Los Angeles Fault Zone

Los Angeles Fault Zone
Name	Los Angeles Fault Zone
Other names	Puente Hills–Coyote Hills system
Location	Los Angeles, Los Angeles County, Orange County, San Gabriel Valley
Coordinates	34°03′N 118°12′W
Type	Blind thrust, reverse fault
Length	~50 km
Plate	North American Plate
Status	Active
Displacement	up to 3–7 mm/yr (est.)
Notable events	1987 Whittier Narrows earthquake (related), prehistoric ruptures

Los Angeles Fault Zone is a network of subsurface thrusts and blind reverse faults beneath the Los Angeles Basin and adjacent uplands. The complex system links buried structures under Puente Hills, Coyote Hills, Whittier Hills, and parts of the San Gabriel Mountains, and contributes to seismic hazard for the City of Los Angeles, Long Beach, and Pasadena. Geologic mapping, paleoseismology, and seismic reflection profiling have progressively revealed its role alongside the San Andreas Fault, Newport–Inglewood Fault, and Sierra Madre Fault Zone in shaping southern California seismic risk.

Geology and Structure

The Los Angeles fault network is composed of stacked imbricate thrusts, blind ramps, and roof thrusts that deform Miocene to Quaternary sedimentary basins such as the Los Angeles Basin and the Inglewood Oil Field. Deep seismic reflection lines tied to wells in the Long Beach Oil Field, Zuma Well, and exploration boreholes show north-dipping fault planes that sole into a regional detachment within the upper crust. Cross-sections integrate data from the United States Geological Survey, California Geological Survey, and industry studies conducted near the Puente Hills Landfill and Whittier Narrows to illustrate ramp-flat geometries and pop-up structures. Stratigraphic relationships between Pliocene conglomerates, Pleistocene alluvium, and modern artificial fills record cumulative fold growth above the blind thrust panels.

Tectonic Setting and Seismotectonics

Situated within the diffuse plate boundary between the Pacific Plate and the North American Plate, the system accommodates crustal shortening, transtension, and transpression associated with slip partitioning along the coastal margin. Interactions with the right-lateral San Andreas Fault transform system and the right-stepping trend of the Newport–Inglewood Fault influence stress transfer and rupture propagation toward the basin. Geodetic networks maintained by Scripps Institution of Oceanography, California Institute of Technology, and Jet Propulsion Laboratory GPS arrays constrain shortening rates, while focal mechanisms from the Southern California Seismic Network and moment-tensor catalogs reveal predominantly reverse-thrust mechanisms with strike-slip components.

Fault Segmentation and Mapping

High-resolution mapping divides the complex into named segments under the Puente Hills, Calle de Baya, and Coyote Hills domains; each segment is characterized by geometry, strike, dip, slip-rate, and rupture potential. Reflection profiles from industry seismic surveys, LIDAR topography near the San Gabriel Fault exposures, and microseismicity clusters identified by USGS and academic networks delineate discrete patches that may rupture independently or cascade. Structural restorations link subsurface segments to surface folds such as the Whittier Hill fold and buried anticlines associated with hydrocarbon accumulations in the Inglewood Oil Field and Long Beach Oil Field.

Seismic History and Notable Events

Direct surface rupture from this blind system is absent in historic times, but paleoseismic trenching in the Puente Hills and stratigraphic offsets indicate multiple large pre-historic earthquakes comparable to the 1857 Fort Tejon earthquake scale. Seismicity catalog analyses tie moderate events like the 1987 Whittier Narrows earthquake and clusters in the 1990s to nearby thrusts and transfer zones. Historic records maintained by the Southern California Earthquake Center, California Earthquake Authority, and municipal archives augment instrumental records to reconstruct recurrence intervals, slip per event, and possible multi-segment ruptures capable of producing magnitudes in the upper 7s.

Hazards and Risk Assessment

Because many segments are blind, the zone poses elevated risk for unexpected strong shaking in densely populated neighborhoods of Downtown Los Angeles, Elysian Park, Baldwin Park, and Montebello. Ground motion modeling by the Federal Emergency Management Agency, USGS ShakeMap studies, and regional seismic hazard models incorporate site amplification, basin effects in the Los Angeles Basin, and potential surface folding that could damage pipelines, lifelines, and structures. Coupled seismic–landslide analyses reference susceptibility in the San Gabriel Mountains foothills, while liquefaction maps for reclaimed land in Long Beach and Harbor City inform urban resilience planning by agencies like Los Angeles Department of Water and Power and Metropolitan Water District of Southern California.

Monitoring and Research

Ongoing investigations employ seismic reflection, dense nodal seismic arrays, ambient-noise tomography conducted by Caltech, permanent broadband stations of the Southern California Seismic Network, and borehole strainmeters installed by USGS and collaborating universities. Interdisciplinary projects such as those funded by the National Science Foundation and coordinated through the Southern California Earthquake Center study rupture propagation, dynamic stress triggering, and paleoseismic chronologies using radiocarbon dating, optically stimulated luminescence, and cosmogenic-nuclide methods. Urban drilling campaigns and microtremor surveys improve velocity models for ground-motion simulations used by California Office of Emergency Services and municipal planners.

Urban Impact and Infrastructure Vulnerability

Critical infrastructure — including the Interstate 5, Interstate 10, Los Angeles International Airport, major rail corridors of Metrolink and Los Angeles County Metropolitan Transportation Authority, water conveyance systems, and petroleum terminals in the Port of Long Beach — overlies or lies near mapped segments and folded zones. Seismic retrofitting programs driven by mandates from the California Building Standards Commission and retrofit efforts for unreinforced masonry, soft-story housing, and lifeline retrofits reference hazard scenarios derived from this fault network. Emergency response agencies such as the Los Angeles Fire Department and Los Angeles Police Department coordinate preparedness exercises informed by scenario ruptures, while insurers and urban planners incorporate downscaling of ShakeMap outputs into resilience strategies.

Category:Geology of Los Angeles County Category:Seismology