Old Faithful Geyser, Yellowstone, was used as a natural laboratory for fluid-flow-induced seismic activity. Pressure measurements within the geyser's water column, obtained simultaneously with seismic measurements on the surface, demonstrated that the tremor observed at Old Faithful results from impulsive events in the geyser. Tremor intensity is controlled by the rate of occurrence of these impulsive events. There is no resonance observed within the water column. The impulsive events are modeled by a collapse of a spherical bubble, including the effects of residual non-condensible gas and damping. The pressure data can be explained by a collapse of a ~5 cm radius bubble driven by a pressure difference of ΔP=0.3¿105 Pa and effective viscosity &ngr;E=0.04 m2/s. Using a quasi-static geyser model, we treat the individual bubble collapses as cooling events that occur when the water column reaches a critical temperature. Their rate of occurrence is controlled by the heating rate dT/dt of the water column. As a result, the intensity of the hydrothermal and seismic activities is determined by the heat and mass input rate into the geyser. It is demonstrated that a sharp widening of the conduit can cause the number of events per unit time to drop (as observed) while the water level is still rising and heat is being input, and thus the tremor intensity can be modulated by variations in the conduit shape. ¿ 1998 American Geophysical Union