Borehole tilt measurements: Aperiodic crustal tilt in an aseismic area

Abstract

Nine months of tilt measurements at tidal sensitivity, with an Askania tiltmeter at a depth of 12m, show aperiodic tilts with time scales of two days and upwards. Statistically significant correlations are found with meteorological variables and, in particular, with variations in the local groundwater table. Typically a variation in the tilt of 1 microradian is associated with a variation in the groundwater table of about 1 m. The seasonal variation over the last two years in groundwater has a range of 3 m about a mean level approximately 2.5 m below the ground surface. These variations have been continuously measured by a Neyrpic tide gauge in a 50 m uncased borehole. In addition to this seasonal variation are episodic events, some in excess of 1m, associated with heavy rainfall (2–3 cm). These events have a rapid onset lasting 1 day followed by a decay lasting 10 days or more. Tilt events associated with similar groundwater variation could, in a seismic region, be easily mistaken for precursory tilt and this work emphasises the necessity of taking a complete account of meteorology in seismic zones where strain and tilt measurements are in progress. Further work is in progress with the Askania tiltmeter, which is now installed in an adjacent borehole at a depth of 30 m and a Hughes TM-3 bubble tiltmeter installed at 12m, with the aim of investigating mechanisms by which meteorology, including groundwater level, affects tilt. Most similar measurements of tilt e.g. in North America, New Zealand and Germany using various techniques at different depths in different geological conditions indicate meteorological perturbations of the tilt signal and it is suggested that continuous monitoring of water-table level will enable-these perturbations to be better understood.