The Response of the Earth to Loading by the Ocean Tides around Nova Scotia

Abstract

Summary The tidal loading effects on gravity and tilt caused by the large amplitude tides in the Bay of Fundy have been separated from the effects of the tides in the larger scale Gulf of St Lawrence and the North Atlantic Ocean. An eight-month series of north-south, east-west tilt observations was made near Rawdon, Nova Scotia, with a pair of Verbaandert-Melchior horizontal pendulums at a site 30 metres below the surface of the ground. The gravity observations were made with a LaCoste-Romberg earth-tide gravity meter and consisted of two short series of observations at the coastal sites of Isle Haute and Victoria Harbour followed by a 48-day series at Berwick, Nova Scotia. The tidal constituents of the observed tilt have been corrected for the theoretical body tilt by assuming a range of values for the global diminishing factor from 0.5 to 0.9. The resulting load tilts at the semi-diurnal frequencies (M2, N2, S2,) have been separated into Bay of Fundy and North Atlantic load tilt components by a least squares method utilizing the known amplitude and phase relationships of the M2, N2, and S2 ocean tide constituents around Nova Scotia. A similar separation of the Bay of Fundy and Gulf of St Lawrence load tilts has been made at the diurnal frequency (O1). The isolated Bay of Fundy tilt has been compared with the measurements of Nishimura and the theoretical studies of Takeuchi and Jobert. The results suggest that a simple two-layer elastic model having rigidities compatible with seismic models for the area would fit the observations within the experimental uncertainties. On the other hand, the tilts due to the tides in the Gulf of St Lawrence and in the North Atlantic Ocean are not compatible with any reasonable laterally homogeneous, elastic models. Notably, the diurnal tilt contribution from the Gulf of St Lawrence is anomalously large at Rawdon, Nova Scotia. The results of the gravity observations on the whole, confirm the tilt results. If there is a deviation of the gravimetric factor from a theoretical value of 1–16 at Berwick, Nova Scotia, it appears to be secondary to the ocean tide effect. The semi-diurnal gravity residuals show that a strong influence from the North Atlantic tides is superimposed on the Bay of Fundy effect, whereas the diurnal residuals are correlated with the diurnal tides in the Gulf of St Lawrence alone. The diurnal gravity results are consistent with the anomalous diurnal tilt and suggest that a large-scale continental structure may be responding coherently to the tidal loading in the Gulf of St Lawrence.