Aquifer permeability change caused by a near‐field earthquake, Canterbury, New Zealand

The MW 7.1 Darfield (Canterbury) earthquake, 4 September 2010, generated widespread hydrological effects in New Zealand ranging from instantaneous changes of piezometric levels, to more sustained postseismic changes in spring flow, river discharge and groundwater levels, and increased turbidity and declined yields of water ed from wells. Four years later, piezometric levels remained elevated in deeper (>40 m) aquifers along the north‐western (upper) side of the Canterbury Plains near the Greendale Fault, with changes in mean piezometric level reaching +13 m. Linear reservoir modeling (eigen modeling) suggests that sustained high groundwater was not the result of changes in ion or land surface recharge. Step‐drawdown tests at six wells within 15 km of Greendale Fault were carried out prior to the earthquake and were retested following fault rupture. Eden‐Hazel analysis of discharge/drawdown relationships discriminates potential sources of head losses, and how these changed (or otherwise) as a result of the earthquake. Objective application of Eden‐Hazel analysis provided confidence levels for the interpretation, including when step tests provide reliable/unreliable estimates of transmissivity change. Increases in both aquifer losses and well losses were observed in four wells, reflecting both a change in sediment transmissivity and decrease in well efficiency. At two locations, the data were unable to provide results that can be interpreted with confidence. As the majority of local groundwater flow occurs through high‐permeability open framework gravel lenses, we suggest that reduction in the permeability of these gravels, due to fine‐sediment incursion, is the cause of the reduction in transmissivity and increase in well losses. Key Points: Piezometric levels showed sustained changes as a result of a Mw 7.1 earthquake Step testing confirmed a reduction in transmissivity in the region of sustained groundwater rise It is proposed that pore space in high‐permeability open framework gravels is reduced by fine‐sediment incursion