The 2016 Nine Mile Ranch Earthquakes: Hazard and Tectonic Implications of Orthogonal Conjugate Faulting in the Walker Lane

Abstract

ABSTRACT The Nine Mile Ranch (NMR) sequence began with three Mw 5.4–5.6 earthquakes within one hour of each other in December 2016 in the remote area of Fletcher Valley, Nevada; only 4 min separated the first and second events. We analyze this complex earthquake sequence in the Walker Lane to determine the geometry and driving mechanism(s), and to improve understanding of deformation and seismic hazard in this region. Field reconnaissance found that these earthquakes caused significant damage to the Nine Mile ranch house but no surface rupture. We precisely relocate 6000+ earthquakes to reveal activated planar structures, unmapped at the surface, including two large, orthogonal, conjugate faults. Moment tensor solutions, focal mechanisms, and relocations show the two conjugate faults to be a vertical, northeast-trending left-lateral strike-slip fault, and a northwest-trending right-lateral strike-slip fault that dips ∼60° to the northeast. The three main events lie at the intersection of both the faults, but the locations and orientations are most consistent with the first (Mw 5.6) and third (Mw 5.5) events rupturing the left-lateral northeast-trending fault plane; the second event (Mw 5.4) ruptured the right-lateral northwest-trending fault plane. Calculated static stress changes support this interpretation. Smaller events and structures show predominantly strike-slip and normal faulting. We calculate the local interseismic strain rate tensor and coseismic displacements using Global Positioning System data to determine whether nearby volcanic centers played a role in causing the fault geometry. Our results, combined with the spatiotemporal development of the sequence and the moment tensor solutions, indicate that regional scale tectonic forces are the dominant driving factors of this sequence. The NMR sequence adds to the documented variety of spatiotemporal patterns and driving mechanisms of earthquake sequences and swarms within the Walker Lane, providing further information and constraints on seismic hazard in this active region.