Seismicity at Old Faithful Geyser: an Isolated Source of Geothermal Noise and Possible Analogue of Volcanic Seismicity

Kieffer, S.W., 1984. Seismicity at Old Faithful Geyser: an isolated source of geothermal noise and possible analogue of volcanic seismicity.A., is a relatively isolated source of seismic noise and exhibits seismic behavior similar to that observed at many volcanoes, including "bubblequakes" that resemble B-type "earthquakes", harmonic tremor before and during eruptions, and periods of seismic quiet prior to eruptions. Although Old Faithful differs from volcanoes in that the conduit is continuously open, that rock-fracturing is not a process responsible for seismicity, and that the erupting fluid is inviscid H20 rather than viscous magma, there are also remarkable similarities in the problems of heat and mass recharge to the system, in the eruption dynamics, and in the seis-micity. Water rises irregularly into the immediate reservoir of Old Faithful as recharge occurs, a fact that suggests that there are two enlarged storage regions: one between 18 and 22 m (the base of the immediate reservoir) and one between about 10 and 12 m depth. Transport of heat from hot water or steam entering at the base of the recharging water column into cooler overlying water occurs by migration of steam bubbles upward and their collapse in the cooler water, and by episodes of convective overturn. An eruption occurs when the temperature of the near-surface water exceeds the boiling point if the entire water column is sufficiently close to the boiling curve that the propagation of pressure-release waves (rarefactions) down the column can bring the liquid water onto the boiling curve. The process of conversion of the liquid water in the conduit at the onset of an eruption into a two-phase liquid-vapor mixture takes on the order of 30 s. The seis-micity is directly related to the sequence of filling and heating during the recharge cycle, and to the fluid mechanics of the eruption. Short (0.2-0.3 s), monochromatic, high-frequency events (20-60 Hz) resembling unsustained harmonic tremor and, in some instances, B-type volcanic earthquakes, occur when exploding or imploding bubbles of steam cause transient vibrations of the fluid column. The frequency of the events is determined by the length of the water column and the speed of sound of the fluid in the conduit when these events occur; damping is controlled by the characteristic and hydraulic impedances, which depend on the above parameters, as well as on the recharge rate of the fluid. Two periods of reduced seismicity (of a few tens of seconds …