For the study of the interactions between earthquakes and groundwater, however, one often needs to compare the responses before and after the earthquake, and well tests become impractical because continuous well tests are too laborious and expensive. The most often used technique is the so-called ‘well test’ where the water level in a well is artificially changed and the subsequent time-dependent recovery of water level measured in the well or in nearby wells is interpreted to estimate the hydraulic properties of the affeted aquifer. Thus the analysis of the response of groundwater level in wells to various kinds of forcing has been a time-honored topic of groundwater research and is an important and effective tool for the study of the hydraulic properties of aquifers. Wells (or boreholes) are hydrogeologists’ ‘telescopes’ to study the properties of groundwater systems. As a consequence, the most often used tides for hydrological studies are the semi-diurnal lunar tide M 2 and the diurnal lunar tide O 1. The solar tides are affected by thermally induced changes in the atmospheric pressure, which are difficult to correct. 5.3), but most of these tides are too small to be clearly recorded in water level data and thus are not used in groundwater studies. In reality, the potential is more complicated due to the facts that the Earth’s orbit about the Sun and the Moon’s orbit about the Earth are both elliptical rather than circular, the Moon’s orbital plane does not align with Earth’s equator and the Earth’s rotation is not aligned with the ecliptic. 5.2 the second term is the diurnal tide due to the inclination of the orbit of the planet (Moon) to the equator the third term is independent of the rotation of the Earth and is thus a constant. The first term inside the parenthesis is the semi-diurnal tide that produces the two symmetrical bulges in Fig. This aspect has not received adequate attention. We review in another separate section the principles of water-level response to seismic waves and the model used in its interpretation and application to earthquake hydrology.įinally, fractures in the shallow crust may significantly affect the response of water level to tides, barometric pressure, and seismic waves. Such efforts may provide useful understanding on the dependence of aquifer properties on the frequency of the forcing mechanisms. 2019, 2020), based on a model developed by Cooper et al. Some recent efforts have made use of this response to estimate aquifer properties (Barbour et al. It has been known for a long time that groundwater responds to seismic waves (e.g., Byerly and Blanchard 1935). We review in a separate section the principles of the barometric response of the water level in wells and the existing applications to earthquake hydrology.Īt the high-frequency end of the continuum is the response of groundwater to seismic waves. A welcoming development is a joint analysis of the tidal and the barometric responses in some recent studies to evaluate earthquake effects on groundwater systems (Barbour et al. Groundwater level also responds to changes of the barometric pressure, and the study of this response can provide important insight to the hydraulic properties of both the aquifer and the aquitard (e.g., Jacob 1940 Rojstaczer 1988 Olding et al. In the following three sections we introduce the principles of tides, the tide-induced deformation of the solid Earth, i.e., the Earth tides, the response of groundwater to the Earth tides, and the models that are used to interpret the tidal responses of water level. The Earth tides, on the other hand, are much better characterized and are therefore more useful in the study of the groundwater system. The ocean tides, though much bigger in amplitude than the solid tides (Earth tides) along the coast, are complicated because they depend sensitvely on the local bathymetry and the configuration of the coast. In addition, the tidal response of aquifers was also found to be particularly sensitive to subsurface disturbances such as mining (Shi et al. ![]() In the past few decades a great amount of work has been done to use the tidal response of aquifers to estimate the hydraulic properties of groundwater systems (e.g., Hsieh et al., 1987 Roeloffs, 1996) and to compare these properties before and after earthquakes in order to quantify earthquake effects (e.g., Elkhoury et al.
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