Short-term groundwater level variations in a seismic region as an indicator of the geodynamic regime

Abstract

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 observations in search of earthquake precursors have been carried out for many years. A great number of groundwater level variations, which reflect active geodynamic processes, were found in wells located in the Main Kopetdag Fault Zone. These variations are a response to relatively rapid movements in the fault zone. A number of specific features of the recent crustal movements were determined from the character of the groundwater level variations. These features are the preferential coincidence 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 level variations proceded or accompanied earthquakes, the rest of the variations were a response to aseismic movements. The characteristics of the precursory variations imply that before the earthquake, in the fracture zone, substantial short-term deformations occur at a rate 1–4 orders larger than the estimated values of deformations. The dependence of the amplitudes of short-term groundwater level variations and earth-tide level fluctuations on the position of observation wells with respect to the fault has been elucidated. These amplitudes and the number of recorded variations are larger near the fault, indicating intensified deformations and enhanced strain sensitivity of the well-reservoir system in the vicinity of major tectonic disturbances. The investigations conducted point to the high information content of special hydrogeological observations for monitoring recent geodynamic activity.