Interest in broad groundwater data

Dunedin GNS scientist Dr Simon Cox points to twin graphics which highlight differences in the monitoring of groundwater changes - through hydrological stations - of the Chi Chi earthquake and fault movement in southwest Taiwan in 1999 (at left), and the monitoring system operating near the 30km-long Greendale Fault, west of Christchurch, during last September's 7.1 magnitude quake (at right). Photo by Peter McIntosh.

Further analysis of the data, which has been highlighted in research by Dunedin GNS scientist Dr Simon Cox, could also help improve future planning to reduce earthquake damage by shedding new light on how groundwater systems respond to big quakes, and on the cause of liquefaction problems.

The September 2010 Canterbury quake caused nearby liquefaction and groundwater changes throughout New Zealand, as far afield as Northland.

Dr Cox, a University of Otago graduate who has worked mainly as a geologist rather than a hydrologist, was recently awarded a New Zealand Hydrological Society best paper prize for his research paper, undertaken with several colleagues, on groundwater changes resulting from the Canterbury earthquake.

The paper focused on the response of subsurface groundwater to the 7.1 magnitude earthquake, whose epicentre was near Darfield.

Groundwater accounted for about half of New Zealand's overall abstractive water needs, and supplied about 80% of water used in agriculture.

After the Canterbury earthquakes, there had been a "desire to understand the source of excess water discharged", and whether aquifers - water-bearing layers of permeable rock, sand or gravel - and the security of the water supply were permanently affected, and whether there was any potential for groundwater pollution.

For liquefaction hazards to be understood and mitigated in Christchurch and other New Zealand cities, the causes and mechanisms needed to be clearly understood.

There had been no previous cases internationally where monitoring equipment had been positioned evenly all around a fault that had ruptured to produce an earthquake, and few examples where borehole monitoring was done so frequently as in the case of these recordings, taken every 15 minutes by Environment Canterbury.

Detailed groundwater changes had been recorded in Taiwan's 7.3 magnitude Chi Chi earthquake in 1999, but in that case quake-related fault movement had occurred to one side of of monitoring devices.

The pattern of groundwater behaviour beneath the Canterbury Plains was "unique in the quality of observations and symmetry of behaviour seen around Greendale Fault".

Data from the Canterbury earthquakes would be of particular interest to international scientists striving to understand the driving mechanisms of fluid movement during earthquakes, and whether changes in dynamic stress - operating like a "seismic pump" - or more static stress, like a broken pipe, were involved, he said.