The earthquake potential of the Pajarito Fault system, New Mexico

Geologic mapping. topographic profiling. drilling. and trenching studies were conducted to investigate the earthquake potential of the Pajarito fault system (PFS) as part of a seismic hazard investigation for Los Alamos National Laboratory. This complex. nonh-striking system of normal faults bounds the Jemez Mountains and forms the active western margin of the Espanola Basin in the Rio Grande rift. New Mexico. The PFS is geomorphically well expressed in early Pleistocene volcanic rocks that erupted from the nearby Valles caldera. but latest Pleistocene and Holocene fault scarps are generally absent. largely because of the sparsity of late Quaternary alluvium along the faults. The main 41-km-long Pajarito fault forms a prominent east-facing escarpment as high a~ I 80 m. offsening the 1.2 Ma Tshirege member of the Bandelier Tuff down to the east. Topographic profiles on the Pajanto fault suggest average and maximum net vertical tectonic displacements fNVTDl of 8 I m and 154m. yielding average and maximum NVTD rates of 0.07 and 0.13 mmlyr. respectively. for the past 1.2 Ma. The shoner Rendija Canyon and Guaje Mountain faults (9 to 14 km) lie within 5 km to the east of the Pajarito fault and dip steeply to the west. The maximum NVTD estimated for the Rendija Canyon fault of 36 m yields a maximum NVTD rate of 0.03 mm/yr. whereas the average NVTD of 22 m yields an average NVTD rate of 0.02 mm/yr for the past 1.2 Ma. The maxtmum NVTD estimated for the Guaje Mountain fault of 27m yields a maximum NVTD rate of 0.03 mm/yr. and an average NVTD of 15 m over the past 1.2 Ma yields an average rate of 0.0 I mm/yr. Shoner-term (past 0.3 Ma I vertical separation rates for the Guaje Mountain fault. estimated from offsets of fluvial terraces. range from 0.0 I to 0.03 mm/yr. Paleoseismic evidence from nine excavation sites on the PFS suggests complex rupture behavior. including variations in rupture panerns through time and independent rupture behavior of closely-spaced faults. Trenches at the base of the Pajanto fault escarpment exposed indirect stratigraphic and structural evidence for as many as five surface-faultmg events. with the youngest event probably having occurred shonly before deposition of the El Cajete pumice. 50 to 60 ka. However. this record is ambiguous and incomplete because the main scarp-forming fault was probably not exposed. Geomorphic. stratigraphic and structural evidence for two or three events on the Guaje Mountain fault since 150 to 300 ka suggests average recurrence intervals between 47 and 300 ka. Stratigraphic and structural evidence for three (possibly four) events on the Rendija Canyon fault since more than 140 ka suggests a preferred recurrence interval of 33 to 66 ka .. although if four events did occur. average recurrence intervals would range from I 6 to 33 ka. Available data suggest independent rupture behavior between the Rendija Canyon and Guaje Mountain faults. However. it is unknown whether these faults are newly forming rift-bounding structures that ruprure independent of the Pajarito fault. or are antithetic faults that rupture dependently with the Pajarito fault.