Hydrological effects of the ...

Hydrological effects of the MW7.1 Darfield (Canterbury) earthquake, 4 September 2010, New Zealand1

SC Coxa*, HJ Rutterb, A Simsc, M Mangad, JJ Weirb, T Ezzye, PA Whitef, TW Hortong and D Scotte

aGNS Science, Dunedin, New Zealand; bAqualinc Research Limited, Christchurch, New Zealand; cDepartment of Geography, University of Otago, Dunedin, New Zealand; dDepartment of Earth and Planetary Science, University of California Berkeley, Berkeley, USA; eEnvironment 5 Canterbury, Christchurch, New Zealand; fGNS Science, Taupo, New Zealand; gGeological Sciences, University of Canterbury, Christchurch,

New Zealand

(Received 7 December 2011; final version received 25 March 2012)

10 The MW7.1 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 15 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) 34 (weak to largely observed) and peak ground acceleration was much B0.01 g. Release of artesian groundwater pressure and groundwater flow are postulated to have played pivotal roles in Christchurch liquefaction.

20 Keywords: earthquake; groundwater; piezometric level; artesian; aquifer; liquefaction