Reevaluation of high slip rates on the Eglington Fault Las Vegas, NV utilizing new chronostratigraphic and geologic evidence

Kathleen B. Springer, & Jeffrey S. Pigati

Submitted August 15, 2016, SCEC Contribution #6911, 2016 SCEC Annual Meeting Poster #098 (PDF)

Poster Image: 
The Eglington fault is a northeast striking monoclinal landform that offsets middle-late Pleistocene groundwater discharge deposits in the Las Vegas Valley by ~14 m vertically. Most of the exposure of the fault has been lost to extensive urbanization, but the scarp is preserved along its northeastern extent. The 2014 USGS National Seismic Hazards Map characterizes the Eglington fault as a Quaternary fault and estimates the slip rate at 0.16 mm/yr. Recent investigations indicate a significantly higher slip rate (0.25 to 0.9 mm/yr, with a preferred rate of 0.6 mm/year). These estimates are based on pre-AMS radiocarbon dates that constrain the age range of a deposit identified as “Unit D” of the Las Vegas Formation (40-18 ka). The preferred offset age of the fault was obtained from deposits that formed on the upper part of “Unit D” (22 ka). Our recent work has redefined the stratigraphy and chronology of the Las Vegas Formation in and around the Eglington fault area, allowing reinterpretation of its slip rate. Specifically, the Eglington fault does not displace the entirety of “Unit D”, but only the middle bed (our Bed D2) from which we obtained radiocarbon dates of 27.58-31.68 ka. Thus, a more accurate revised slip rate is ~0.44 - 0.50 mm/year. Additionally, we show that the 22 ka date that previously defined the minimum displacement age for “Unit D” across the Eglington scarp applies not to that unit but to a newly recognized unit (Member E, Bed E0) that so far, has not been shown to be offset by the fault. Additional investigation is necessary to determine offset of this unit and other younger deposits in the sequence. While calculations based on our revised stratigraphy and high-precision chronology broadly corroborate previous high contemporary slip-rate estimates for the Eglington fault, they also significantly increase the precision of earlier slip rate estimates. The revised rates also affect probable earthquake-recurrence intervals and reinforce the need for further study of this fault with respect to public safety in the Las Vegas Valley.

Key Words
Eglington Fault, high contemporary slip rate, Las Vegas Valley, high resolution chronology

Citation
Springer, K. B., & Pigati, J. S. (2016, 08). Reevaluation of high slip rates on the Eglington Fault Las Vegas, NV utilizing new chronostratigraphic and geologic evidence. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology