Recent hydraulic experiments at the KTB site have shown that seismicity induced by long-term fluid injection directly into a continental crustal fault remains guided by this fault. The seismicity is triggered by pressure perturbations as low as 0.01--1 bars at the hypocenters. A combination of sequential one-year fluid extraction (2002/2003) and one-year fluid injection (2004/2005) experiments has shown that only positive pore pressure perturbation (i.e., injections) was able to induce the seismicity. Moreover, the onset of seismicity roughly coincides with the time of compensation of the extracted fluid volume by the following injection. This confirms that the pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections. The probed fault shows a significant anisotropy and non-linearity of its hydraulic behaviour. Its hydraulic diffusivity is up to one order of magnitude larger than that of surrounding rocks.