Nontidal tilt and water table variations in a seismically active region in Quebec, Canada

Abstract

An array of three borehole tiltmeters was installed in 1981 in the Charlevoix region of Quebec, Canada, an area of high intraplate seismic activity. A magnitude 7.0 earthquake struck the area in 1925, and the last major event was a magnitude 5.0 event which occurred in 1979. The nontidal tilt from two 47 m deep boreholes correlates strongly with transient and seasonal water table fluctuations. This suggests a dominant influence of pore pressure effects on that part of the tilt signal. The episodic nature of such effects makes it difficult to separate them from potential earthquake precursors during the build-up of tectonic stress. We present both an empirical and a deterministic approach to tackle this problem. The former consists of removing pore pressure effects from the tilt signal by using simple empirical relations. The latter is an attempt to model the physical process through finite element calculations based on Biot's consolidation theory for elastic porous media. Although in our case some boundary conditions are poorly known, it is believed that Biot's theory is adequate to describe the observed phenomena.