Spatially and temporally systematic hydrologic changes within large geoengineered landslides, Cromwell Gorge, New Zealand, induced by multiple regional earthquakes

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Abstract
Geoengineered groundwater systems within seven large (23 × 104–9 × 106 m2), deep‐seated (40–300 m), previously slow‐creep (2–5 mm/yr.) schist landslides in the Cromwell Gorge responded systematically to 11 large (Mw > 6.2) earthquakes at epicentral distances of 130–630 km between 1990 and 2013. Landslide groundwater is strongly compartmentalized and often overpressured, with permeability of 10−17 to 10−13 m2 and flow occurring primarily through fracture and crush zones, hindered by shears containing clayey gouge. Hydrological monitoring recorded earthquake‐induced meter‐ or centimeter‐scale changes in groundwater levels (at 22 piezometers) and elevated drainage discharge (at 11 V notch weirs). Groundwater level changes exhibited consistent characteristics at all monitoring sites, with time to peak‐pressure changes taking ~1 month and recovery lasting 0.7–1.2 years. Changes in weir flow rate near instantaneous (peaking 0–6 h after earthquakes) and followed by recession lasting ~1 month. Responses at each site were systematic from one earthquake to another in terms of duration, polarity, and amplitude. Consistent patterns in amplitude and duration have been compared between sites and with earthquake parameters (peak ground acceleration (PGA), seismic energy density (e), shaking duration, frequency bandwidth, and site amplitude). Shaking at PGA ~0.27% g and e ~ 0.21 J m−3 induced discernable gorge‐wide hydrological responses at thresholds comparable to other international examples. Groundwater level changes modeled using a damped harmonic oscillator characterize the ability of the system to resist and recover from extrinsic perturbations. The observed character of response reflects spectral characteristics as well as energy. Landslide hydrological systems appear most susceptible to damage and hydraulic changes when earthquakes emit broad‐frequency, long‐duration, high‐amplitude ground motion.
Funding Information
  • Victoria University of Wellington

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