Paleoseismicity of the Alamogordo fault along the Sacramento Mountains, southern Rio Grande rift, New Mexico
D. J. Koning and F. J. Pazzaglia


 The Alamogordo fault of the southern Rio Grande rift, New Mexico, is a major rift-bounding structure that has down-dropped the Tularosa Basin on the west relative to the Sacramento Mountains on the east. The distribution of late Quaternary rupture activity of the Alamogordo fault adjacent to the Sacramento Mountains was investigated by measuring fault scarp profiles at 40 localities, mapping proximal piedmont deposits, and describing and dating exposures of offset Quaternary sediment.

The timing of four late Quaternary surface rupture events is constrained by stratigraphic relationships and C-14 radiometric determinations. An additional older, poorly constrained event possibly happened at about 20-30 ka. The two older of the four constrained rupture events probably occurred within a time span of 1000-2000 years before 12,600 (radiocarbon years). The estimated average displacement for these two events is 1-3 m. North of the city of Alamogordo, the youngest interpreted surface rupture event occurred between 10,500 and 11,300 radiocarbon years. South of Alamogordo, the youngest surface rupture likely occurred in the early Holocene during a period of major alluvial fan aggradation. This event probably had an average displacement of 0.5 tol m. The difference in the timing of the youngest surface rupture event relative to location supports the interpretation that the southern Alamogordo fault consists of two segments, with the common segment boundary located near Alamogordo. Based on empirical length-displacement-magnitude relations, the four constrained ruptures are estimated to have been associated with ancient earthquakes having seismic moment magnitudes of 6.7 to 7.3.


  1. Koning, D. J.; Pazzaglia, F. J., 2002, Paleoseismicity of the Alamogordo fault along the Sacramento Mountains, southern Rio Grande rift, New Mexico, in: Geology of White Sands, Lueth, Virgil W.; Giles, Katherine A.; Lucas, Spencer G.; Kues, Barry S.; Myers, Robert; Ulmer, Scholle, Dana S., New Mexico Geological Society, Guidebook, 53rd Field Conference, pp. 107-119.

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