LONG-TERM HYDROGEOLOGICAL EFFECTS ON THE CANTERBURY PLAINS
AQUIFER SYSTEM FROM THE DARFIELD 2010 EARTHQUAKE
Rutter, H K1 and Cox, S C2
1
Aqualinc Research
2
GNS Science
Aims
A number of wells across mid-Canterbury have shown a sustained rise in water levels since
the Mw7.1 Darfield earthquake on 4 September 2010, some exceeding previous record high
levels. This paper examines long-term (>3 year) changes that might have occurred in the
aquifer system.
Method
To provide an overview of these changes, we derived mean piezometric levels from all
available groundwater level data over the period three years before (pre-) versus the three
years after (post-) this earthquake, then carried out statistical assessment of these data. Any
wells with less than 10 months of post-earthquake data were removed from the assessment.
The spatial pattern described above included all wells, at depths from a few metres to over
300 m. T tests were carried out to examine the significance of the differences in these data.
Results
When changes were examined with respect to well depth, within 10 km of the fault line, we
found only 44% (12/27) of the shallow (<40 m depth) wells had a statistically significant
difference in groundwater levels before versus after the earthquake, with the mean rise being
1.0 m. Deeper wells (>40 m depth) had a much greater proportion that showed significant
changes (76% or 26/34), with an average rise in water level of 7.8 m. Figure 1 shows the
distribution of sustained changes for > 40 m deep wells, overlain on a coloured grid of this
change (using a natural neighbours interpolation in ArcGIS). The figure illustrates the northeast to south-west orientation of sustained high groundwater levels along the upper, northwestern edge of the Canterbury plains, from approximately 20 km north east of the Greendale
Fault to 30 km south west. The style of the response varied between wells, with the increase
in mean piezometric level for the most part caused by either: (i) an increase in the rate of
groundwater level recovery after the September earthquake in 2010, (ii) a lack of a recession
following the event, (iii) an offset or (iv) some combination of these.
As the changes in ground elevation were between one- and two- orders of magnitude smaller
that the changes in mean groundwater level, and were centred about the western end of the
newly ruptured Greendale Fault, the fault offset alone cannot account for the observed spatial
pattern in groundwater change.
The reasons for the increase in water level are being explored in a separate study, through
examination of pre- and post-earthquake aquifer testing and comparison with static and
dynamic shaking. Increases appear to be at least in part, due to a decrease in transmissivity,
possibly as a result of re-sorting of sediment, with fine-grained sediment being moved into the
open framework gravels (OFGs) and consequent and clogging, re-sorting of the gravel clasts
within the OFGs, or truncation of these higher permeability flow paths. The analysis suggests
that aquifer deformation and dynamic shaking only account for part of the response. The scale
of the response may also be in part a reflection of the position in the groundwater catchment,
with greater responses occurring at the head of the Canterbury plains.
Figure 1 Distribution of sustained changes for > 40 m deep wells