Using new paleoseismic data to reevaluate fault system interdependence: a case study from the Pajarito fault system, Rio Grande rift, northern New Mexico

The Pajarito fault system is an ~50km long region of extensional faults located on the western boundary of the Rio Grande rift in northern New Mexico. The fault system includes the east-dipping Pajarito fault (thought to be the master structure), and the west-dipping Rendija Canyon, Guaje Mountain, and Sawyer Canyon faults. Paleoseismic studies conducted on the Pajarito fault system near Los Alamos, NM prior to 2002 suggested that potentially up to 3 Holocene surface-rupturing events occurred on this system (Lewis et al., 2009). Based on established scaling relationships from Wells and Coppersmith (1994), this indicates that potentially 3 M5+ earthquakes occurred within a 50 km zone in the past 11,000 years. Previous work has interpreted this to mean that all faults in the Pajarito fault system rupture simultaneously, three times in the Holocene (Lewis et al., 2009). Given that this fault system is thought to have an 0.1 mm/yr average slip rate, a large number of large surface rupture events occurring so close in time seems unusual. We present data from new paleoseismic studies on multiple strands of the Pajarito fault that suggest a single mid- to late-Holocene event. New age constraints on the most recent event (MRE) of the Pajarito fault, when compared to the MRE from previous work on the Rendija canyon and Guaje mountain fault, located ~4.5km northeast of this study, do not overlap. These new data support the interpretation of two M5+ events within the Holocene, one on the Pajarito fault and one on the Rendija Canyon and Guaje Mountain faults; however, it appears that no single fault records both events. We discuss several possible interpretations for the apparent numerous moderate scale events in this low-extension fault system, including the following: 1. Traditional fault scaling relationships may not apply in this system and the slip events are too small to activate other faults in the system; 2. The Pajarito fault ruptures in moderate-sized events independently from the Rendija Canyon and Guaje Mountain faults which may rupture as the southern tip of the nearby Santa Clara/Embudo strike-slip system, thus no fault slips more than once in the Holocene; 3. The Pajarito, Rendija Canyon and Guaje Mountain faults all ruptured together in a 2-3 event Holocene cluster, but the paleoseismic evidence is not recorded on all of the faults.