Late Holocene rupture history of the Ash Hill fault, Eastern California Shear Zone

Christine Regalla, Hannah Pangrcic, Eric Kirby, & Eric McDonald

Published August 25, 2016, SCEC Contribution #6840, 2016 SCEC Annual Meeting Poster #092

Recent paleoseismic investigations in the Eastern California Shear Zone (ECSZ) suggest that the Panamint, Garlock, and Owens Valley faults may have experienced a late Holocene cluster of seismicity, concurrent with an ongoing <~1.5 ka cluster documented in the southern ECSZ. The spatial and temporal patterns in seismicity are complex, however, and constraints on earthquake recurrence along faults in the northern ECSZ are limited. Here we present new mapping of alluvial deposits offset by the Ash Hill fault, a ~50km long, near-vertical, right-lateral transtensional fault in the western Panamint Valley, that provide evidence for three ruptures since ~2-4 ka. We develop an alluvial stratigraphy from field observations, air photos, and LiDAR topography based on the relative development of desert varnish, desert pavement, modification of bar and swale morphology, and surface clast weathering. Single and multi-event fault scarps within the youngest five generations of alluvium occur along >30 km of fault length. Vertical and lateral offsets of alluvium measured at >80 locations define three apparent clusters of displacement magnitude, with the largest displacements restricted to the oldest units. At one site, a single continuous surface rupture laterally displaces three generations of alluvium, with 1-1.5m, 1.6-2.6m, and 2.7-4.9m of offset in the youngest, intermediate, and oldest surfaces. Ages of faulted surfaces, currently determined using a locally calibrated soil chronosequence, suggest that the most recent rupture occurred within the past ~500-800 years, and that the previous two ruptures occurred since 2-4 ka. When combined with displacement magnitudes, these data indicate late Holocene slip rates on the order of 0.5–1 mm/yr, with a ~5:1 ratio of right lateral to extensional slip, and ~0.9-2m of displacement per event. The timing of events on the Ash Hill fault overlaps with age estimates for late Holocene ruptures on the Panamint and Owens Valley faults, and suggests the potential for strain transfer and temporally clustered seismicity across both the southern and northern ECSZ.

Key Words
ECSZ, tectonic geomorphology, paleosesmology, clustering

Citation
Regalla, C., Pangrcic, H., Kirby, E., & McDonald, E. (2016, 08). Late Holocene rupture history of the Ash Hill fault, Eastern California Shear Zone. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology