Abstract: Through the study of several earthquake cases, it can be held that the same decreases of groundwater levels in different wells, though not many, were recorded before several strong earthquakes. These decreases indicate that the groundwater anomaly before earthquakes has the character of recurrence especially in the period from short-term to imminent earthquake occurrence. The time of earthquake occurrence, usually about 20 days or shorter after these anomaly become break, is relatively steady, so it can be regarded as the important time index for prediction of earthquake occurrence. Therefore, it appears extremely important to study the basic characteristics and the usage of the anomaly in earthquake analysis and prediction.

Key words: Strong earthquake, Groundwater, Groundwater anomaly, Period from short-term to imminent earthquake occurrence