Tidal and secular tilt from an earthquake zone: Thresholds for detection of regional anomalies

Abstract

Tiltmeter data from an array of three boreholes at the Charlevoik observatory in the Charlevoix seismic region of Quebec have been analysed for evidence of tectonically related signals. The secular tilt is dominated by water table induced effects which can be substantially removed by linear regression of the water level on the tilt. Short-term (days to months) anomalies are shown to be detectable at the 0.3 mu rad to 1 mu rad level depending on the depth of the measurement. Long-term changes in the linear drift as small as 0.1 mu rad/yr would be detectable in all of the boreholes. Large spatial anomalies in the mean tidal admittance among boreholes preclude its use in refining either models of the regional crustal structure or the adjacent marine tide distribution. Strongly coherent time variations in the tidal admittance among the observations of all the major tidal constituents are shown to be generated by corresponding variations in marine tidal loading in the St. Lawrence Estuary.