San Fernando Fault Zone

San Fernando Fault Zone
Name	San Fernando Fault Zone
Location	San Fernando Valley, Los Angeles County, California, United States
Coordinates	34.3°N 118.4°W
Length	~? km
Type	Reverse, thrust, right-lateral oblique
Plate	North American Plate, Pacific Plate
Status	Active

San Fernando Fault Zone is a complex active fault system in the western Transverse Ranges of Southern California centered in the northern Los Angeles metropolitan region. It accommodates crustal shortening and strike-slip deformation between major structural elements, producing strong historical earthquakes and persistent concern for urban seismic hazard across Los Angeles County, California, San Bernardino County, California, Ventura County, California, and adjacent communities such as San Fernando, California, Sylmar, California, Pacoima, California, and Saugus, California. The zone interacts with the broader network of faults that includes the San Andreas Fault, San Gabriel Fault, Hollywood Fault, Verdugo Fault, Santa Susana Fault, Puente Hills Fault, and Garlock Fault.

Geology and Fault Mechanics

The fault zone consists of a series of thrusts, reverse faults, and oblique-slip strands cutting Neogene and Quaternary sedimentary rocks and metamorphic basement exposed in the Transverse Ranges and adjacent Sierra Madre Mountains (California). Stratigraphic relations among units such as the Puente Formation, Sespe Formation, Topanga Formation, and older crystalline rocks reveal Plio-Pleistocene uplift and folding driven by motion on thrust ramps and blind faults. Structural geometries include ramp-flat-ramp profiles, imbricated thrust slices, and flower structures similar to those mapped on the Anza-Borrego Fault and San Jacinto Fault. Kinematic indicators record mostly reverse slip with significant right-lateral shear comparable to oblique slip on the Hayward Fault and hanging-wall anticlines like those on the Mount Diablo Thrust. Surface rupture expressions are discontinuous; paleoseismic trenches across alluvial fans and colluvial wedges expose multiple Holocene events correlated by tephrochronology, radiocarbon dating, and optically stimulated luminescence.

Tectonic Setting and Plate Interactions

The San Fernando Fault Zone lies within the boundary zone between the Pacific Plate and the North American Plate where relative plate motion is partitioned among the San Andreas Fault system, interconnected strike-slip systems such as the San Jacinto Fault Zone and the Elsinore Fault Zone, and contractional structures in the Transverse Ranges and Peninsular Ranges (California). Regional transpression resulting from a bend in the San Andreas Fault and block rotation of crustal slivers like the Western Transverse Ranges produces uplift documented by GPS networks operated by institutions like the United States Geological Survey and the Southern California Earthquake Center. Interactions with lithospheric heterogeneities such as the Mojave Desert crustal block, the Los Angeles Basin, and inherited basement fabrics influence rupture propagation and segmentation patterns observed in historical events.

Earthquake History and Notable Events

The zone produced significant earthquakes in the 20th century, most notably the 1971 event that caused extensive damage across Sylmar, California and prompted major changes in seismic building codes. Ground failures, landslides on the San Gabriel Mountains slopes, and strong-motion recordings influenced retrofits of structures including hospitals at Olive View–UCLA Medical Center and bridges on Interstate 5 (California), Interstate 210, and state routes. Prehistoric paleoearthquakes identified on the fault correlate with rupture histories on neighboring thrusts and serious events recorded on the New Madrid Seismic Zone and Cascadia Subduction Zone that illustrate the potential for multi-fault ruptures. Historic shaking recorded by networks such as the California Institute of Technology seismological arrays and archived in the Southern California Earthquake Data Center provides datasets used in ground-motion prediction studies.

Hazard Assessment and Seismic Risk

Seismic hazard analyses incorporate fault geometry, slip rates estimated from trenching and geomorphic offsets, and recurrence intervals derived from probabilistic models developed by the USGS National Seismic Hazard Model and regional agencies like the California Geological Survey. Urban exposure includes essential facilities such as Los Angeles International Airport, water conveyance structures in the Los Angeles Aqueduct, and lifeline infrastructure managed by utilities including the Los Angeles Department of Water and Power and Southern California Edison. Building inventories subject to retrofit ordinances include high-risk classes cataloged under statutes like the Alquist-Priolo Earthquake Fault Zoning Act and influenced by standards from the American Society of Civil Engineers. Scenario-based assessments explored by emergency planners from Los Angeles County Fire Department, California Office of Emergency Services, and the FEMA illustrate cascading hazards—liquefaction in valley-fill deposits, landslides in steep canyons, and urban fire following shaking.

Monitoring, Instrumentation, and Research

Continuous and campaign geodetic monitoring using GPS stations operated by the USGS, Scripps Institution of Oceanography, and the University of California, Los Angeles along with dense seismic arrays including the Southern California Seismic Network, High Resolution Seismic Network (HRSN), and temporary deployments by the Earthquake Engineering Research Institute provide constraints on fault creep, stress changes, and aftershock sequences. Paleoseismology teams from institutions like the California Institute of Technology, University of California, Berkeley, and USC perform trenching, radiocarbon dating, and tephrostratigraphy to build event chronologies. Advanced modeling using finite-element codes and dynamic rupture simulations applied by groups at the SCEC and Lawrence Livermore National Laboratory explore rupture propagation, ground-motion prediction, and rupture directivity with inputs from geology, geomorphology, and historical seismicity catalogs.

Interaction with Nearby Faults and Structures

The fault zone’s proximity to major structures fosters complex stress transfer and potential for multi-fault rupture linking to faults such as the San Andreas Fault, San Gabriel Fault, Hollywood Fault, Santa Susana Fault, and blind thrusts beneath the Los Angeles Basin including the Puente Hills Fault. Historic and modeled stress interactions implicate compound hazard scenarios similar to joint rupture hypotheses examined for the Hayward Fault–Calaveras Fault system and the San Jacinto–San Andreas interactions. Urban development atop alluvial fans and basin sediments alters surface expressions and complicates paleoseismic interpretation; infrastructure intersections with fault traces—rail corridors of Metrolink (California), freeways like Interstate 5 (California), and water mains—remain focal points for mitigation projects coordinated by regional planning bodies including the Southern California Association of Governments and local municipalities.

Category:Geology of California Category:Seismic faults of California