Abstract
Zuojiazhuang and Baodi are two adjacent wells (~50 km apart) in northern China. The large 2008 M w 7.9 Wenchuan and 2011 M w 9.1 Tohoku earthquakes induced different co-seismic water-level responses in these far-field (>1000 km) wells. The co-seismic water-level changes in the Zuojiazhuang well exhibited large amplitudes (~2 m), whereas those in the Baodi well were small and unclear (~0.05 m). The mechanism of the different co-seismic hydraulic responses in the two wells needs to be revealed. In this study, we used the barometric responses in different frequency domains and the phase shifts and amplitude ratios of the tidal responses (M2 wave), together with the well logs, to explain this inconformity. Our calculations show that the co-seismic phase shifts of the M2 wave decreased or remained unchanged in the Baodi well, which was quite different from the Zuojiazhuang well and from the commonly accepted phenomena. According to the well logs, the lithology of the Baodi well is characterized by the presence of a significant amount of shale. The low porosity/permeability of shale in the Baodi well could be the cause for the unchanged and decreased phase shifts and tiny co-seismic water-level responses. In addition, shale is one of the causes of positive phase shifts and indicates a vertical water-level flow, which may be due to a semi-confined aquifer or the complex and anisotropic fracturing of shale.
Highlights
Different kinds of hydrologic responses to earthquakes have been documented (Wang and Manga 2010)
We found shale in the aquifer lithology of the Baodi well, where there was no increase in the co-seismic phase shift
Shale has a unique lithology with low porosity/permeability, as well as anisotropy, fragility, and possibly complex fractures
Summary
Different kinds of hydrologic responses to earthquakes have been documented (Wang and Manga 2010). In the low- and high-frequency bands, the influence from the Earth tide is small, so we only calculated the transfer functions of the water-level response to barometric pressure, from Eq (3). Permeability estimation for the Baodi well (with shale) Given that positive phase shifts were observed in the Baodi well (which has shale in its aquifer lithology), Hsieh’s model (Hsieh et al 1987) may not be generally applicable to describe the water-level response to Earth tides for this well, and the permeability (transmissivity) should not be directly calculated on the basis of Hsieh’s model. Shales are always deemed as traps (closed reservoirs) Within this low permeability (transmissivity 10−9–10−6 m2/s) range, phase shifts at the frequency of the M2 wave will continue to decrease or remain unchanged rather than increase, even when the permeability (transmissivity) increases (Appendix Fig. 11; Doan et al 2006). Considering that the co-seismic water level changed with a very low amplitude in the Baodi well, the permeability might be harder to enhance in this well than in the Zuojiazhuang well, because of the compact structure of the shale
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