Pajarito Fault

Pajarito Fault
Name	Pajarito Fault
Location	Jemez Mountains, Rio Grande Rift, New Mexico, United States
Length	~35 km
Type	normal? (predominantly) strike-slip? (context-dependent)

Pajarito Fault The Pajarito Fault is a fault zone in the Jemez Mountains region of northern New Mexico associated with the western margin of the Rio Grande Rift near the Los Alamos National Laboratory and Bandelier National Monument. The zone lies in proximity to the Valles Caldera, Frijoles Canyon, and settlements such as Los Alamos, New Mexico and Santa Fe, New Mexico. It has been the subject of investigations by agencies and institutions including the United States Geological Survey, University of New Mexico, and Los Alamos National Laboratory because of its relevance to volcanic, seismic, and societal risk in the southern Rocky Mountains.

Geology and Structure

The Pajarito Fault system transects volcanic and sedimentary units of the Jemez Volcanic Field, cutting Tschicoma Formation lavas, Bandelier Tuff, and older Santa Fe Group sediments near the Española Basin. Structural mapping links splays to the Puye Formation and to local graben structures adjacent to the Valles Caldera ring fracture. Geologists from New Mexico Institute of Mining and Technology and the National Park Service documented fault scarps, echelon segments, and bedrock offsets that show relationships to regional normal faults mapped by the USGS and regional transects by researchers at Los Alamos National Laboratory. Stratigraphic correlations reference units described by the Geological Society of America and regional surveys by the New Mexico Bureau of Geology and Mineral Resources.

Tectonic Setting and Regional Context

The fault lies within the extensional regime of the Rio Grande Rift, a structure that links to broader plate interactions involving the North American Plate and mantle processes beneath the Basin and Range Province. Regional tectonics relate the Pajarito Fault to the Sangre de Cristo Mountains uplift, the Taos Plateau volcanic field, and interactions with the Jemez Lineament. Academic programs at Stanford University, California Institute of Technology, and the University of Arizona have modeled strain partitioning across the rift, comparing the Pajarito Fault to features like the Socorro Seismic Zone and the Wasatch Fault. Paleogeographic reconstructions reference contributions from the American Geophysical Union, the National Science Foundation, and comparative studies in the Great Basin.

Seismic History and Activity

Instrumental seismicity catalogues maintained by the USGS, New Mexico Tech Seismological Observatory, and the Northern New Mexico Seismic Network record microearthquake swarms and background activity near the Pajarito Fault alongside events in Los Alamos County. Historic accounts from early 20th-century settlers and records compiled by the New Mexico Historical Review complement modern detections using broadband networks funded by the National Earthquake Hazards Reduction Program. Comparative analyses reference seismic sequences like the Albuquerque Basin and notable events logged by the International Seismological Centre. Research collaborations including Sandia National Laboratories and universities have assessed earthquake recurrence in context with volcanic unrest at the Valles Caldera.

Geomorphology and Surface Expression

Surface mapping shows linear escarpments, terrace offsets, and alluvial fan displacements along canyons draining the Jemez Mountains into the Rio Grande corridor near Los Alamos Canyon and Frijoles Canyon. The geomorphic character has been described in publications by the Geological Society of America, the American Association of Petroleum Geologists, and regional field guides from the National Park Service for Bandelier National Monument. Remote-sensing efforts using imagery from the Landsat program, ASTER, and airborne LiDAR collected in partnership with the USGS Earth Resources Observation and Science (EROS) Center reveal fault-related lineaments analogous to those studied in the Wasatch Front and San Andreas Fault systems.

Paleoseismology and Slip Rates

Trenching studies near populated areas conducted by teams from Los Alamos National Laboratory, University of New Mexico, and consultants for the Department of Energy have exposed stratigraphic evidence used to bracket late Quaternary surface ruptures. Radiocarbon ages derived from detrital charcoal and organic soils were analyzed alongside optically stimulated luminescence samples processed at labs associated with Texas A&M University and University of California, Berkeley to estimate recurrence intervals and vertical slip rates similar to other rift border faults such as the Socorro Fault. Published estimates in technical reports for the USGS and the DOE place slip rates in a range consistent with moderate Holocene activity, though values vary among studies.

Hazards and Risk Assessment

Hazard analyses prepared by the USGS, Los Alamos County, and emergency planners at the National Nuclear Security Administration evaluate seismic shaking, surface rupture, landslide initiation in steep canyon walls, and secondary effects such as liquefaction along the Rio Grande floodplain. Infrastructure at risk includes facilities at Los Alamos National Laboratory, transportation corridors connecting Interstate 25 and local highways, and cultural resources managed by the National Park Service and Pueblo of San Ildefonso. Comparative risk frameworks draw on methods developed by the Federal Emergency Management Agency, National Institute of Standards and Technology, and the Pacific Northwest Seismic Network.

Research and Monitoring Techniques

Ongoing research employs dense seismic arrays deployed by the USGS and UNM, continuous GPS stations part of the Plate Boundary Observatory and regional networks, InSAR analyses using data from Sentinel-1 and TerraSAR-X, and paleoseismic trenching following protocols by the Quaternary Research Association. Collaborations involve Los Alamos National Laboratory, Sandia National Laboratories, academic groups from Massachusetts Institute of Technology and Colorado School of Mines, and international partners citing methodologies from the International Union of Geological Sciences. Data integration leverages repositories at the Incorporated Research Institutions for Seismology and contributes to hazard models used by the USGS and regional planners.

Category:Geology of New Mexico Category:Seismic faults of the United States