Following a M L 5.1 earthquake in eastern France, a post-seismic 12-day long electrical conductivity increase was recorded in the water discharging from a karst aquifer located 3 km from the epicentre. We attribute this to a permeability enhancement which allowed long-residence time water from low-permeability fractures located in the saturated zone to be expelled. The permeability enhancement shows that shallow aquifers can be significantly deformed for several days by moderate magnitude earthquakes.