Idaho Geological Survey Responds to Stanley Earthquake

On March 31, 2020, a magnitude (M) 6.5 earthquake occurred approximately 30 kilometers (18.64 miles) north-northwest of Stanley, Idaho. This was the second largest earthquake to occur within Idaho in recorded history.

The mainshock was followed by an aftershock sequence that continues to produce M3 and M4 earthquakes. Immediately after the mainshock, the Idaho Geological Survey secured and deployed seismic instrumentation – obtained via an instrumentation grant through the Incorporated Research Institutions for Seismology Portable Array Seismic Studies of the Continental Lithosphere (IRIS PASSCAL).

Shaking from the M6.5 Stanley earthquake was felt widely across the western U.S. and triggered numerous snow avalanches. Shaking also caused rockfall and toppled several rock formations in the Sawtooth Mountains that hosted popular climbing routes. Liquefaction and lateral spreading caused by the mainshock or early aftershocks was observed at Stanley Lake, where the inlet delta collapsed and disappeared into the lake.

Data collected have been shared with the international community and are contributing to defining the location and geometry of previously unknown tectonic structures and to a deeper understanding of the regional stress field as a tool for better hazard mitigation.