We analyzed micro seismic data observed during a recent hydraulic experiment at the KTB site, Southeastern Germany. Following a one-year fluid extraction phase in 2002-2003, fluid was injected directly into a continental crustal fault at 4 km depth over a period of one year in 2004-2005. Seismicity was monitored by a borehole sensor and a near-surface seismic network. Precise location of 150 micro earthquakes indicates that the seismicity remains guided by this fault. The seismicity is triggered by small positive pressure perturbations (<1 bar). Moreover, the onset of seismicity approximately coincides with the time of compensation of the extracted fluid volume by the following injection. This confirms that pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections. We estimated the hydraulic diffusivity using the spatio-temporal characteristics of the induced events. The hydraulic diffusivity of the probed fault is about one order of magnitude larger than that of the surrounding rocks. The fault shows a significant anisotropy and non-linearity of its hydraulic behaviour.