Hydrogeologic effects of the Earthquake
The Darfield earthquake of 4th September 2010 resulted in widespread hydrogeologic effects.
Liquefaction and sand volcanoes are now well-known phenomena which have been widely reported in the media. Close to the fault and earthquake epicenter, the spring-fed Hororata River burst its banks and developed a new course across farmland. At least five wells in the Hororata area became artesian shortly after the earthquake, flowing to the surface at places where water levels are normally at least 30m deep. Increased river levels were recorded in the Selwyn, Heathcote, Avon, and Styx Rivers for about one day following the earthquake. Groundwater as far away as Marlborough, Tasman and Hawkes Bay also recorded small changes in level, while hot springs at Copland River and Hanmer cooled slightly.
Changes in the amount, direction, and rate of water flow in streams and in fluid pressure in the subsurface have been documented following large earthquakes for thousands of years. Observations of such phenomena were once considered as curiosities but are now being more closely examined for the unique insight they may provide on hydrogeologic processes during an earthquake cycle. Excess flows of water are caused by either an increase in hydraulic conductivity (permeability) or an increased head gradient (water released from storage). Hypotheses proposed to explain observations include:
1. breaching barriers or seals, or ruptured subsurface reservoirs.
2. release by increased permeability from fracturing.
3. expulsion of water by strain and compression.
4. release by compaction, consolidation and liquefaction of sediment.
5. uplift and subsidence of the water table through fault offset.
The extent to which the effects will be transient or permanent depends upon which of the above mechanisms is responsible for the change. While seasonal groundwater levels in Canterbury are currently high, there is potential for issues to arise when irrigation is needed later in the summer months.
GNS Science and other research partners are now working to capture as much relevant data on earthquake-related behaviour of wells, rivers and springs. If you are willing to share information on the response of pumps and wells, such as which wells went artesian or have unusual levels, it would be greatly appreciated. It is particularly useful where observations can be precisely located, either by farm address, rapid number, well number or GPS coordinate. We are also interested to hear of sites where there have been no obvious problems or changes. A survey form can be downloaded from here