Shallow creep and confined ruptures in frictionally unstable mica schist.

Crustal faults across geological settings are often enriched in phyllosilicates because of the hydrothermal alteration of wall rocks. The frictional behavior of these altered rocks at mid-crustal conditions remains poorly understood. Here, we show that mica-schist gouge under crustal hydrothermal conditions can host mixed behavior promoting shallow aseismic creep and confined seismic ruptures. We conduct triaxial deformation experiments on natural samples from the East Anatolian Fault, Turkey, and simulate the resulting dynamics on the Pütürge segment during seismic cycles using a physical model of rock friction and a realistic thermal structure. Mica schist exhibits potentially unstable weakening between 300° and 400°C, promoting depth-confined earthquakes. The frictional properties of mica schist are compatible with the 2020 moment magnitude (Mw) 6.8 Elazığ earthquake and the northern rupture arrest of the 2023 Mw 7.8 Kahramanmaraş event, illustrating how the thermal and lithological structure affects rupture style and segmentation.