Abstract

The La Jencia fault in the central part of the Rio Grande rift has a remarkable history of segmented movement during the late Pleistocene and Holocene. After a long period of relative quiescence in the Pleistocene, movement on the reactivated fault created new scarps along five of its six segments. Movement occurred at varying intervals of several ka to more than 10 ka, and at what appears to be decreasing recurrence intervals from the late Pleistocene into the Holocene. Surface rupturing has been in such a manner to fill in intervening, unbroken segments of the fault and to produce continuous scarps along 30 km of the fault's 35 km length. Many of the segments have scarps that show characteristic and statistically significant relations between their height and their maximum slope angle.

The Holocene and upper Pleistocene fault scarps were formed by prehistoric earthquakes having Richter magnitudes of 6.8 to 7.1 (estimated from two relations of earthquake magnitude to displacement and length of ground rupture) and moment magnitudes of 6.6 to 6.7 (estimated from one relation of magnitude to area of source rupture). The geologic and pedologic evidence derived from trenching demands a minimum of five separate instances of faulting during the past 33 ka, although as many as six such events are required if movement along different segments of the fault was not synchronous. The average recurrence interval is about 7.5 ka in the former case and about 6 ka in the latter case. Although earthquakes of Richter magnitude 6 or greater have not been felt in the Rio Grande rift in historic times, the abundance of Quaternary faults suggests that large earthquakes have been prevalent during the Quaternary. If these faults have histories of offset and paleoseismicity such as has been documented for the La Jencia, then the region may have a more significant potential for earthquake hazards than previously considered.

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