Low‐Rate Faulting on the Margin of the Colorado Plateau and Rio Grande Rift in North‐Central New Mexico

Abstract

In northern New Mexico, seismic hazard outside of the Rio Grande Rift (RGR) is generally considered low due to limited historical seismicity and few mapped Quaternary faults. Although the region has a complex tectonic history, nearly all known faults are considered inactive because of a lack of faulted Quaternary deposits. Analysis of lidar and existing geologic and geomorphic mapping permits identification and characterization of four faults on the Colorado Plateau/RGR margin. The Gallina, Willow Creek, and East and West Brazos Peak faults are normal or dextral‐normal faults that strike NW‐SE with lengths from 15 to 50 km. The Willow Creek fault has scarp heights <0.5–6.4 m on five Quaternary surfaces, with displacement increasing with relative surface age. An ∼5‐m‐high scarp on the ∼250 ka Brazos basalt yields an ∼0.02 mm/yr minimum dip‐slip rate. Oblique slip is recorded by dextral offset (∼40 m) of a middle Quaternary (?) terrace riser, a linear and en‐echelon stepping fault pattern, and a consistent down‐to‐the‐northeast fault expression. The West Brazos Peak fault displaces late Quaternary (?) surfaces by <4 m down‐to‐the‐southwest. We interpret these faults are Laramide‐aged faults reactivated as normal or dextral‐oblique faults in the Quaternary, consistent with regional stress and geodetic data. Although the faults have low slip rates (<0.1 mm/yr), the fault lengths suggest they may rupture in M6‐7 earthquakes and may pose a previously unrecognized seismic hazard to the region. Dextral transtension within the eastern Colorado Plateau is consistent with geodetic strain rates and earthquake focal mechanisms.

Advancing the Limits of InSAR to Detect Crustal Displacement from Low-Magnitude Earthquakes through Deep Learning

“…This earthquake occurred less than 70 km from several northern New Mexico communities, as well as the Valles Caldera National Preserve and Bandelier National Monument, which together hold geological and cultural significance. A better understanding of seismicity along this fault system is critical, as a recent study provided evidence that the Gallina fault is currently active [19] and supported earlier, poorly constrained estimates that this fault may be capable of accommodating an Mw 6-7 event in the future [20].…”

“…In two probabilistic seismic hazard analyses for the Los Alamos National Laboratory [20,26], and a study on the nearby El Vado Dam [27], both the Gallina and Nacimiento faults were conservatively interpreted to be active, despite no clear evidence of Quaternary offset, largely due to microseismicity observed within and east of these faults [28,29]. More recently, the first clear geologic evidence for Quaternary deposit offset was provided across the northern part of the Gallina fault and also extended the fault 14 km further north [19]. No similar studies have been performed near the southern end of the Gallina fault or the northern end of the Nacimiento fault, near the location of the 30 July 2020 earthquake.…”