Verdugo Fault

Verdugo Fault
Name	Verdugo Fault
Location	Los Angeles County, California, United States
Coordinates	34.18°N 118.25°W
Type	right-lateral strike-slip (primarily)
Length	~13 km (surface trace)
Status	Quaternary active
Notable events	Several historic small earthquakes; studied during 20th–21st centuries

Verdugo Fault is a Quaternary active right-lateral strike-slip fault located in the eastern San Fernando Valley and western Verdugo Mountains region of Los Angeles County, California. It lies within a complex network of faults that includes major structures such as the San Andreas Fault system, the Santa Monica Mountains, and the Transverse Ranges. The fault plays a role in regional strain partitioning among the Garlock Fault, San Gabriel Fault, and other Southern California structures, and has been the subject of geological, geophysical, and seismic-hazard investigations by institutions including the United States Geological Survey, California Institute of Technology, and the Southern California Earthquake Center.

Geology and Structure

The Verdugo rupture zone traverses bedrock units mapped in association with the Topanga Formation, the Tujunga Formation, and late Cenozoic alluvium related to the Los Angeles Basin stratigraphy. Structural mapping links the fault to folding and uplift recorded in the Verdugo Mountains, adjacent to the San Gabriel Mountains and the Santa Susana Mountains. Crosscutting relationships with igneous bodies emplaced during the Neogene link to exposures correlated with the Sierra Madre block. Stratigraphic offsets observed across the trace correlate with measured stratigraphic units similar to those studied along the Puente Hills Fault and the Whittier Fault. Slickensides and fault gouge within mylonitic zones mirror deformation styles seen along the Palos Verdes Fault and the Raymond Fault.

Tectonic Setting and Fault Mechanics

Regionally, the Verdugo structure operates within the transtensional and transpressional regime of the Pacific Plate–North American Plate plate boundary in Southern California, interacting with transform and oblique-slip faults such as the nearby San Jacinto Fault Zone and the Elsinore Fault Zone. Geodetic measurements from arrays maintained by the Plate Boundary Observatory and the GPS Geodesy community record localized strain accumulation consistent with right-lateral slip partitioning, comparable to observations on the Rose Canyon Fault and the Mission Creek Fault. Mechanical behavior includes strike-slip shear with secondary dip-slip components, influenced by crustal heterogeneities described in studies by Wesley Ward and colleagues. Modeling efforts using finite-element techniques by teams at Scripps Institution of Oceanography and Stanford University simulate stress transfer between the Verdugo trace and neighboring structures like the San Fernando Fault and the Hollywood Fault.

Seismology and Historical Activity

Seismic catalogs maintained by the United States Geological Survey and the Southern California Seismic Network document microseismicity clustering along the Verdugo alignment, analogous to patterns observed before events on the Northridge earthquake rupture and aftershock distributions following the 1994 Northridge earthquake. Historical instrumental records and paleoseismology trenches reveal Holocene displacement episodes similar in recurrence to those estimated for the Imperial Fault and the Cucamonga Fault. Studies by researchers affiliated with Caltech and UCLA have analyzed focal mechanisms from local events, comparing nodal planes to mechanisms reported for the San Andreas Fault and the Garlock Fault. Paleoseismic evidence from trenching and colluvial wedge analysis has been compared with findings on the Sierra Madre Fault Zone to constrain timing of surface-rupturing events.

Geographic Extent and Surface Expressions

The surface trace of the fault is mapped across the foothills of the Verdugo Mountains, adjacent to communities in Glendale, California, Burbank, California, and parts of the San Fernando Valley such as North Hollywood, Los Angeles. Topographic scarps, linear drainages, and sag ponds comparable to geomorphic features on the Elsinore Fault and the Anza Gap indicate surface deformation. Urban infrastructure in proximity includes transportation corridors like the Interstate 5 and the Golden State Freeway, utilities serving Los Angeles Department of Water and Power, and municipal land managed by the City of Glendale. Correlations between mapped fault traces and lidar-derived topography, used extensively in studies of the Puente Hills Fault and Hollywood Fault, assist in defining the fault’s geometry beneath alluvial cover.

Hazard Assessment and Risk Mitigation

Seismic hazard models by the USGS National Seismic Hazard Model and regional assessments by the California Geological Survey incorporate Verdugo Fault parameters when evaluating site-specific shaking for urbanized sectors of Los Angeles County and the San Fernando Valley. Scenario earthquakes used by agencies such as the Federal Emergency Management Agency and the California Office of Emergency Services consider potential rupture lengths and ground-motion prediction equations similar to those applied for the San Andreas Fault and the Puente Hills Fault. Engineering studies by firms collaborating with the American Society of Civil Engineers assess impacts on critical lifelines including Los Angeles Aqueduct components and rail infrastructure operated by Metrolink (California). Mitigation measures referenced in county seismic ordinances reflect guidance from FEMA and the National Institute of Standards and Technology on retrofit priorities for structures in fault-rupture hazard zones.

Research History and Investigations

Investigation of the Verdugo trace accelerated in the mid-20th century with geological mapping by teams from the United States Geological Survey and universities such as UCLA and Caltech. Paleoseismic trenching projects led by researchers affiliated with the Southern California Earthquake Center and the California Institute of Technology provided insight into Holocene activity, while airborne lidar surveys by the National Aeronautics and Space Administration and high-resolution topographic mapping advanced fault characterization comparable to studies on the San Andreas Fault and Puente Hills Fault. Ongoing multidisciplinary research involves contributions from the Scripps Institution of Oceanography, the USGS Earthquake Science Center, and international collaborations with institutions like the British Geological Survey to refine rupture scenarios, recurrence intervals, and hazard models.

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