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

摘要

Earthquakes have been inferred to induce hydrological changes in aquifers on the basis of either changes to well water-levels or tidal behavior, but the relationship between these changes remains unclear. Here, changes in tidal behavior and water-levels are quantified using a hydrological network monitoring gravel aquifers in Canterbury, New Zealand, in response to nine earthquakes (of magnitudes M-w 5.4 to 7.8) that occurred between 2008 and 2015. Of the 161 wells analyzed, only 35 contain water-level fluctuations associated with Earth+Ocean (7) or Ocean (28) tides. Permeability reduction manifest as changes in tidal behavior and increased water-levels in the near field of the Canterbury earthquake sequence of 2010-2011 support the hypothesis of shear-induced consolidation. However, tidal behavior and water-level changes rarely occurred simultaneously (2%). Water-level changes that occurred with no change in tidal behavior reequilibrated at a new postseismic level more quickly (on timescales of 50min) than when a change in tidal behavior occurred (240min to 10days). Water-level changes were more than likely to occur above a peak dynamic stress of 50kPa and were more than likely to not occur below 10kPa. The minimum peak dynamic stress required for a tidal behavior change to occur was 0.2 to 100kPa.