Tidal and coseismic well-level observations at the Charlevoix Geophysical Observatory, Quebec

A systematic change in well tide amplitude and phase during the period 1981–1986 suggests a progressive attenuation of tidal strain in the azimuth range 135°–165° east of north. This may be related to an increasing concentration of tidal strain at one or more nearby NE-striking faults or to a change in local elastic properties due to the nonlinear response of the rock to increasing tectonic tectonic stress in the azimuth range. Only four of twenty local earthquakes experienced during the period caused an identifiable response in well evel. The response in at least one of these cases was much larger than that expected on the basis of dislocation theory and triggered aseismic slip on faults located much closer to the wells than the hypocentres is a possible cause. The size of unexplained residual well-level changes after correction for tides, barometric effects and estimated hydrological effects probably rules out the detection of precursory phenomena associated with the earthquakes ( M ≤ 5.0) experienced during the period.