Whittier Fault

Whittier Fault
Name	Whittier Fault
Country	United States
State	California
Region	Los Angeles County
Coordinates	33.97°N 118.03°W
Length km	35
Type	Strike-slip and oblique-slip
Plate	North American Plate, Pacific Plate
Status	Active

Whittier Fault The Whittier Fault is a right-lateral, strike-slip fault in eastern Los Angeles County, California, linked to the complex fault network of the Los Angeles Basin and the Transverse Ranges. It accommodates distributed motion between the San Andreas Fault system and offshore faults such as the Palos Verdes Fault and interacts with buried structures beneath Los Angeles, Long Beach, and Santa Ana. Studies of the fault draw on investigations by the United States Geological Survey, the California Geological Survey, and academic groups at institutions like the California Institute of Technology and the University of California, Los Angeles.

Geology and Structure

The fault lies within late Cenozoic sedimentary strata of the Los Angeles Basin and juxtaposes Tertiary and Quaternary deposits, alluvium, and Holocene fan surfaces near the Puente Hills and San Gabriel Valley. Geologic mapping by the California Division of Mines and Geology and structural analyses published in journals linked to the Geological Society of America show a complex, segmented trace with en echelon strands, stepovers, and blind thrust components that connect to the nearby Elysian Park Fault and Sierra Madre Fault Zone. Cross sections constrained by seismic reflection profiles acquired by the United States Geological Survey and industry partners reveal anastomosing strands with variable dip angles and localized transpressive folds similar to those described for the Santa Monica Fault and Compton–Norwalk Fault. Basement involvement includes Cretaceous to Paleogene crystalline rocks akin to exposures in the San Gabriel Mountains.

Tectonic Setting and Fault Type

The Whittier Fault lies in the plate boundary zone between the Pacific Plate and the North American Plate, where right-lateral shear is partitioned across a network that includes the San Andreas Fault, the Newport–Inglewood Fault, and the Elsinore Fault. Kinematic indicators and focal mechanisms from the Southern California Seismic Network indicate dominantly right-lateral strike-slip motion with oblique reverse components, consistent with transpressional regimes observed along the Transverse Ranges and the Peninsular Ranges. Its behavior is influenced by regional compression related to the Big Bend of the San Andreas Fault and by sedimentary basin structures comparable to those of the Santa Barbara Channel region.

Seismic History and Notable Events

Instrumental seismicity recorded by the California Integrated Seismic Network and historic catalogs compiled by the United States Geological Survey show moderate seismicity near the fault, with clusters correlated to activity on neighboring structures such as the 1987 Whittier Narrows earthquake and the 1994 Northridge earthquake, which altered stress fields across southern California. Paleoseismic trenches and geomorphic studies link Holocene surface-rupturing events to fault segments, and recorded microseismicity has been used to associate fault behavior with regional sequences including the Long Beach earthquake of 1933 contextually. The fault has not hosted an independently named, large historic earthquake comparable to the San Fernando earthquake but is considered capable of generating damaging events given its connectivity to regional rupture pathways identified in scenarios developed by the Southern California Earthquake Center.

Slip Rates, Recurrence Intervals, and Paleoseismology

Geodetic measurements from the Global Positioning System campaign networks and continuous stations operated by the Plate Boundary Observatory and USGS suggest slip rates that are modest relative to the San Andreas Fault yet significant for metropolitan exposure, typically reported in the range of a few millimeters per year. Paleoseismic trenching studies, correlated with radiocarbon ages and stratigraphic markers used in research at the Scripps Institution of Oceanography and the University of Southern California, indicate recurrence intervals for surface-rupturing events on individual strands on the order of hundreds to thousands of years, with complexity introduced by multi-strand rupture interactions similar to those inferred for the Puente Hills Fault and Elysian Park Fault. Cosmogenic nuclide dating and luminescence dating employed in regional studies have refined timing of Holocene displacements.

Hazard Assessment and Risk to Communities

Hazard models by the United States Geological Survey and probabilistic seismic hazard assessments by the Southern California Earthquake Center incorporate the Whittier Fault as part of multi-fault rupture scenarios that elevate shaking intensities for Los Angeles County, Orange County, and port facilities in Long Beach and San Pedro. Vulnerability analyses referencing inventories from the Federal Emergency Management Agency and Cal OES highlight exposure of critical infrastructure including freeways such as Interstate 5 and Interstate 605, water conveyance systems like the California Aqueduct corridors, and lifeline facilities serving Los Angeles International Airport and regional hospitals. Building-stock risk is evaluated against standards promulgated by the American Society of Civil Engineers and state seismic retrofit programs.

Monitoring, Research, and Mitigation Efforts

Monitoring is conducted by networks including the Southern California Seismic Network, continuous GPS arrays, and temporary paleoseismic programs supported by the National Science Foundation and state agencies. Research collaborations among the California Geological Survey, USGS, and university groups have produced community earthquake scenario planning, fault-specific rupture models, and urban seismic resilience studies in partnership with municipalities such as City of Los Angeles and City of Long Beach. Mitigation efforts emphasize retrofit programs guided by the ASCE 7 standards, emergency preparedness coordinated with FEMA and Cal OES, and public education initiatives promoted by the ShakeOut exercise and regional hazard communication campaigns.

Category:Faults of California Category:Seismology Category:Los Angeles County, California