5:42 am, a small earthquake struck the Castaic area north of Los Angeles. Just looking at the maps, it seems near the San Gabriel Fault but it is a somewhat complex area. It was also a very deep earthquake as they go, 18.1 km. Shaking was felt in a wide area but wasn’t very strong.

Shake map for the earthquake. Now a M 3.5.

For more information, check out the USGS Summary Page of the event.

https://earthquake.usgs.gov/earthquakes/eventpage/ci39079535/executive

M 3.6 – 4km NNE of Granada Hills, CA

Shakemap from USGS showing the extent of shaking and major fault lines

On January 21, 2020, just a few days after the 26th anniversary of the Mw 6.7 Northridge Earthquake, we had another small jolt strike the San Fernando Valley. This one, while small, was felt over a wide area. If you are among the many that did feel it, the USGS wants to hear from you! These “Did You Feel It?” reports help them to understand earthquakes further.