Mechanism of co-seismic water level change following four great earthquakes – insights from co-seismic responses throughout the Chinese mainland

Abstract

We analyze the co-seismic groundwater level responses to four great earthquakes recorded by China's network of groundwater monitoring wells. The large number of operational wells (164 wells for the 2007 Mw 8.5 Sumatra earthquake, 245 wells for the Mw 7.9 Wenchuan earthquake, 228 wells for the Mw 9.0 Tohoku earthquake and 223 wells for 2012 Mw 8.6 Sumatra earthquake) and co-seismic responses provide an opportunity to test hypotheses on mechanisms for co-seismic water level changes. Overall, the co-seismic water level responses are complex over large spatial scales, and there is great variability both in the sign and amplitude of water level responses in the data set. As shown in previous studies, permeability change, rather than static strain, is a more plausible mechanism to explain most of the coseismic responses. However, we find through tidal analysis of water level responses to solid Earth tide that only one third of these wells that showed a sustained post-seismic response can be explained by earthquake-induced permeability change in aquifers, and these wells had sustained (>30 days) water level changes. Wells that did not show sustained changes are more likely affected by permeability changes only immediately adjacent to the wellbore.

Disruption of groundwater systems by earthquakes

“…Figure b shows that the phase shift of the tidal response of well water exhibits many transient increases (becoming less negative) up to 15° relative to the tidal strain, both before and after the Wenchuan earthquake; these transient increases are associated in time with earthquakes (Table S3 in the supporting information) with seismic energy density of ~10 −5 J/m 3 at the Jiangyou well, which is sufficient to cause water level changes [ Weingarten and Ge , 2014], and are followed by a gradual return to the pre‐earthquake value. Similar changes in the phase shift have been reported previously and interpreted to reflect changes in aquifer permeability, due perhaps to unclogging of preexisting cracks by seismic waves followed by postseismic healing of these cracks [ Brodsky et al ., ; Elkhoury et al ., ; Xue et al ., ; Shi et al ., ]. Following these studies, we estimate the transmissivity ( T ) of the aquifer by fitting the tidal data with equations (S3) and (S6) and setting S = 10 −4 ; results show that T increased from 1.6(±0.6) × 10 −5 m 2 /s before the Wenchuan earthquake to 5.2(±3.0) × 10 −5 m 2 /s after the earthquake, which corresponds to an increase in horizontal permeability from 2.4(±0.8) × 10 −15 m 2 to 7.8(±4.5) × 10 −15 m 2 (Text S5 in the supporting information).…”

“…Various mechanisms have been proposed to explain the hydrologic responses, including changes in aquifer permeability [ Rojstaczer et al ., ; Wang et al ., ; Manga et al ., ], changes in static strain [ Muir‐Wood and King , ; Jonsson et al ., ], consolidation or liquefaction of saturated sediments [ Manga , ; Manga et al ., ], rupturing of geothermal reservoirs [ Wang et al ., ], opening of deep fractures [ Sibson and Rowland , ], and water shaken out of soils [ Mohr et al ., ]. Estimates of the coseismic change in permeability [ Elkhoury et al ., ; Xue et al ., ; Shi et al ., ] have been limited to horizontal flow to wells, because permeability in the horizontal direction of a layered groundwater system is normally orders of magnitude greater than that in the vertical direction. But earthquakes may also breach the hydraulic barriers between aquifers and increase vertical permeability [ Wang et al ., ]; relatively little study has been done on this aspect of the earthquake effect.…”

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

“…Once climatic, tidal and anthropogenic anomalies have been identified, distinct positive and negative groundwater level changes, and increased groundwater discharge associated with large intermediate and far‐field earthquakes can be readily identified (Figures ). The amplitudes of the observed earthquake‐related groundwater level responses are of the order of 10 −2 to 10 1 m (i.e., centimeters to meters in scale), which include many >1 m scale responses that are relatively large in comparison to previously reported international examples of intermediate and far‐field responses [e.g., Brodsky et al ., ; Chia et al ., ; Roeloffs , ; Shi et al ., , ]. The durations of the groundwater level changes, persisting for several months to more than a year, are also relatively long.…”

“…To date, much research into earthquake‐induced hydrologic phenomena has involved either data from a single site that span multiple earthquakes [ Roeloffs , ; Matsumoto et al ., ; Shi and Wang , ; Weingarten and Ge , ] or data from many sites recording a single earthquake [ Wang et al ., ; Chia et al ., ; Cox et al ., ; Yan et al ., ]. A few studies have now started utilizing continent‐wide records of multiple earthquakes at multiple sites [ Parvin et al ., ; Shi et al ., , ]. By eliminating variables, it may be possible to clarify the importance and interactions of various extrinsic (earthquake and tectonic), intrinsic (local hydrogeological), and anthropogenic (human‐related) influences on responses observed.…”

Comparison of Regression Methods to Compute Atmospheric Pressure and Earth Tidal Coefficients in Water Level Associated with Wenchuan Earthquake of 12 May 2008

“…), water level fluctuates regularly with the earth tide. In China, about 80 % of the water level wells show earth tidal effect (SHI et al 2015b;YAN et al 2014). …”