The water level in an observation well (EDY) has been monitored with high frequency sampling (1/sec). Large changes in the water level are observed in this well, which correspond to dynamic strain changes due to the passage of long period seismic waves. For example, fluctuations of more than 200 cm were observed after a M 8.1 earthquake, 1200 km from the observation site. The response characteristic of the water level to strain change was investigated by comparison with nearby volumetric strain changes: the response curve has a peak of sensitivity and a phase transition at periods of 18~19 seconds. At the resonant period, the sensitivity is as large as 15 mm per 10-9 strain.¿ 1997 American Geophysical Union