Nano variations in gravity due to seasonal groundwater movements: Implications for the gravitational detection of tectonic movements

Six nanogravity surveys have been made at two coastal areas of eastern Canada in order to study the gravity changes associated with seasonal groundwater fluctuations. The repeated surveys made with a LaCoste and Romberg model D gravimeter, D6, demonstrate that (1) seasonal changes in gravity differences (∼100 nm/s2, 10 µgal) at Cap Pelé, a region of relatively simple hydrogeology, are in good agreement with well and piezometer measurements, and (2) the corresponding changes (∼120 nm/s2, 12 µgal) at York Point; a region of complex hydrogeology, suggest a different interpretation of seasonal groundwater flow from that inferred from piezometer measurements alone. The results demonstrate that normal variations in groundwater may obscure gravity changes associated with tectonic movements and earthquake precursory effects. When normal variations in groundwater are known or are not significant, the results demonstrate that tectonic gravity changes as small as 20–30 nm/s2 (2–3 µgal) may be detected at the 90% confidence level with the LaCoste and Romberg D meter. Such a gravity change corresponds to a ‘free air’ change in elevation of ∼1 cm.