Strong earthquakes, even with distant epicentres, can induce changes in the groundwater system as evidenced by changed discharge rates of groundwater springs or fluctuations in groundwater levels in boreholes (Nagra 2024i, Wang & Manga 2010). Such changes may be the result of stress changes and related dilation, or contraction of cracks or enhanced permeability produced by seismically induced fractures. The effect generally diminishes over the timescale of a few years. It is thus not expected that effects of single earthquakes would fundamentally and persistently change the flow system.

Accumulated slip along faults can also affect aquifer pathways. Fundamental persisting changes to the aquifer flow system due to fault slip on regional or local faults are not anticipated during the period under consideration as deformation rates are generally low (Section 6.2). Nevertheless, the impact of flow across and along faults on the deep aquifer system and on discharge paths and the effects on flow paths and discharge areas are discussed in Nagra (2024n).