A laboratory method is devised to study acoustic emission induced by critically stressed planes and to understand field micro-seismic activity upon minor pore pressure increase, much below the hydraulic fracturing injection pressure. The experiment relies on generating a shear failure plane, re-loading the sample to the near-failure axial stress that closes/narrows the fracture zone, and finally reopening the fracture with a pore pressure increase. The entire process is controlled based on the acoustic emission response, which determines the reopening early enough to prevent the sliding. A change of the emission event energy distribution exponent is observed during fracture generation/activation and fracture sliding, combined with an increase of acoustic emission activity upon the dislocation. The results indicate that even small pore pressure increases, up to 2 MPa and in extreme cases as low as 0.4 MPa, are sufficient to activate a pre-existing fracture and finally to fail the outcrop and field sandstone sample. This lab observation supports the field findings of micro-seismicity even for low pore pressure increments due to fluid injection below the frac gradient. The field response may arise from geologically created critically stressed planes, which are not sliding at the current stresses, but are suddenly activated with minimal pressure increase.