Novel approaches to identifying earthquakes along bedrock fault scarps using quartz luminescence: an example from the Hurricane Fault, UT

Margo Odlum, Alexis K. Ault, & Tammy M. Rittenour

Published September 10, 2023, SCEC Contribution #13281, 2023 SCEC Annual Meeting Poster #081

Quantifying coseismic temperature rise from exhumed brittle faults can identify past earthquakes, with implications for modern seismic hazards. We use microstructural analysis and low temperature thermochronometry on the seismically active Hurricane fault, Utah, to understand faulting processes and timing. At our study site, the fault scarp is characterized by large silica fault mirrors (FMs) that cut the Rock Canyon Conglomerate of the Moenkopi Formation. Previous (U-Th)/He (He) thermochronometry from hematite patches along the silica FMs provide are consistent with the up-dip propagation of earthquake ruptures through ~300 m depth ~0.65-0.4 Ma (Taylor et al., 2021, GRL). New scanning and transmission electron microscopy of the silica FMs indicate highly localized deformation, and partially to fully amorphous but still polygonal particles within the ~5 μm of the slip interface. Apatite He dates >4 cm away from the FM are ~8-12 Ma and record exhumation related cooling. We test quartz optically stimulated luminescence (OSL) and thermal luminescence (TL) as new tools to fingerprint temperature rise and/or material transformations associated with shallow seismic slip. Pulsed annealing linearly modulated quartz OSL and TL from thin (~2 mm) slabs parallel to a mirrored slip surface indicate that trap depths and lifetimes, as well as TL sensitivity are lowest within 4 mm of the fault plane and increase away from the fault plane. The sensitivity corrected TL signals exhibit patterns consistent with signal loss at the fault surface. The physical transformations of quartz, assisted by mechanical, fluid-mediated, and thermal processes during slip along the fault surface, likely contribute to the different luminescence properties. Collectively, micro to nanoscale observations and luminescence data illustrate mechanical processes, fluids, and/or elevated temperatures during fault slip work constructively to transform fault materials and affect the quartz luminescence properties and signals in natural fault rocks. These processes may facilitate the up-dip propagation of potentially large earthquakes along thin slip surfaces in the shallow crust. Fault rock luminescence may be a powerful approach to identifying earthquakes, with potential for quantifying coseismic temperatures and earthquake timing, along young (<300 Ka) faults in California.

Key Words
luminescence, fault rock, microstructure

Citation
Odlum, M., Ault, A. K., & Rittenour, T. M. (2023, 09). Novel approaches to identifying earthquakes along bedrock fault scarps using quartz luminescence: an example from the Hurricane Fault, UT . Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology