Characterisation of the Hydrogeological Properties of the Ntane Sandstone Aquifer Using Co-Seismic and Post-Seismic Groundwater Level Responses to the Mw 6.5 Moiyabana Earthquake, Central Botswana

Abstract

The 3 April 2017 Mw 6.5 Moiyabana earthquake (Central Botswana) had a significant impact on groundwater levels; a gradual co-seismic increase and a stepwise decline in groundwater levels were observed in response to the earthquake at boreholes MH2 and Z12836, respectively. In this study, we investigated the response of groundwater levels to Earth tides by computing the amplitude and phase shift of the M2 tidal constituent to estimate the temporal variations of the storativity, transmissivity, and permeability of the Ntane sandstone aquifer (the main aquifer system) prior to and after the earthquake event. The storativity and permeability computed for borehole MH2 showed a decrease in magnitude of 3.17432 × 10−4 and 1.85 × 10−13 m2 respectively, indicating that strong ground shaking at borehole MH2 might have consolidated the aquifer material, thus resulting in decreased aquifer permeability. The aquifer coefficient of storativity decreased by 2.85 × 10−4 while permeability was enhanced by 0.047 × 10−13 m2 at borehole Z12836. Enhanced permeability might have resulted from increased/enhanced fracturing of the aquifer, fracture clearing and dynamic shaking.