1986 年 39 巻 4 号 p. 635-643
Variations of the level of ground water were observed at four stations near the epicenter of the shallow earthquake (M4.6) which occurred in the southern part of the Lake Biwa which has been recently a low seismicity region.
About ten days before the occurrence of the earthquake anomalous changes of the water level were observed. Considering the relation between the variation of water level and rainfall in the ordinary period these anomalous changes were precursors of the earthquake.
Co-seismic steps of the water level were observed at two stations. One of them was a sudden ascent which was observed at the region where initial P motions were compressional. At another station which was in the dilatational P region sudden descents were recorded.
Such characteristic variations of the ground water level are detectable by using deep wells when an earthquake occur conspicuously in the low seismicity region, even if its magnitude is small.