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

Abstract

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~50 min) than when a change in tidal behavior occurred (~240 min to 10 days). Water-level changes were more than likely to occur above a peak dynamic stress of~50 kPa and were more than likely to not occur below 10 kPa. The minimum peak dynamic stress required for a tidal behavior change to occur was~0.2 to 100 kPa.

Sustained Changes in Well Water Levels Following a Large Earthquake: Possible Evidence of Permeability Decreases in a Shallow Groundwater System

“…Studies using pumping tests (Rutter et al., 2016) and the tidal responses of well water levels (Weaver et al., 2019) have shown that earthquakes can decrease aquifer permeability by shear‐induced consolidation, thereby mathematically supporting our proposed hypothesis. However, field studies using these methods have only shown that permeability changes occur near observation wells (usually <∼100 m).…”

Heterogeneous Permeability Changes along a Fault Zone Caused by the Xingwen M5.7 Earthquake in SW China

“…Earthquake‐induced water level changes are often attributed to changes in permeability, and the polarity of the water level change (increase or decrease) in a well may be influenced by permeability changing either up or down the head gradient of that well, with almost instantaneous responses occurring in the vicinity of the well (Shi & Wang, ; Weaver et al, ). The magnitude of the change in permeability does not show a direct correlation with the magnitude of water level response (Rutter et al, ), and a higher preexisting permeability may cause a larger water level change and shorter decay time (Shi et al, ).…”

Different Sensitivities of Earthquake‐Induced Water Level and Hydrogeological Property Variations in Two Aquifer Systems

“…Four types of co‐seismic response are generally observed during earthquake shaking; abrupt changes (step‐like), gradual changes, oscillation, and oscillation accompanied by other responses such as oscillation‐abrupt and oscillation‐gradual changes (Shi, Wang, Manga, & Wang, 2015). Post‐seismic responses after earthquake shaking include transient, persistent, and no apparent change (Weaver, Doan, et al., 2019; 2020). Only post‐seismic responses are examined in this study because co‐seismic responses lasting minutes or seconds would not have been properly observed due to the hourly sampling rate.…”

“…Weaver, Doan, et al. (2019) showed that permeability changes occurred in only nine of 35 tidal‐sensitive wells following multiple intermediate‐ and far‐field earthquakes. Furthermore, some wells, such as Devils Hole in Nevada (Weingarten & Ge, 2014), show extreme sensitivity to seismic events regardless of earthquake distance, while other nearby wells show unique responses despite their proximity (King et al., 1999; Sun et al., 2019; Yan, Wang, & Shi, 2016).…”

“…Wang, Liao, et al (2016) examined responses in fifty multi-level monitoring wells located near and at intermediate distances from the 1999 Chi-Chi earthquake (Ms 7.7), finding that only 10% of these wells manifested vertical enhancement of permeability, with most of the wells with changed permeability located beyond the near-field. Weaver, Doan, et al (2019) showed that permeability changes occurred in only nine of 35 tidal-sensitive wells following multiple intermediate-and far-field earthquakes. Furthermore, some wells, such as Devils Hole in Nevada (Weingarten & Ge, 2014), show extreme sensitivity to seismic events regardless of earthquake distance, while other nearby wells show unique responses despite their proximity (King et al, 1999;Yan, Wang, & Shi, 2016).…”