Hydrological effects of the MW 7.1 Darfield (Canterbury) earthquake, 4 September 2010, New Zealand

The M(W)7.1 Darfield (Canterbury) earthquake on 4 September 2010 generated widespread hydrological effects ranging from near-instantaneous coseismic liquefaction and changes of groundwater levels in boreholes, to more sustained (days to weeks) post-seismic changes in spring flow, river discharge and groundwater piezometric levels, to longer term shifts in groundwater level one year after the earthquake. Groundwater piezometric responses include local groundwater level increases of >20 m around the Greendale Fault, particularly in deep aquifers (>80 m), whereas decreases occurred in coastal confined aquifers beneath Christchurch city. Increases of up to 5 m persisted within 20 km of the fault 12 h after the earthquake. Groundwater levels and springs were affected throughout New Zealand, from >350 km away in Southland to nearly 1000 km away in Northland, even where shaking intensities were less than Modified Mercalli Intensity (MM) 3-4 (weak to largely observed) and peak ground acceleration was much <0.01 g. Release of artesian groundwater pressure and groundwater flow are postulated to have played pivotal roles in Christchurch liquefaction.