Effects of Seismic Waves in Water Level Variations in a Well: Empirical Data and Models
The high-precision water level measurements with a time interval of 5–10 min were carried out in 1996–2017 in the YuZ-5 well, Kamchatka. In the obtained time series, water level variations caused by the passage of seismic waves (hydrogeoseismic variations—HGSV) during 19 earthquakes with Mw = 6.8–9.1 that occurred at the epicentral distances of 80–14.6 thousand km are identified. Based on the HGSV morphological features, four main types of these variations are distinguished: oscillations (I); short (up to tens of hours) rises (II) in the water level superimposed on the fluctuations; short rises (III); and long (1.5–3 months) falls in the water level ( IV). The dependence of the occurrence of the revealed GHSV types on the earthquakes’ parameters (magnitude and distance), on the specific energy density and maximum velocities of seismic waves, and on the amplitude-frequency content of ground motion according to the records at the nearest seismic station is considered. Hydrogeodynamic processes of HGSV formation are analyzed by several examples using numerical modeling. It is shown that the forced and free fluctuations amplitude in the water level amplitude (types I and II) can arise due to the intensification of the groundwater pressure variations in the well-water-bearing rock system during the passage of surface seismic waves with the periods corresponding to the resonant frequency of the well (τ = 44.6 s). The rise in the water level in well lasting for tens of minutes to hours (HGSV variation types II and III) is due to the short increase in pressure when the steady state of water flow in the direct vicinity of the well is violated; the strong local earthquakes accompanied by ground shaking with intensity Imsk-64 ≥ 5 cause long falls in the water level (type IV) due to pressure drop with amplitudes up to 0.1 bar within a radius up to several hundred meters from the well.