San Andreas Fault (southern)

San Andreas Fault (southern)
Name	San Andreas Fault (southern)
Location	Southern California, United States
Length	~300 km (southern section)
Plate	Pacific Plate; North American Plate
Type	Right-lateral strike-slip
Status	Active
Notable events	1857 Fort Tejon earthquake; 1906 implications; 1992 Landers (related); 2019 Ridgecrest (regional strain)

San Andreas Fault (southern) The southern segment of the San Andreas Fault is a major right-lateral strike-slip fault system in Southern California, forming a primary plate boundary between the Pacific Plate and the North American Plate. It links plate-bounding structures near the Mendocino Triple Junction and the Gulf of California transform with regional faults including the Garlock Fault and the San Jacinto Fault Zone, and it influences urban regions such as Los Angeles, San Bernardino, and San Diego. This section has produced large historic earthquakes and remains central to seismic hazard analyses by institutions like the United States Geological Survey and the California Institute of Technology.

Overview and Geologic Setting

The southern San Andreas traverses diverse physiographic provinces from the Transverse Ranges to the Peninsular Ranges and neotectonic basins adjacent to the Salton Trough, crossing jurisdictions including Kern County, Los Angeles County, Riverside County, San Bernardino County, and San Diego County. It accommodates relative plate motion estimated from geodetic networks such as Global Positioning System campaigns and satellite-based measurements from agencies like NASA and Jet Propulsion Laboratory. Regional geology includes crystalline basement exposures near the San Gabriel Mountains, Cenozoic sedimentary basins like the Coachella Valley, and volcanic and fault-related geomorphology recorded in the Anza-Borrego Desert State Park region.

Fault Geometry and Segmentation

The southern segment comprises linked fault strands and structural complexities including step-overs, bends, and creep zones observed near features such as the San Gorgonio Pass and the Indio Hills. Major geometric segments commonly cited include the Mojave segment, the Carrizo Plain-equivalent southern reach, and the Coachella Valley section which projects toward the Salton Sea. Nearby systems like the San Jacinto Fault and subsidiary strands such as the Banning Fault interact via stress transfer and geometric junctions at notable tectonic nodes like the Big Bend of the San Andreas. Fault trace mapping by the California Geological Survey and field studies by university groups provide fine-scale segmentation maps used in rupture-propagation models developed at UC Berkeley and Scripps Institution of Oceanography.

Seismicity and Earthquake History

Historic earthquakes associated with the southern fault include the 1857 Fort Tejon earthquake, which produced extensive surface rupture across the Antelope Valley and the Tehachapi Mountains; paleoseismic investigations indicate multiple late Holocene surface-rupturing events. Seismic catalogs maintained by the Southern California Earthquake Center record a mix of large characteristic events and distributed ruptures that also involve the Landers earthquake and the 1992 Big Bear earthquake sequence through static and dynamic interactions. Instrumental seismicity shows clustering near the Salton Sea geothermal field and triggered sequences following remote events like the 2011 Tōhoku earthquake and the 2019 Ridgecrest earthquakes, highlighting the fault’s sensitivity to regional stress perturbations documented by the International Seismological Centre.

Slip Rates, Creep, and Deformation

Geodetic studies combining campaign and continuous GPS sites, interferometric synthetic aperture radar (InSAR) processed by European Space Agency and JAXA datasets, and geological trench measurements yield slip-rate estimates varying along strike, commonly in the 10–20 mm/yr range for the southern system overall, with local variations near the San Bernardino Mountains and Coachella Valley. Portions of the fault exhibit aseismic creep measurable at surface markers and creeping offsets recorded by long-term leveling and paleoseismic offsets in trenches at sites studied by researchers at USC and UC Riverside. Contemporary deformation is partitioned into strike-slip and localized transtensional components accommodated by splays such as the Corral Canyon Fault and transfer zones into the Salton Trough.

Hazards and Risk to Communities

The southern San Andreas poses significant seismic hazard to metropolitan areas including Los Angeles, Riverside, San Bernardino, Indio, and Palm Springs, as assessed in statewide seismic hazard maps produced by the California Geological Survey and probabilistic models by the USGS National Seismic Hazard Model. Infrastructure at risk includes major transportation corridors like Interstate 10, Interstate 15, State Route 111, water conveyance systems tied to Metropolitan Water District of Southern California, and lifeline assets within the Greater Los Angeles Area. Scenario rupture studies, community emergency plans by counties and city offices, and insurance loss estimations by organizations such as the California Earthquake Authority inform mitigation including building codes under the California Building Standards Commission.

Monitoring, Instrumentation, and Research

Dense seismic networks including the California Integrated Seismic Network, broadband stations from the ANSS backbone, continuous GPS arrays operated by UNAVCO, and borehole observatories like those installed through collaborations with the Southern California Earthquake Center and USGS enable real-time monitoring and research on rupture nucleation, foreshock sequences, and aftershock decay. Experimental campaigns using temporary deployments, paleoseismic trenching supported by the National Science Foundation, and numerical modeling at institutions such as Stanford University and Caltech probe rupture dynamics, fault friction, and ground-motion predictions applied in operational earthquake forecasting frameworks.

Tectonic Evolution and Paleoseismology

The southern fault’s long-term evolution reflects Miocene and Pliocene reorganization of the Pacific–North America plate boundary, initiation of the modern transform linked to rifting in the Gulf of California, and interaction with the Transverse Ranges uplift. Paleoseismic trenching, cosmogenic nuclide dating conducted in collaboration with laboratories at Scripps Institution of Oceanography and optically stimulated luminescence studies led by university teams have constrained late Quaternary slip histories, recurrence intervals, and segmented rupture behavior that feed into seismic source models used by the USGS and Southern California Earthquake Center for hazard assessment.

Category:Geology of California Category:Seismic faults of the United States