Fault Slip Reversals Preceded the Intraplate 2021 Earthquake Series in the Vienna Basin (Central Europe)

Abstract Fault‐slip reversals indicate transient stress changes and may illuminate earthquake preparation processes. Here we present high‐resolution observations of sub‐millimetric creep and slip reversals on the Altaquelle Fault, a Quaternary normal fault in the southern Vienna Basin, preceding a regionally significant intraplate earthquake sequence in spring 2021. Continuous three‐dimensional monitoring in a cave detected two slip reversals 15 and 4 days before the Mw 4.2 mainshock. These events showed oblique reverse‐dextral slip and near‐vertical dilation, opposite to the fault's long‐term oblique normal sinistral kinematics. Precursory displacement stress inversions agree with stress regimes derived from earthquake focal mechanisms. InSAR vertical motion revealed contemporaneous, spatially coherent deformation evolving from butterfly‐shaped lobes to a lenticular uplift centered on the epicentral area. Together, fault‐scale and regional data indicate transient stress perturbations expressed as near‐surface slip reversals and subtle crustal deformation. Integrated creep, InSAR, and seismic observations thus uncovered short‐term stress reorganization preceding these intraplate earthquakes.