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

On March 31, 2020, an Mw 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. Plain Language Summary: The northwest of Stanley, Idaho, is struck by an Mw6.5 earthquake on March 31, 2020, which is the largest event in Idaho since the 1983 Borah Peak earthquake. This event has several intriguing aspects: (1) the epicenter located by USGS is not on a mapped fault and the source region has little historical seismicity over the past 50 years; (2) it predominantly involves strike‐slip faulting, which is inconsistent with the extensional strain accommodated by the closest Sawtooth fault; (3) long‐period point‐source solutions have more than 35% non‐double‐couple components, indicating a more complex source than slips on a planar fault. Based upon an integrative analysis for both seismological and remote sensing data, we prescribe an opposing‐dip two‐fault model to reconcile all observations. The rupture initiated near the USGS epicenter, and then propagated to the southeast about 20 km along the northerner subfault trajectory. When reaching the northwestern terminus of the Sawtooth fault, the rupture changed its original trajectory and moved southwestward, traversing a 10‐km‐wide step‐over. After passing the mapped Sawtooth fault terminus, it propagated to the southeast about 25 km along the southern subfault, which is subparallel to the Sawtooth fault scarp. Key Points: An integrative analysis is applied for the seismological and geodetic data of the 2020 Stanley, Idaho earthquakeWe found that the Stanley earthquake ruptured a pair of opposing‐dip faults offset by a 10‐km‐wide step that narrows with depthThis study reveals that complicated rupture processes appear to be typical for the earthquakes located near the northern Centennial Tectonic Belt boundary [ABSTRACT FROM AUTHOR]