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Abstract We recorded a M_WR3.6 earthquake in Idaho (USA) on 7 April 2020 with a network of six three‐element infrasound arrays and co‐located broadband seismometers situated within 25 km of the hypocenter. Infrasound array processing is used to identify the arrival of seismic‐to‐atmospheric coupled phases and as much as 90 s of infrasound coda. Apparent velocities ranging from seismic speeds to subhorizontal atmospheric sound speeds are attributed to a superposition of coincident waves arriving at the arrays. We find that the arriving infrasound originates from a broad range of back azimuths that deviates from epicentral back azimuth and indicates the ubiquity of secondary radiators for this relatively small earthquake. Secondary radiators, which often locate in regions of elevated topography, are identified using backprojections and earthquake initiation time. Analysis of infrasound sources from proximal earthquakes can be used to map ground shaking distributions, which are important for assessment of earthquake hazards. 

Abstract We report on the tectonic framework, seismicity, and aftershock monitoring efforts related to the 31 March 2020 Mw 6.5 Stanley, Idaho, earthquake. The earthquake sequence has produced both strike-slip and dip-slip motion, with minimal surface displacement or damage. The earthquake occurred at the northern limits of the Sawtooth normal fault. This fault separates the Centennial tectonic belt, a zone of active seismicity within the Basin and Range Province, from the Idaho batholith to the west and Challis volcanic belt to the north and east. We show evidence for a potential kinematic link between the northeast-dipping Sawtooth fault and the southwest-dipping Lost River fault. These opposing faults have recorded four of the five M≥6 Idaho earthquakes from the past 76 yr, including 1983 Mw 6.9 Borah Peak and the 1944 M 6.1 and 1945 M 6.0 Seafoam earthquakes. Geological and geophysical data point to possible fault boundary segments driven by pre-existing geologic structures. We suggest that the limits of both the Sawtooth and Lost River faults extend north beyond their mapped extent, are influenced by the relic trans-Challis fault system, and that seismicity within this region will likely continue for the coming years. Ongoing seismic monitoring efforts will lead to an improved understanding of ground shaking potential and active fault characteristics.