Potential Changes to Aquifer Properties caused by a Near-Field Mw7.1 Earthquake, Canterbury, New Zealand

Abstract:

The M_W 7.1 Darfield (Canterbury) earthquake on 4 Sept 2010 in New Zealand generated widespread hydrological effects ranging from instantaneous changes of groundwater levels in wells, to more sustained (days to weeks) post-seismic changes in spring flow, river discharge and groundwater levels, as well as increased turbidity of water abstracted from wells and decreased well yields. Three years later, piezometric levels in deep (>70 m) aquifers were still elevated around the newly ruptured Greendale Fault. Eigen modelling suggests sustained high water levels are a real artefact, not due to changes in abstraction or land surface recharge. Investigation of the phenomenon include examining approaches to assess the types of responses as well as definition of a sustained rise/elevated levels. Step drawdown testing was carried out to determine whether the effects are related to a change in aquifer properties (transmissivity and storativity) or effects adjacent to the well. Other approaches, including recession analyses, have been explored to look for evidence of changes in aquifer properties. Step drawdown testing highlights potential changes in the various components of head losses both before and after the earthquake, and possible reasons for changes between tests. Sustained high groundwater and reduced well performance in some of the wells appear to be due to both a change in aquifer properties, and a decrease in well efficiency. There are different mechanisms occurring at different locations, with some reduction in permeability, and possibly storativity, at all locations tested. This paper investigates how earthquake-generated disturbances affected the hydrologic functioning of the Canterbury Plains aquifer system, using field measurements, data analysis, and modelling.