Abstract

Permeability is commonly assumed to be a geometrically controlled constant that is fixed over human timescales. However, continuous measurements of permeability from tidal responses reveal that the apparent permeability can change significantly over time. Seismic waves from regional earthquakes can increase the permeability by factors of up to 3-4 and the permeability can recover slowly over a period of years. Permeability change is correlated to peak ground motion and we can recreate this fundamental observation with laboratory experiments. We have demonstrated through image analysis and variations in adhesive behavior with the ionic strength of the fluid that the mechanism of these experimental permeability enhancements is the clearing of clogged fractures. The fluid velocity during the oscillations is the best predictor of the resultant permeability enhancement. Transient permeability effects are particularly important in fault zones where the hydrogeology is coupled to the strength of the fault. For instance, we have directly measured the permeability recovery within a major fault following the 2008 M7.9 Wenchuan earthquake and showed that the evolution of the system continues for years after the mainshock.