Borehole tilt measurements from Charlevoix, Québec

Abstract

An array of three borehole tiltmeters near Québec City in eastern Canada is designed to study the tidal and secular response of the crust in the Charlevoix seismic zone. The objectives of this study of the first year of data from two boreholes of the array are to investigate the spatial coherency of the tidal observations and determine whether there are time variations in the tidal amplitudes and phases and to describe the main features of the secular tilt signal. The tidal analysis was done using a modified version of the HYCON harmonic analysis program with which the time‐varying tidal amplitudes and phases were determined by the sequential analysis of overlapping 2‐monthly subsets of the data. The admittance observed for the major semidiurnal (M2) and diurnal (O1) constituents varies by up to 10 and 30%, respectively, and is strongly correlated between boreholes. Comparison with admittance variations determined from two nearby tide gauges indicates a strong correlation in the amplitude fluctuations, pointing to a predominantly marine loading source for the time‐varying tilt admittance. Differences of up to 20% in amplitude and 5° in phase were found between the mean M2 results determined from boreholes 1 and 2, located only 80 m apart, indicating small‐scale distortion of the local tilt field by lateral inhomogeneities. The secular tilt from both boreholes correlates strongly with transient and seasonal water table fluctuations, suggesting the dominant influence of pore pressure effects on the nontidal tilt. A preliminary estimate of the detectability of long‐term regional trends in tilt is 0.4 μrad/yr.