Aseismic slip along the evaporite-rich Katouna-Stamna fault in Greece

The left-lateral, NW-trending Katouna–Stamna Fault (KSF) in Western Greece marks the NE-boundary of the Ionian-Akarnania block. The Global Navigation Satellite System (GNSS) displacement field suggest the potential presence of aseismic slip, which is corroborated by the minimal seismicity in the area. To better understand the current kinematics of the KSF, we integrate full-coverage surface displacement rates derived from 7-yr-long Interferometric Synthetic-Aperture Radar (InSAR) time-series with the GNSS rates, field observations, and structural analyses.

While previous geodetic studies suggested a strike-slip rate of ∼10 mm/yr, our distributed-slip model indicates a strike-slip rate of up to 19 ± 1 mm/yr and a dip-slip rate of up to 11 ± 7 mm/yr. In particular, sinistral slip localises in the right-stepping south-central fault segment, while the highest dip-slip value is found in the northwestern part. In the model, aseismic slip reaches the surface, but the highest slip rates are found below 5 km depth. Furthermore, only the northernmost part of the fault appears locked and accumulating elastic strain, which also corresponds to the location of an earthquake occurred in 2014, supporting our model.

Field evidence indicates complex fault kinematics with multiple deformation phases. The younger generation of NNW-trending striae shows mostly oblique motion with dominating strike-slip component, in agreement with geodetic observations. Such agreement indicates that the geological kinematic regime under which they formed may be relatively recent and possibly still in place.

Overall, our geological observations highlight several possible drivers of aseismic slip. The fault bounds a so-called ‘salt wall’, represented by an elongated evaporite – mainly gypsum – diapir intruding the carbonate bedrock. Although dry gypsum does not display aseismic behaviour on its own, the interaction between evaporites and fluids could promote pressure-solution creep in wet gypsum. Pressure-solution creep in the fault gouge has been reported for other creeping faults, namely the San Andreas Fault, the North Anatolian Fault and the Longitudinal Valley Fault.

Structural evidence of ductile shear deformation within the evaporite-bearing rock along the KSF suggests that, at least in the shallow part, slip may occur with predominantly ductile creeping. This process is facilitated by the high solubility of evaporites and by the presence of fluids, and could explain both ductile deformation and abundant veining observed on the field. Furthermore, the fact that pressure-solution is slower between identical minerals, due to healed boundaries, but faster between different minerals, notably halite and calcite, would explain why the deformation is localised along the contact between evaporites and carbonates, possibly on both sides of the salt wall.