Effects of Seismic Waves in Water Level Changes in a Well: Empirical Data and Models

Abstract

The high-precision water level measurements with a sampling 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 Мw = 6.8–9.1 which occurred at epicentral distances of 80–14.6 thousand km are revealed. Based on the HGSV morphological features, four main types of these variations are distinguished: oscillations (I); short (up to tens of hours) water level rises superimposed on oscillations (II); short rises (III); and long (1.5–3 months) drawdowns (IV). The dependence of the occurrence of the revealed GHSV types on earthquake parameters (magnitude and distance), specific energy density and maximum seismic wave velocity, and the amplitude-frequency content of ground motion is analyzed based on the records at a nearest seismic station. Based on several case studies, hydrogeodynamic processes of HGSV formation are investigated using numerical modeling. It is shown that the forced and free amplitude fluctuations in the water level (types I and II) can arise due to the enhancement of groundwater pressure variations in the well–water-bearing rock system during the passage of surface seismic waves with periods corresponding to the resonant frequency of the well (τ = 44.6 s). The rise in the water level in well lasting for tens minutes to hours (types II and III of HGSV variation) is caused by the short increase in pressure under violation of the steady water flow in the direct vicinity of the well; strong local earthquakes accompanied by ground shaking with intensity Imsk-64 ≥ 5 cause sustained drawdowns (type IV) due to pressure drop with the amplitudes up to 0.1 bar within a radius of up to a few hundred meters from the well.