Puente Hills Fault

Puente Hills Fault The Puente Hills Fault is a major subterranean thrust fault system beneath the eastern Los Angeles Basin, notable for its potential to produce large earthquakes beneath densely populated Los Angeles, Orange County, and adjacent communities. Identified by geologists and seismologists in the late 20th century, it links to broader tectonic processes involving the Pacific Plate, North American Plate, and the complex network of Southern California faults including the San Andreas Fault, Whittier Fault, and Santa Monica Fault. Research institutions, government agencies, and universities have prioritized study of its geometry because of its proximity to critical infrastructure in the Greater Los Angeles Area, Long Beach, and Pasadena.

Geology and Structure

The fault is a blind, low-angle reverse (thrust) structure situated beneath the sedimentary fill of the Los Angeles Basin and the Puente Hills anticline, with strands extending roughly east–west beneath Downey, Whittier, Montebello, and Commerce. It is part of the regional step-over between the San Gabriel Mountains uplift and the basin depocenters tied to plate boundary interactions among the San Andreas Fault system, the Transverse Ranges, and the Peninsular Ranges. Stratigraphic relationships expose deformation in Pliocene and Quaternary strata near the Santa Ana River, the Los Angeles River, and buried paleochannels, indicating repeated folding and blind thrusting. Subsurface imaging from seismic reflection profiles, borehole data from the United States Geological Survey, and geomorphic studies by researchers at California Institute of Technology and University of Southern California reveal multiple linked thrust ramps and a ramp-flat geometry that concentrates uplift beneath urban basins such as Pomona and Montebello Hills.

Seismic History and Notable Earthquakes

Although not expressed at the surface, paleoseismological evidence and studies of uplifted marine terraces and liquefaction deposits indicate past large events. The fault produced a widely studied Mw 5.9–6.0 earthquake in 1987 and was implicated as the source of a larger Mw ~6.1–6.6 event inferred from aftershock patterns and waveform modeling in the late 20th century, prompting re-evaluation by teams at Southern California Earthquake Center, USGS, and state earthquake centers. Historical ties have been examined with seismicity catalogs including events recorded at Rinconada Observatory and seismic arrays operated by Caltech Seismological Laboratory, with paleoseismic trenches near La Habra and sediment core analyses correlating periods of folding to regional sequences of elastoplastic failure documented alongside earthquakes on the Garlock Fault and Newport–Inglewood Fault.

Hazard Assessment and Recurrence Rate

Hazard models produced by the United States Geological Survey and the California Geological Survey assign high earthquake probability to the fault, estimating a recurrence interval for major ruptures (Mw ≥ 7.0) on the order of several hundred to a thousand years but with considerable uncertainty due to its blind nature and incomplete paleoseismic record. Probabilistic seismic hazard assessments incorporate slip rates inferred from geodetic studies by Scripps Institution of Oceanography, University of California, Los Angeles GPS networks, and stress-transfer modeling linked to regional events on the San Jacinto Fault and Elsinore Fault. Scenario-based risk analyses prepared for Los Angeles County offices and the Federal Emergency Management Agency consider cascading impacts from a large rupture, including strong ground shaking, induced landslides in the Verdugo Mountains, ground failure in reclaimed harbor areas like Long Beach Harbor, and secondary hazards affecting the Port of Los Angeles and Los Angeles International Airport.

Impact on Urban Areas and Infrastructure

Because the fault lies directly beneath heavily urbanized areas, a significant rupture would threaten residential neighborhoods in Montebello, Whittier, and Downey as well as commercial centers in Commerce and Norwalk. Critical lifelines at risk include freeway interchanges on Interstate 5, Interstate 10, and Interstate 605, freight corridors serving the Ports of Los Angeles and Long Beach, water conveyance in the Los Angeles Aqueduct system, petroleum pipelines feeding facilities near El Segundo and Long Beach Oil Field, and high-voltage transmission lines supplying the Los Angeles Department of Water and Power. Urban seismic vulnerability studies referenced by the Los Angeles County Metropolitan Transportation Authority and emergency plans from the City of Los Angeles emphasize potential for building collapse, liquefaction in reclaimed wetlands near San Pedro Bay, and disruption to hospitals such as UCLA Medical Center and Cedars-Sinai Medical Center.

Monitoring and Mitigation Efforts

Monitoring combines dense seismic networks operated by Caltech, the USGS, and the Southern California Earthquake Center with continuous GPS stations from UNAVCO and borehole strainmeters to resolve fault slip and crustal deformation. Mitigation strategies promoted by state agencies include retrofitting of critical structures under programs administered by the California Office of Emergency Services and the National Institute of Standards and Technology recommendations for lifeline resilience. Urban planning tools used by Los Angeles Department of City Planning incorporate proximity to blind thrust hazards for permitting and zoning, while community preparedness initiatives coordinated with Red Cross chapters, Los Angeles County Fire Department, and municipal emergency managers emphasize earthquake drills and public education modeled on past responses to events like the Northridge earthquake. Ongoing research partnerships among USGS, Caltrans, Southern California Association of Governments, and university laboratories continue to refine rupture scenarios, early warning algorithms compatible with the ShakeAlert system, and engineering guidance for new construction in the Los Angeles Basin.

Category:Geology of Los Angeles County Category:Seismic faults of California