The Fluctuation in Groundwater Level prior and after the Miyagi Oki Earthquakes.

Preseismic and coseismic changes in groundwater level at the earthquake on February 20 and June 12, 1978, both of which occurred off the coast of Miyagi prefecture, are studied. The study was conducted by the data recorded at wells installed for the protection of groundwater resources in Miyagi prefecture. The record of groundwater level change is so complicated due to superimposing of various kinds of noises, that processed trend lines instead are mainly analyzed. The trend of groundwater level for 30 days, 10 days and 0.5-1 days preceding each earthquake showed normal pattern, without any signs of preseismic effects. The coseismic change in groundwater level is detected at all wells. The direction of the change did not coincide with stress distribution which is estimated from initial shock of seismic wave. The amplitude of level changes decreased with an increase of depth of strainers. This phenomenon is regarded as a result of sediment compaction by