Paleoseismology of the Zia Fault and Its Impact on Seismic Hazard for Albuquerque, New Mexico, USA

ABSTRACT The Zia Fault is one of three major east-dipping normal faults on the western margin of the Rio Grande Rift in the northern Albuquerque Basin. Our trenches were located about 250 m south of the northern rim of the Llano de Albuquerque (LdA), where a 100-m-wide graben has formed east of the main, 5-m-high fault scarp. The middle trench across the steepest, highest part of the scarp revealed evidence for four to five faulting events. Displacement in the latest three events ranged from 0.9 to 2.6 m, with an average of 1.8 m, and cumulative down-to-the-east throw of 4.6 m. Age control for faulted deposits in the trench came from thermoluminescence and infrared stimulated luminescence (IRSL) dating, and by calibrated accumulation rates of secondary carbonates. If the 4.6 m of throw accumulated over the past 32 ka, as indicated by secondary carbonates, then the average long-term slip rate is 0.14 mm/yr. This slip rate is greater than that inferred for the two other major faults on the LdA, the County Dump Fault (0.016–0.018 mm/yr) and the Calabacillas Fault (0.0053–0.0072 mm/yr). Luminescence ages were also in correct stratigraphic order but were roughly 5–10 times older than the carbonate rate ages. These ages are interpreted as overestimates due to anomalous fading, which is common with IRSL feldspar dating of sediments older than 100 ka.