The 31 March 2020 Mw 6.5 Stanley, Idaho, Earthquake: Seismotectonics and Preliminary Aftershock Analysis

We report on the tectonic framework, seismicity, and aftershock monitoring efforts related to the 31 March 2020 M-w 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 M-w 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.

Automated summary

This article discusses the tectonic framework, seismicity, and aftershock monitoring efforts related to the 6.5 magnitude earthquake in Stanley, Idaho on March 31, 2020. It highlights the possible kinematic link between different fault systems in the region and the historical seismicity patterns in Idaho. The ongoing seismic monitoring efforts are expected to lead to a better understanding of the ground shaking potential and active fault characteristics in the area.