In search of earthquake precursors in the water-level data of 16 closely clustered wells at Tono, Japan

Water‐level data recorded during 1989–1999 at 16 closely clustered wells near the Tono mine in central Japan are searched for possible premonitory changes before earthquakes. The results suggest that some water‐level drops at a sensitive well before an M5.8 local earthquake on 1997 March 16 and some large distant earthquakes during 1994–1995 in Japan may have been premonitory in nature. The high sensitivity of this well to earthquake occurrence is attributed to the near‐critical hydrological conditions and the relatively high permeability of the tapped aquifer.