Earthquake-induced hydrologic changes in the geoengineered schist landslides of Cromwell Gorge, Central Otago
Geoengineered groundwater systems located within seven large (> 100 ha surface area), deep-seated, slow-creep schist landslides in Cromwell Gorge (Otago, New Zealand) are observed to respond systematically to 10 large (>Mw6.2), regional earthquakes at epicentral distances of 130-630 km. The permeabilities of the schist landslides have previously been reported to be c. 1 x 10⁻¹⁷ - 4 x 10⁻⁶ m2 and the permeability structure is dominated by large fracture zones. Of the 315 hydrological instruments in the gorge for which data have been analysed, 21 monitoring well piezometers record repeated metre- or centimetre-scale groundwater level changes, and 12 underground V-notch weirs record elevated flow rates induced by the same earthquakes. Groundwater level changes exhibit consistent temporal characteristics at all monitoring sites, namely a time to peak pressure change on the order of one month and a subsequent recovery period on the order of one year. Changes in weir flow rate are near-instantaneous with maximum flow rates reached within 0-6 hours, followed by recession periods on the order of one month. Hydrological responses to different earthquakes at each monitoring site are systematic in terms of polarity and amplitude. This comprehensive dataset enables consistent patterns in the amplitude, time to peak pressure change and recovery time of groundwater level changes, and elevated weir discharge volumes in response to earthquake shaking to be documented. Earthquakes inducing hydrological responses have been categorised into five categories based on shaking characteristics (duration, bandwidth and amplitude). Larger hydrological responses and proportionally shorter time to peak pressure change and recovery time are associated with long duration (25-50 s or longer), high-amplitude, broad bandwidth shaking. The larger amplitudes of hydrological response and proportionally shorter times to peak pressure change and recovery times, are interpreted to represent greater temporary enhancement of the landslides hydraulic properties, particularly permeability. Understanding how earthquakes can enhance or otherwise affect hydraulic properties such as permeability in fractured reservoirs is intrinsically important and may prove of economic utility for both the geothermal and hydrocarbon energy sectors.