Precursory and coseismic anomalies in well water levels observed for the February 2, 1992 Tokyo Bay Earthquake

Possible precursor water level changes of a moderate earthquake were detected by the long‐term groundwater observation sites operated over 5 years in Japan. Three observation wells showed anomalous changes: rises and a fall in water levels of 3–10 cm which began simultaneously 1–1.5 days prior to the February 2, 1992 Tokyo Bay earthquake (M5.9). The distance from the hypocenter to the wells were 90–110 km. These possible precursors were followed by coseimic water level changes of 1–20 cm. Some portions of rises may be attributed to effects of concurrent precipitation, however, the water level fall of 10 cm observed at one well indicates that the change is anomalous and is highly likely to be an earthquake precursor.