When tectonic plates collide and separate, fractures spread across Earth’s crust. Beneath the planet surface, these fractures act like a natural plumbing system, carrying water down from the surface. The hydrogeological characteristics of these fault zones—the properties that affect how groundwater moves around the fault—are hugely important for faulting processes and stability. However, the hydrogeological properties of faults are difficult to measure and remain somewhat poorly defined. Fortunately, well systems can provide a window into the unseen hydrological processes going on underground. Tidal forces influence the hydraulic head of aquifers—a measure of water pressure and elevation in groundwater that determines the direction of water flow—which, in turn, influences the height of water in wells. Here Xue et al. take advantage of this relationship, measuring well water responses to Earth’s tides in order to examine and assess the hydrogeologic properties of the San Andreas Fault.
The researchers looked at four monitoring wells in a quartz gabbro quarry in Aromas, Calif., located at varying distances from the San Andreas Fault. Between April 2014 and July 2015, the team used pressure sensors to measure the water level in each well every 10 minutes. The sensors picked up both water and barometric pressure; to account for barometric pressure separately, the researchers placed a sensor inside a well above the water level.
The water level response to tidal forcing within wells is a product of the permeability of the crust material around a well and its storativity—the amount of water that is released from an aquifer as the hydraulic head declines. The researchers found that the two wells closest to the fault were roughly 10 times more permeable than the two wells farthest away. The specific storage capacity of the closer wells was also larger. The observed storage structure is novel. Taken together, however, the permeability and storativity of all four wells result in a relative uniform hydraulic diffusivity measure of 0.01 square meters per second. The uniform diffusivity structure suggests that the permeability contrast might not efficiently trap fluids near faults during the interseismic periods. The team also examined data from a broadband seismic station to assess the effect of earthquakes on the permeability of wells’ surrounding formation. They found that quakes increased the permeability of some wells by as much as 160%.
The measures of specific storage are particularly relevant for understanding fault mechanics, as storage capacity can influence the fault’s response to stress. The study shows that well water response to tidal forcing can be a valuable proxy for hydrogeological structures beneath Earth’s surface. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2015GC006167, 2016)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), The role of water in Earth’s tectonic plumbing systems, Eos, 97, doi:10.1029/2016EO050619. Published on 19 April 2016.