SHORT-TERM GROUNDWATER LEVEL VARIATIONS IN A SEISMIC REGION AS AN INDICATOR OF THE GEODYNAMIC REGIME

An analysis of short-term (up to 5 days) variations in the groundwater level has been made for a group of wells in Turkmenistan, where obser vations in search of earthquake precursors have been carried out for m any years. A great number of groundwater level variations, which refle ct active geodynamic processes, were found in wells located in the Mai n Kopetdag Fault Zone. These variations are a response to relatively r apid movements in the fault zone. A number of specific features of the recent crustal movements were determined from the character of the gr oundwater level variations. These features are the preferential coinci dence of the directions of rapid and slow movements, variation in the deformation rates in different portions of the fault, and a change in the movement rate with time, About 56% of the short-term groundwater l evel variations preceded or accompanied earthquakes, the rest of the v ariations were a response to aseismic movements. The characteristics o f the precursory variations imply that before the earthquake, in the f racture zone, substantial short-term deformations occur at a rate 1-4 orders larger than the estimated values of deformations. The dependenc e of the amplitudes of short-term groundwater level variations and ear th-tide level fluctuations on the position of observation wells with r espect to the fault has been elucidated These amplitudes and the numbe r of recorded variations are larger near the fault, indicating intensi fied deformations and enhanced strain sensitivity of the well-reservoi r system in the vicinity of major tectonic disturbances. The investiga tions conducted point to the high information content of special hydro geological observations for monitoring recent geodynamic activity.