The spatio-temporal correlation of micro-earthquakes occuring in a mining-induced seismic system (Creighton mine, Ontario, Canada) is investigated. It is shown that, when considering only the after-events correlated to a main event, i.e., not accounting for the uncorrelated regime of 'background' activity, the spatial distribution of these after-events occurring at t after the main event change with t. This change takes the form of an expanding pattern, characterized by a typical scale Lc(t) varying as Lc(t)~tH, H being estimated to 0.18. This diffusion exponent is found to increase when considering only a subset of the most energetic events as mainshocks. We interpret this result as the indication of a stress (sub-)diffusion mechanism, involving propagation on the heterogeneous fractal fault network. ¿ 1999 American Geophysical Union