Sierra Madre Fault

Sierra Madre Fault
Name	Sierra Madre Fault
Location	San Gabriel Mountains, Los Angeles County, California, United States
Type	Thrust fault / blind thrust
Length	~40–70 km (variable by study)
Plate	North American Plate
Status	Active
Notable earthquakes	1987 Whittier Narrows earthquake (associated region)

Sierra Madre Fault The Sierra Madre Fault is a prominent crustal thrust system beneath the southern flank of the San Gabriel Mountains and the northern Los Angeles Basin in Los Angeles County, California. It forms part of a network of blind and emergent thrusts that accommodate shortening between the San Andreas Fault system and the Transverse Ranges, contributing to seismic hazard in the Greater Los Angeles Area, including Pasadena, Glendale, and Burbank. Studies of the fault integrate results from institutions such as the United States Geological Survey, the California Geological Survey, and university groups at California Institute of Technology and the University of Southern California.

Geology and Tectonic Setting

The Sierra Madre Fault lies within the structural province of the Transverse Ranges, formed by transpression related to the right-lateral motion of the Pacific Plate relative to the North American Plate as expressed along the San Andreas Fault. It is a frontal thrust of the Sierra Madre deformed belt that uplifts bedrock units including the San Gabriel Mountains crystalline basement and Cenozoic sedimentary sequences such as the Sierra Madre Formation and the Monrovia Formation. The fault system interacts with nearby structures like the San Fernando Fault, the Santa Susana Fault, and the Puente Hills Fault; collectively these structures influence basin subsidence and uplift history tied to Los Angeles Basin evolution and regional slip partitioning documented in paleomagnetic and structural studies by researchers at Stanford University and UC Berkeley.

Fault Geometry and Segmentation

Seismological, seismic reflection, and geological mapping reveal a complex fault geometry with multiple splays and ramp-flat architectures typical of blind thrust systems studied in the Los Angeles region. High-resolution seismic reflection lines collected by the USGS and academic consortia image north-dipping fault ramps beneath the San Gabriel Valley and downlap onto foreland strata in the San Fernando Valley area. Geomorphic indicators near Sierra Madre and La Cañada Flintridge suggest segmentation on scales from a few kilometers to tens of kilometers; these segments are compared with segmented thrusts such as the Hollywood Fault and the Santa Monica Fault to infer rupture propagation limits and slip accumulation. Kinematic models incorporating GPS data from networks run by Scripps Institution of Oceanography and the Southern California Earthquake Center constrain dip angles and maximum potential rupture lengths.

Seismic History and Paleoseismology

Instrumental catalogs show seismicity clustering near the southern front of the Sierra Madre system with historic earthquakes such as events in the 20th century attributed to the broader Sierra Madre-San Fernando structural zone and contemporaneous with the 1971 San Fernando earthquake and the 1987 Whittier Narrows earthquake. Paleoseismic trenching performed across suspected fault scarps near Pasadena and Arcadia has uncovered stratigraphic evidence for multiple Holocene surface-rupturing events, charcoal and detrital charcoal samples dated by laboratories at USGS Menlo Park and university groups provide radiocarbon ages that constrain recurrence intervals. These studies are coordinated with fault-slip studies by researchers affiliated with Caltech and the California Institute of Technology Seismological Laboratory to establish slip rates and coseismic deformation magnitudes comparable to other major southern California thrusts.

Earthquake Hazard and Risk Assessment

Because the Sierra Madre Fault is a blind thrust beneath densely populated suburbs and critical infrastructure, hazard assessments by the USGS and the California Earthquake Authority incorporate probabilistic seismic hazard models that account for potential multi-segment rupture scenarios. Ground-motion modeling links rupture scenarios on Sierra Madre-related structures to shaking patterns across Interstate 210 (I-210), Interstate 5 (I-5), the Los Angeles River corridor, and major lifelines serving Downtown Los Angeles and Pasadena. Urban risk studies by RAND Corporation-affiliated researchers and state emergency planners map vulnerability of structures including unreinforced masonry buildings listed in inventories by the City of Los Angeles and lifeline assessments coordinated with Los Angeles County offices. Scenario planning often references large events on nearby thrusts such as a hypothetical multi-segment rupture analogous to historic behavior on the San Fernando Fault.

Monitoring and Instrumentation

Continuous and campaign GPS stations operated by the Scripps Institution of Oceanography, the University of California, Riverside geodesy groups, and the USGS monitor crustal strain accumulation across the Sierra Madre region. A network of strong-motion accelerometers maintained by the California Integrated Seismic Network and the Southern California Seismic Network provides real-time shaking data for rapid intensity mapping and early warning systems integrated with the ShakeAlert program developed by UC Berkeley, Caltech, and the USGS. Seismic reflection and passive seismic experiments, including deployments of broadband stations by the Earthquake Engineering Research Institute and university consortia, have improved imaging of blind thrust geometry and guided targeted paleoseismic trenching.

Mitigation and Preparedness Measures

Mitigation strategies combine engineering, land-use planning, and public preparedness. Building codes enforced by the State of California and updated in consultation with engineering groups at ASCE and PEER reflect lessons from thrust-related earthquakes; retrofit programs in Los Angeles County target vulnerable inventories such as unreinforced masonry and soft-story buildings. Emergency management exercises coordinated by the California Governor's Office of Emergency Services, Los Angeles County Office of Emergency Management, and municipal agencies test response for disruptions to Metrolink, Los Angeles International Airport, and regional hospitals. Public outreach campaigns run by USGS and local governments promote preparedness actions among residents of Pasadena, Monrovia, and adjacent communities.

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