Hybrid Frictional Behavior of Hydrothermally‐Altered Fault Zones in Long‐Lived Polyphase Geothermal Reservoirs

Abstract Geothermal reservoirs commonly record seismic and creep events, constituting earthquake hazards in enhanced geothermal systems. To understand the geological controls on mixed‐mode frictional behaviors in these settings, we performed friction experiments on fault gouges from the polyphase‐altered carbonate Massenkalk Formation in Germany, a regional priority target for geothermal energy production. The experiments, performed at conditions consistent with ∼4 km deep geothermal reservoir, yield evolved friction coefficients of ∼0.48 for limestone and ∼0.61 for dolostone, whereas the silicate‐carbonate fault gouges exhibit ∼0.55. Dolostone gouges exhibit unstable sliding at low displacement rates, whereas the limestone and silicate‐carbonate gouges are stable at all investigated conditions, with the dolostone and silicate‐carbonate samples exhibiting the most significant microstructural strain localization. These results suggest that the geochemical alteration history of geothermal systems may facilitate hybrid frictional behaviors of seismic and aseismic slip within the same reservoir, and that cataclastic strain localization style is independent of frictional behavior.