Interaction between the hydrothermal system and the axial magma chamber at a mid-ocean ridge spreading centre takes place in a boundary layer of crust that separates circulating sea water from basaltic melt(1). The nature of heat now through this region is Critical because it determines the pressure-temperature conditions of the water-rock interaction and regulates the total heat flux through the system(2). Here we combine seismic, thermal and chemical time-series data from high-temperature vents on the East Pacific Rise axis at 9 degrees 50.2＇N to Link a microearthquake swarm with changes measured in vent fluids. Four days after the earthquake swarm opened fractures near the base of the circulation system, a sudden increase in fluid temperature in the overlying ＇Bio9＇ black-smoker vent was observed Temperatures peaked at the vent 11 days after the swarm and gradually declined back to just above pre-swarm levels (365 degrees C) over the next 70 days. These observations ape consistent with the Bio9 hydrothermal system tapping a previously isolated region of crust, and an upflow fluid residence time of 4 days, compared to previous lower-resolution estimates of 3 years or less(3).