LEFT-LATERAL FAULTING BENEATH THE MONTE CRISTO RANGE, WEST-CENTRAL NEVADA

Earthquakes of M6.5 or smaller are difficult to interpret in the geologic record. However, events of this size can drive seismic hazard due to their relatively high probabilities of occurrence. We investigate one such earthquake, the 15 May 2020 M_w 6.5 Monte Cristo Range (MCR) earthquake of west-central Nevada, a partially blind left-lateral event. Small surface breaks associated with the 2020 event were previously documented along the western half of the rupture. We now focus on the eastern half of the rupture, where InSAR and aftershock observations indicate that an east-west trending, left-lateral fault ruptured at depth but without significant surface faulting. We assess the presence or absence of evidence of recurrent left-lateral deformation through the MCR. The youngest faulted unit observed in the MCR is a Tertiary basalt flow with a pre-existing age of 7.2 Ma. Previously mapped minor faults in the MCR do not show evidence of Quaternary motion. We did not observe a left-lateral fault zone in bedrock or pediment surfaces in Quaternary alluvium along the projection of deep slip inferred from the InSAR observations or the surface projection of fault planes defined by aftershocks. To assess if minor faults were activated in 2020, we revisited discontinuities originally observed on the wrapped coseismic interferogram, particularly those aligned with previously mapped north-south trending bedrock faults. InSAR discontinuities cut across steep topography without leaving a geomorphic trace and do not clearly align with mapped faults, suggesting that these features, if real, do not represent reactivation of major bedrock faults within the MCR. At present, we lack positive evidence tying deep 2020 rupture to Quaternary left-lateral deformation at the surface in the MCR. This leads us to some tentative conclusions. First, deep slip in the 2020 earthquake seems decoupled from surface faulting in the MCR. Second, the MCR may represent a region where smaller, distributed structures accommodate the transition between neighboring, major, conjugate strike-slip faults. Finally, in the U.S. National Seismic Hazard Model, the 2020 rupture would not be represented as a fault source given the on-fault minimum magnitude threshold of M6.5, and thus is a true 'background' event captured by other methods, such as gridded seismicity.