Deceleration-phase restrengthening in compositionally zoned fault materials under transient slip

Moderate intraplate earthquakes often nucleate at depth and may not produce surface rupture. Yet the frictional response during fault slip deceleration remains poorly constrained, even though this stage may influence whether slip ceases locally or continues. Here, motivated by the 2024 Buan earthquake in South Korea (Mw 4.2), we combine low-velocity friction experiments that constrain nucleation-relevant behavior with high-velocity friction experiments that quantify frictional recovery during transient, decelerating slip. Low-velocity experiments on intact granite and granular wall-rock powder show near-neutral to weak velocity-weakening behavior and measurable healing, consistent with nucleation-relevant instability in crystalline basement. High-velocity pulse-like loading reveals a strong material contrast during deceleration: granular wall-rock powder restrengthens as slip rate decays, whereas clay-rich gouge remains persistently weak. These results show that, under prescribed transient slip histories, frictional evolution depends not only on dynamic weakening at peak slip rate, but also on composition-dependent frictional recovery during deceleration. This provides experimental constraints on how material contrasts may influence transient slip evolution in compositionally zoned fault materials.