The 2020 Mw 6.5 Stanley, Idaho, Earthquake and Aftershock Sequence: Complex Faulting at the Northern End of the Basin and Range Province

ABSTRACT The magnitude 6.5 Stanley, Idaho, earthquake occurred on 31 March 2020 in a sparsely populated region north of the Sawtooth normal fault. We used seismic data from temporary and permanent stations to derive a 1D velocity model and relocate 1401 M ≥ 2.4 earthquakes with hypoDD, including a foreshock, the mainshock, and 3 yr of aftershocks. We used broadband data to determine seismic moment tensors for 173 Mw≥3.1 earthquakes. Combining locations and mechanisms shows the mainshock ruptured an unmapped north-trending, steeply west-dipping, left-lateral strike-slip fault. Rupture initiated near the bottom of the seismogenic zone and propagated upward and bilaterally for ∼20 km north and ∼3–5 km south, where the fault likely ends and deformation changes to extension. There, the rupture may have jumped west to another unmapped blind fault accommodating oblique extension. Support for a late mainshock rupture on a northwest-trending, likely east-dipping fault comes from normal faulting aftershocks. The total rupture length is ∼25–30 km, because oblique fault activity ends at the latitude of the northern terminus of the Sawtooth fault, but its trace, if it reached the surface, would be ∼6 km to the west. The Sawtooth fault was not active, even though aftershock clusters indicate that short strike-slip and normal faults are abundant in its footwall and hanging wall. Extension in the northern Basin and Range seems to terminate where major normal faults reach the inactive Eocene Trans-Challis fault system (TCFS), suggesting the TCFS exerts structural control. The deformation north of the TCFS is low, and the strike-slip character was unknown before the Stanley earthquake. Faults rupturing in the Stanley earthquake lack surface expression and are immature with low cumulative displacement. Complex transitions between tectonic regimes are common and may result in blind ruptures on unknown, immature faults, posing an underrated hazard.