Oscillatory phenomena in the geyser system at Onikobe, Japan

We conducted experimental observation at the Onikobe geyser, Japan, to understand hot-water effusion mechanisms, which are considered to be similar to the volcanic eruptions. In this study, we present simplicities and complexities in oscillatory phenomena found in geyser system by examining ground tilt motions and acoustic waves excited by effusion. The Onikobe geyser effuses hot water every about 10 minutes for about 1 to 1.5 minute during our observation in April 2004. Such cyclic signals are clearly recorded as saw-teethed time sequence in tilt motions. After an effusion, the tilt meters start to record a gradual increase of the pressure in chambers until the following effusion, which indicate a constant supply of hot-water to the geyser system. Carefully examining the data, we find that the gradual increase of the pressure tends to become weak about 1 min before the following effusion, and that about 10 percent of pressure loss is observed just before effusion. This suggests that a bulk density of the hot-water decreases before effusion and vesiculation process proceeds in chambers of the geyser system. During the pressure decrease, hot water escapes from a side of the vent, and the temperature of water increase from 80 to 100 C. And then, the effusion starts and the tilt motion start to show a de-pressurization of the chambers. Since the tilt vectors indicate a directional change about 20 s after the start of effusion, we infer two chambers beneath the vent. The geyser system sometimes becomes unstable. Time interval between nearby effusions changes from 10 min to 6 min and vice versa, and the two time intervals are irregularly repeated. In these unstable periods, as duration time of the effusion becomes shorter, interval time to the following effusion becomes longer. This implies that the Onikobe geyser system is a _gtime predictable system_h, that is, we can predict the time of next effusion by measuring the duration time of effusion. It should be noted that the tilt vectors does not change its direction when effusions with a short duration occur. However, significant differences are not recognized in tilt motions before each effusion, hence duration of effusion cannot be easily predicted although more accurate measurements may find out some difference in the data. Our acoustic sensor installed close to the vent records large signals associated with a start of effusion as well as short period signals of hot water falling to the ground. The observed acoustic signal is also characterized by a long-period oscillation with a period of a few to ten seconds. This long-period signals are often observed a few to tens of seconds after the beginning of effusion, but the waveforms are not always the same. We infer that the periodic oscillations in air-waves are caused by a Helmholtz resonance in the chamber system. Our observation revealed that chamber system beneath the vent significant role on oscillatory and cyclic phenomena in the geyser system, which may help us to understand the complexity found in active volcanoes.