Revisiting Wyoming’s Greys River Fault: A Newly Recognized Northern Extent

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The Greys River fault is a west-dipping, >50 km-long normal fault in western Wyoming’s fold and thrust belt thought to be capable of producing M7 earthquakes. The fault cuts through folded sedimentary rock in steep, heavily vegetated terrain, making detection of scarps by traditional means difficult. Despite evidence for Holocene activity at a site in the central section, much of the northern Greys River fault has been only approximately located and there are discontinuities in its mapped trace across a quadrangle boundary. This study uses photogrammetry, structure-from-motion digital surface models (DSMs) acquired from an unmanned aerial vehicle, and ground-based laser rangefinder surveys to better constrain the location of the northern Greys River fault. Previously unrecognized fault scarps in hillslope deposits at the base of Deadman Peak were mapped using post-wildfire stereo imagery. DSMs and their derivatives show a scarp in Quaternary glacial deposits <0.3 km south of the Little Greys River, 8 km north of the fault’s previously mapped terminus. Beyond this point the scarp is less apparent in Pinedale glacial outwash and is not visible north of the Little Greys River. Surveyed scarp profiles throughout the northern Greys River fault show vertical surface offsets of 2-6 m across Pinedale glacial and younger debris flow deposits and 4-11 m across Bull Lake glacial deposits, with offsets decreasing abruptly near the Little Greys River. Surface offsets suggest average late-Pleistocene to Holocene slip rates of <0.5 mm/year, slower than the rate calculated from a paleoseismic study along the central fault section. This work increases the mapped length of surface rupture along the Greys River fault by ~13%, which has implications for modeling of regional ground motions associated with a characteristic seismic event and may inform modifications to the National Seismic Hazard Map.

Presenting Author: James P. Mauch

Authors