Borehole Tilt Measurements from Charlevoix, Quebec.
Scientific rept. no. 1,
DALHOUSIE UNIV HALIFAX (NOVA SCOTIA) DEPT OF OCEANOGRAPHY
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An array of three borehole tiltmeters near Quebec 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 first study 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 explore the main features of the secular tilt signal. The tidal analysis was done using a modified version of the HYCON harmonic analysis program of Schuller 1977, 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 variations observed for the major constituents M, N2, S2K2, O1, P1S1K1 show remarkable agreement between boreholes, suggesting that they are regional in origin. Differences of up to 20 in amplitude and 5 degrees in phase were found between the mean M2 results determined from boreholes 1 and 2, 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 non-tidal tilt. A preliminary estimate of the detectability of long term regional trends in tilt is 0.4 microradyr. Originator-supplied keywords Geophysics, Earth tides, Tiltmeters.
- Geology, Geochemistry and Mineralogy