Hydrogeological processes inside fault zones

Water level response to periodic natural loadings

Understanding the response of well water levels to natural periodic loadings, such as seismic waves, tides, and barometric loadings, can provide an innovative approach to quantitatively measure the in-situ hydrogeologic properties and architecture of fault zones.

1.     The response of well water levels to seismic waves

We develop an automatic detection of water level changes caused by seismic waves. This method combines a short-term to long-term averaging ratio in the time domain with spectral analysis in the frequency domain. Applying our detection algorithm to a decade’s worth of high-rate pore-pressure data recorded near Parkfield, we find that coseismic water-level oscillations are primarily influenced by the seismic energy density. Additionally, based on our observations, we modify the empirical equation of seismic energy, distance, and magnitude, a widely used relationship in hydroseismology.

Cao, M., Xue, L.* and Zhao, L. (2024). Investigating well water level oscillations caused by seismic waves using automatically detected responses. Journal of Hydrology, 639, p.131628.

2. The response of well water levels to barometric pressure and tidal loading 

We develop a new approach that combines well water-level responses to barometric and tidal loadings to better constrain in-situ hydrogeological properties. By constructing the barometric response of water levels using a transfer function in the frequency domain, we can assess the subsurface flow system based on multi-frequency responses. Responses at high frequencies (> 1 cycle/day) can precisely constrain hydrogeologic properties in the confined aquifer, either independently or in combination with tidal responses. The response at low frequencies (<0.1 cycle/day) can be used to constrain the hydrogeologic properties of the confining layer. We are testing this method at the pore pressure stations of the Plate Boundary Observatory. This method enables the monitoring of in-situ hydrogeologic properties at any well, regardless of the presence of a tidal response.

Zhang Y., Xue L. and Sun, X. Joint inversion of in-situ hydrogeological properties by combining the well water level responses to Earth tides and barometric pressure, in preparation.

Fault Zone hydrogeologic architecture

Fault zone hydrogeologic architecture has an important role in controlling the elevated pore pressure that can reduce a fault’s strength to encourage earthquake failure. However, the in-situ fault zone hydrogeological architecture is hard to quantitatively measure. We investigate the fault zone hydrogeologic properties and architecture by using water level response to Earth tides.

1. Decreasing permeability records a healing process inside the Wenchuan earthquake fault zone

Permeability is a good proxy for rock damage after an earthquake. By using water level tidal response inside a borehole through a fault main rupture zone, we have observed a decreasing permeability for most time in the aftermath of the 2008 Wenchuan Earthquake. This decreasing permeability is also partially reset by the seismic waves of remote large earthquakes, indicating complex healing and damaging processes after a major earthquake. Our work provides the first continuous hydrogeological record of the post-seismic healing process.

Xue, L., Li, H.B., Brodsky, E.E., Xu, Z.Q., Kano, Y., Wang, H., Mori, J.J., Si, J.L., Pei, J.L., Zhang, W. and Yang, G., 2013. Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone. Science, 340(6140), pp.1555-1559.

2. Uniform diffusivity structure of the San Andreas Fault near Logan Quarry