Fluid‐Induced Magnetic Enhancement in Sandstone Friction Experiments: Implications for Coseismic Fault Temperature Estimates

Abstract Constraining peak temperature during seismic slip is essential for quantifying earthquake energy budgets and fault weakening. Rock magnetic methods provide a sensitive means of estimating shear‐induced coseismic temperature rise; however, the role of fluids in friction‐induced magnetic alterations remains poorly constrained. Here we present the first experimental investigation into fluid effects, performing rotary‐shear experiments (peak slip rate of 5 cm/s) under dry and fluid‐saturated conditions on simulated fault gouges prepared from Slochteren Sandstone (Groningen gas field, The Netherlands), followed by comprehensive rock magnetic analysis. Dry shearing produced negligible magnetic changes, whereas fluid‐saturated shearing generated pronounced magnetic enhancement, grain growth of magnetic minerals, and the formation of metallic iron in the simulated fault gouge. Our results demonstrate that fluid‐mediated magnetic alterations occur in the absence of strong frictional heating. These findings highlight the need to assess the role of fluids in modifying the magnetic signatures of fault rocks.