Late Holocene Rupture History of the Ash Hill Fault, Eastern California Shear Zone

Christine Regalla, Eric Kirby, Shannon A. Mahan, & Eric McDonald

Submitted August 15, 2021, SCEC Contribution #11471, 2021 SCEC Annual Meeting Poster #073

Several recent, large-magnitude earthquakes in the Eastern California Shear Zone (ECSZ) help serve as a reminder of the role that intraplate faults play in accommodating plate boundary deformation. Understanding how seismogenic strain is distributed and transferred across faults within the ECSZ requires knowing rupture histories for both well-established and distributed fault systems, but our knowledge of the timing, kinematics, and extent of paleoseismic ruptures for many faults in the ECSZ remains limited. Here, we help fill an existing data gap in the rupture history of the Ash Hill fault, a ~50km long, right-lateral transtensional fault in the western Panamint Valley, using new tectonogeomorphic mapping, geochronological dating, and fault offset measurements. We completed high-resolution mapping of alluvial fans and fault scarps in an ~20 km2 area at a scale of 1:4k, and developed an alluvial stratigraphy based on surface morphology and weathering parameters that allow us to constrain the number and relative timing of late Holocene ruptures. We determined the magnitude of offset for each event from a set of >100 measurements of 2D and 3D fault offset using field data, lidar DEMs, and backslipped reconstructions of drone-based structure from motion digital surface models. Finally, we constrain the ages of ruptures using soil profile development indices (PDI), a locally calibrated soil chronofunction, and feldspar infrared luminescence (IRSL) in alluvial and aeolian deposits. Collectively, our data show that the Ash Hill fault experienced three earthquakes in the past ~4 ka, with an average late Holocene slip rate of ~0.7-1.5 mm/yr. Each rupture has average lateral and vertical offsets of ~1.0 and ~0.15 m, suggesting earthquakes magnitudes of ~6.9-7.0. Soil age estimates suggest that the most recent earthquake occurred <0.5-0.8 ka, and the previous two earthquakes occurred in the past 2-4 ka, while IRSL data suggest the most recent earthquake occurred 0.16 to 2.9 ka, and the previous earthquakes occurred at 1.5-3.1 ka and >2.2 ka. The timing of late Holocene earthquakes on the Ash Hill fault appear to overlap in time with those on the adjacent Panamint Valley fault, suggesting the potential that these two fault systems rupture in the same or closely temporally related earthquakes. We are further testing these ideas by refining our mapping and age control of offset surfaces along the Ash Hill and Panamint faults.

Key Words
Panamint Valley, paleoseismology, tectonic geomorphology

Citation
Regalla, C., Kirby, E., Mahan, S. A., & McDonald, E. (2021, 08). Late Holocene Rupture History of the Ash Hill Fault, Eastern California Shear Zone. Poster Presentation at 2021 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology