Multifault Opposing-Dip Strike-Slip and Normal-Fault Rupture During the 2020 Mw 6.5 Stanley, Idaho Earthquake

On March 31, 2020, an M-w 6.5 earthquake struck near Stanley, Idaho. More than 35% nondouble-couple component in long-period point-source solutions indicate a more complex source than slip on a planar fault. Using an integrative analysis of seismological and geodetic data, we find that the Stanley earthquake ruptured a pair of opposing-dip faults offset by a 10-km-wide step, including an unmapped northern subfault with predominantly strike-slip faulting and a southern subfault subparallel to the Sawtooth fault with predominantly normal faulting. This converging fault geometry allowed the rupture to traverse a surficial 10-km-wide step, which is greater than the limiting dimension (3-4 km) that commonly ceases earthquake ruptures. This study reveals that a composite rupture process with strike-slip and normal faulting is typical for earthquakes located near the northern boundary of the Centennial Tectonic Belt (CTB), which is distinct from the predominantly normal faulting in the central CTB.