HOLOCENE RUPTURE HISTORY OF THE ASH HILL FAULT, EASTERN CALIFORNIA SHEAR ZONE

Recent paleoseismic investigations in the Panamint Valley and Mojave sections of the Eastern California Shear Zone (ECSZ) reveal an ongoing cluster of seismicity since <1.5 Ka. The spatial and temporal patterns in seismicity are complex, however, and few constraints exist on the timing of ruptures along faults in the northern section of the ECSZ. 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 at least one, and likely two ruptures since the middle Holocene. We define five alluvial units based on the relative development of desert varnish, desert pavement, modification of bar and swale morphology, and surface clast weathering identified from field mapping and interpretation of airphotos and LiDAR topography. Estimates of absolute surface ages are obtained from correlation to previously mapped surfaces in the Panamint Valley, whose ages are constrained from C¹⁴, cosmogeinc dating, and a locally calibrated soil chronosequence.

Fresh surface scarps without re-varnished surface clasts that offset the youngest, Qf7, surface are present for ~32km of fault length. Discontinuous scarps in Qf7 suggest that the most recent rupture may have occurred during Qf7 deposition, likely within the latest Holocene. At multiple locations, fault scarps in older Qf6a have at least twice the displacement as in adjacent Qf7 deposits, requiring at least one additional event since the middle Holocene. An older rupture that offsets Qf5 but not Qf6b requires a third event near the early Holocene. Measurements of surface offset indicate that single-event fault scarps are characterized by ~0.8 to 1.9 m of right lateral and ~0.2 to 0.4m of vertical displacement, while multi-event scarps are characterized by >0.7m of vertical displacement. Collectively, these new data indicate that the Ash Hill fault has produced multiple ~M 6.5-7 earthquakes during the Holocene, and may rupture at similar times as the nearby Panamint Valley and Owens Valley faults.