Water-level oscillations caused by volumetric and deviatoric dynamic strains

Abstract

Travelling seismic waves and Earth tides are known to cause oscillations in well water levels due to the volumetric strain characteristics of the ground motion. Although the response of well water levels to S and Love waves has been reported, it has not yet been quantified. In this paper we describe and explain the behaviour of a closed artesian water well (Gomé 1) in response to teleseismic earthquakes. This well is located within a major fault zone and screened at a highly damaged (cracked) sandstone layer. We adopt the original Skempton approach where both volumetric and deviatoric stresses (and strains) affect pore pressure. Skempton's coefficients B and A couple the volumetric and deviatoric stresses respectively with pore pressure and BKu and N are the equivalent coupling terms to volumetric and deviatoric strains. The water level in this well responds dramatically to volumetric strain (P and Rayleigh waves) as well as to deviatoric strain (S and Love waves). This response is explained by the nonlinear elastic behaviour of the highly damaged rocks. The water level response to deviatoric strain depends on the damage in the rock; deviatoric strain loading on damaged rock results in high water level amplitudes, and no response in undamaged rock. We find high values of N = 8.5 GPa that corresponds to -0.5 < A < -0.25 expected at highly damaged rocks. We propose that the Gomé 1 well is located within fractured rocks, and therefore, dilatency is high, and the response of water pressure to deviatoric deformation is high. This analysis is supported by the agreement between the estimated compressibility of the aquifer, independently calculated from Earth tides, seismic response of the water pressure and other published data.