An experiment on the uses of shear-wave splitting as an imaging tool in fracture-controlled geothermal reservoirs was conducted at Krafla, Iceland. Fifteen days after the beginning of the seismic recording the injection was stopped for eleven days and then restarted, a sequence designed to determine whether shear-wave splitting measurements can detect the transient response of the subsurface crack system to changes in fluid pressure. It was observed that time delays between the fast and slow split shear waves changed significantly and promptly with the stoppage and resumption of injection. Large time delays occurred only during injection, decreased substantially during the stoppage phase, and increased again as injection restarted. Comparisons of these results with similar observations at the Coso geothermal field in California strongly suggest that the time delay of split shear waves can be a useful proxy to monitor fluid pressure in the cracks and changes in crack density.