Interactions between pore pressure, strain and seismicity were analyzed in a 30-m-thick shallow fault zone that was subjected to a small overpressure of 80 kPa to produce small changes in effective stress. Simultaneous pressure-strain and seismic measurements taken in the fractured damage zone during the pressurization indicated that seismicity is triggered along low-permeable, highly rigid, low-dip angle, mesoscale-inherited fractures where deformation is controlled by the hydromechanical behavior of the surrounding high-permeability, not-so-rigid, aseismic, sub-vertical, fault-related fractures. Using a three-dimensional distinct-element representation of the fractures network, we show that the seismicity observed is mainly due to shear-slip along inherited planes in response to the mechanical loading induced by the opening by a few microns of the surrounding subfaults.