Relationships between seismic activity and piezometric level changes in the Arax basin (SW Armenia): Attempt at a typology of seismically induced piezometric anomalies

We analyze the relationships between seismicity in the Arax basin, and the water level changes of two deep confined fresh water reservoirs, monitored by two wells with hourly records of piezometric levels. The reservoirs are alimented by the northern faulted border of the basin and discharge on the southern border. In the vicinity of the recording wells, the water flow may be considered as stationary on a time scale of months. Tectonic and neotectonic studies indicate that the basin is located in an area where the state of stress varies, with the minimum stress (sigma(3)) Oriented horizontally and E-W to NE-SW and the maximum stress (sigma(1)) varying from vertical to horizontal. Analysis and filtering of hydrographs yields cleaned curves where seismically induced piezometric level changes can be identified. The most relevant observations are the following: Each well records events occurring only in the tectonic block where it is located, independently of earthquake magnitude. Ten types of seismically induced piezometric anomalies are identified, each of them resulting from a peculiar reaction of the reservoir to a local change of the strain state induced by the earthquake. The duration of the deformation may either be short (about 1 week) or long (about 2 months). Level changes of the aquifer often postdate earthquakes and are not precursors. We attempt to correlate the state of stress and the anomalous behavior of the piezometric levels in the two wells. In the northern reservoir, sigma(3) is perpendicular to the flow lines, the hydraulic gradient is lower than in the south, the rocks are fractured and the permeability is mainly of a fracture type. Seismic energy and water level changes are well correlated. In the southern reservoir, sigma(3) and the flow lines are parallel and the water-bearing rocks consist in a continuous medium where the permeability is mainly of a pore type. There is a poor correlation between seismic energy and water level changes.