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

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.

Rain and small earthquakes maintain a slow-moving landslide in a persistent critical state

“…We examined the influence of three different mechanisms, i.e., undrained loading 10 , the variation of the water table 44 , and the damage of the soil due to earthquake shaking 17 , upon the dv/v in the 3-8 Hz frequency range, thanks to poroelastic models 45,46 . Supplementary Figs.…”

Tidal Behavior and Water‐Level Changes in Gravel Aquifers in Response to Multiple Earthquakes: A Case Study From New Zealand

“…The removal of colloidal blockages in heterogeneous aquifers (Brodsky et al, ) may alter flow pathways significantly. Fracture‐scale permeability enhancement, either subvertical (Wang, ; Wang et al, ) or subhorizontal (O'Brien et al, ), may connect hydraulically isolated pore pressure zones. Shear‐induced dilation (Wang et al, ) may occur if cyclic shear strain exceeds ~10 −4 (Vucetic, ).…”

Seismological and Hydrogeological Controls on New Zealand-Wide Groundwater Level Changes Induced by the 2016 Mw7.8 Kaikōura Earthquake

“…The aquifers mostly occur in young sedimentary deposits that have varying levels of consolidation, with few aquifers in igneous or metamorphic rocks. A notable exception are the geoengineered schist landslides in Cromwell Gorge [34], which have fracture permeability and water tables that have been displaced from their natural condition by drainage tunnels. In contrast to these examples from New Zealand, many international studies of earthquake hydrology provide datasets on aquifers in well-consolidated and crystalline rocks (e.g., [35][36][37]).…”

“…400 km south of the epicentre (Figure 8), monitoring wells in Cromwell Gorge screened in schist rock exhibited water level changes of~±3 m. Yet, monitoring wells within 200 km of Cromwell Gorge, mainly screened in unconsolidated deposits, responded persistently with small amplitudes of less than 1 m. Cromwell Gorge monitoring wells are known to be sensitive to hydrological change from multiple earthquakes. Cromwell Gorge groundwater levels are depressed below equilibrium levels by pumping and gravity drainage due to infrastructure, and the sensitivity may in part reflect anthropogenic modification of the groundwater regime [34]. Devils Hole, Nevada, is another example of a site that is sensitive to change, with responses occurring at low seismic energy densities of 10 -6 J/m 3 [75].…”

“…Therefore, regardless of the magnitude of the earthquake, it appears that monitoring wells may have a predisposition to show water level changes of certain amplitude relative to dynamic shaking, based on the strength of the rock they are screened in. This concept was adopted by O'Brien et al [34] who concluded that aquifer systems have the ability to resist and recover from dynamic shaking which is consistent between earthquakes. WLC/PGA varies over several orders of magnitude at any given depth or Vs 30 .…”