Carbon capture and storage in geological reservoirs is essential to mitigate carbon dioxide emissions, but is only effective if long-term storage is ensured and no leakage occurs, for example through unknown faults in the caprock that seals the reservoir. To test how the integrity of a caprock can be monitored non-invasively, we conducted an injection experiment directly into a faulted caprock analog at the Mont Terri Rock Laboratory. We monitored the experiment using active seismic techniques, along with strain, fluid pressure, and microseismicity measurements. Pulse injection with pressures of up to 4.8 MPa caused a perturbation of effective stress, which we observed through a reduction of seismic P-wave velocities by around 1 %. This perturbation was not sensed by means of microseismicity, even though a sudden pressure drop indicated that the injection caused the opening of some fractures within the fault zone. Substantial rock deformation was also only detected at the injection borehole but not in surrounding monitoring boreholes. Thus, with seismic velocity monitoring, processes can be detected within a faulted caprock for which other methods may be blind. This demonstrates the value of seismic velocity monitoring as a tool to complement a monitoring system.