Faulting and Earthquakes

Southern California is an area well known for its seismicity. The region sits astride the boundary between two plates – the North American and the Pacific. Movement along this boundary has created numerous faults and other features. This website will cover the major faults, their features, and some of their seismic history.

While most of the major fault lines have some surface expression here, there are many that do not. These faults can also produce large earthquakes, such as the M6.7 Reseda/Northridge Earthquake of January 17, 1994. That earthquake originated on a previously unknown fault known as a Blind Thrust Fault. This type of fault exists only as an anticline near the surface and has not yet reached the surface.

Sierra Madre Fault – Cucamonga Fault

Sierra Madre Fault scarp near Etiwanda Ave.
Cucamonga Fault scarp near Etiwanda Ave.

Runs along the base of the San Gabriel Mountains and has been responsible for a few earthquakes in recent times. A M5.5 in Upland in 1990 was attributed to this fault. While most of the fault trace has been developed, some of the fault scarp with springs can be seen near the northern end of Etiwanda Ave in the Rancho Cucamonga area.

San Gabriel Fault

San Gabriel Fault between Red Box Gap and Clear Creek Saddle.
San Gabriel Fault between Red Box Gap and Clear Creek Saddle.
San Gabriel Fault exposure in Little Tujunga Canyon. Fault is visible as the boundary between the lighter and darker rocks.
San Gabriel Fault exposure in Little Tujunga Canyon. Fault is visible as the boundary between the lighter and darker rocks.

This fault passes through the heart of the San Gabriel Mountains for much of its eastern half. Large canyons, such as Pacoima and Big Tujunga Canyon, follow the trace of this fault. These canyons were created through the erosion of fault-weakened rocks. The San Gabriel Fault is a part of the San Andreas “family” of faults and may have been a proto-San Andreas Fault itself during the late Miocene and Pliocene. The western half of the fault passes through the Santa Clarita Valley and follows I-5 toward the Frazier Mountain area. The eastern half has a split near central Big Tujunga Canyon. The southern branch heads toward Altadena and the northern branch stays within the mountains, passing through Clear Creek Junction (Junction of Angeles Crest and Angeles Forest Highways), Red Box Gap, and both forks of San Gabriel Canyon.

San Jacinto Fault

Fault controlled valley along Gilman Springs Road just south of Jackrabbit Trail near Hemet, CA.
Fault trace visible as a line of trees above the orange grove east of Hemet, CA along State 74.

This fault is a major branch of the San Andreas Fault. It has actually been more active than the San Andreas south of its junction near Cajon Pass and Lytle Creek. In the past couple hundred years, this fault has been the source of multiple M6.0+ earthquakes, giving rise to the theory that it may be, in effect, the “new San Andreas Fault” as it takes up the slack from the locked segment of the SAF.

The faults northern segment passes through a well populated area, including underneath the I-10/I-215 Interchange in Colton.  From there it passes through the hills south of Loma Linda to Moreno Valley.  In Moreno Valley, it runs along the south side of the Moreno Badlands, passing to the north of Hemet.  The fault also passes through Anza and Borrego Springs on its way to the Salton Basin.

Raymond Hill Fault

Starting in the Highland Park area of Los Angeles, this fault trends east-west roughly along York Blvd until South Pasadena, where it passes its namesake – Raymond Hill. The upthrown side of the fault forms the low San Marino Hills. Near the Santa Anita Racetrack, the fault turns toward the mountains and blends into the Duarte Fault.

Santa Monica Fault

  • Follows the base of the Santa Monica Mountains from the Los Feliz area to Malibu.

Northridge Hills Fault

As the name implies, this fault runs through the Northridge area of the San Fernando Valley. It is one of the faults that helps accommodate the compression of the San Fernando Valley between the Santa Susana Mountains and the Santa Monica Mountains. This is not the fault that was responsible for the M6.7 January 17, 1994 Reseda-Northridge earthquake. It does, however, reveal itself as an abrupt rise in the terrain. This rise can be seen north of Devonshire St between Wilbur Ave and Mason Ave. Another good place to see this uplift is near the intersection of Plummer St and Balboa Blvd.

Newport-Inglewood Fault

Passing through a major portion of the city, this is one of the more dangerous fault lines in the Los Angeles area. It produced the M6.4 March 10,1933 Long Beach earthquake which caused a lot of damage throughout the area. This fault runs from the West Los Angeles area, through the Baldwin Hills, Dominguez Hills, and Signal Hill before heading offshore. Staying somewhat parallel with the coast, it eventually becomes the Rose Canyon Fault in the San Diego area.

Uplift along this fault line has also created some of the more prolific oil fields in the Los Angeles region. Oil is still being pumped from the Signal Hill, Dominguez Hills, and Signal Hill fields. These fields were created by the stratographic traps created by the compressional uplifts along the fault.

While the 1933 Long Beach earthquake was quite destructive, it provided the impetus for improving building codes throughout California and the world. The first major legislation for seismic codes, the Field Act of 1933, created building codes to strengthen key structures such as schools and hospitals. These codes have since been modified and enhanced after each major earthquake.

Chino Hills Fault

A northern extension of the Elsinore Fault, this fault bounds the northern side of the Chino Hills. Some recent seismicity has been attributed to this fault, such as the 2008 M5.4 Chino Hills earthquake which caused damage in the Chino area and as far as downtown Los Angeles.

Puente Hills Fault

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