Figure 3: (a) Records of electric field anomalies on 11 October, 1990 related to the small volcanic activities in the Izu-Oshima Island by the borehole antenna [25, 28]. Slow speed recording shows numerous pulse-like signals (left part of (a) before 6:42 JST). Rapid recording by 60 times (right part of (a) after 6:42 JST) shows the particular waveform similar to the geyser height evolution. (b) A model of confined pressure change is proposed to explain the electric field changes based on the electrokinetic effect.