The offshore Castor Underground Gas Storage (UGS) project had to be halted after gas injectiontriggered three M4 earthquakes, each larger than any ever induced by UGS. The mechanisms thatinduced seismicity in the crystalline basement at 5-10 km depth after gas injection at 1.7 kmdepth remain unknown. Here we propose a combination of mechanisms to explain the observedseismicity. First, the critically stressed Amposta fault, bounding the storage formation, crept bythe superposition of well-known overpressure effects and buoyancy of the relatively lightinjected gas. This aseismic slip brought an unmapped critically stressed fault in the hydraulicallydisconnected crystalline basement to failure. We attribute the delay between induced earthquakesto the pressure drop associated to expansion of areas where earthquakes slips cause furtherinstabilities. Earthquakes occur only after these pressure drops have dissipated. Understandingtriggering mechanisms is key to forecast induced seismicity and successfully design deepunderground operations.