ABSTRACT

An Mw 5.8 earthquake occurred on 6 July 2017 at 12.2‐km depth, 11 km southeast of Lincoln in west‐central Montana. No major damage or injuries were reported; however, the widely felt mainshock generated a prolific aftershock sequence with more than 1200 located events through the end of 2017. The Lincoln event is the latest in a series of moderate to large earthquakes that have affected western Montana. We characterize the spatiotemporal evolution of the sequence using matched filter detection and multiple event relocation techniques. Moment tensor solutions and aftershock locations indicate faulting occurred on a 9‐km‐long north‐northeast‐striking, near‐vertical, strike‐slip fault antithetic to the Lewis and Clark Line, the main through‐going fault system. Seismicity primarily occurs between 6‐ and 16‐km depth, consistent with seismicity in the Intermountain Seismic Belt. We estimate a fault rupture area of 64  km2 and 30  cm of average fault displacement. We identified four foreshocks during the three days before and 3005 aftershocks in the three weeks after the mainshock. The supplemented catalog frequency–magnitude distribution has a b‐value of 0.79 and a minimum magnitude of completeness of 0.7. The overall decay rate is consistent with a modified Omori decay law p‐value of 0.76 and c‐value of 0.32. This event demonstrates that unmapped faults antithetic to major geologic structures play a role in accommodating regional strain in western Montana and can host significant earthquakes.

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