Rocks rupture in myriad ways. People often think of great faults cracking the ground open (like the unrealistic chasm in the movie San Andreas) but rocks also break at much smaller scales. Fractures, which are narrow, mechanical breaks, are small but mighty gaps that often facilitate the flow of groundwater. As water squeezes through these spindly pathways, the rock can break down more easily.
There are several outstanding questions when it comes to fractures, including how they’re created and how they change over time, says Benjamin Gilbert, a geochemist at Lawrence Berkeley National Lab. However, addressing such questions proves tricky because for one, fractures are hidden under the ground, beneath our feet. Plus, says Gilbert, “most fractures are created and move without any detectable acoustic energy emission, so they’re not only invisible, they’re silent.”
But hydrologists must understand how fractures form because they can connect surface waters like lakes and rivers with deep-seated groundwater. In a new study published in Nature Communications, scientists from Lawrence Berkeley National Lab, led by Gilbert, found that measurements of the trace element thorium provide a proxy indicator of bedrock fracturing in certain catchments in Colorado. Moreover, the team noted a correlation between thorium concentrations and the passing of seismic waves from distant earthquakes, which suggests a plausible scenario in which remote earthquakes influence Colorado hydrology.
Difficulty finding fractures
When bedrock breaks and groundwater flows, the fluid picks up chemical elements from the rock, analogous to physical weathering in which rivers can pluck rocks and sand from their shores. Within fractures, this fluid-driven process of chemical weathering slowly transforms rock to soil while simultaneously connecting the groundwater—now enriched in various elements—to surface water. In this way, fractures function as a liaison between rocks and any waters they may hold and Earth’s surficial layers that are rich with soils and teeming with life. “Creating an ecosystem starts with fracturing,” Gilbert says. “Modest-scale fractures are really important.”
To find fracture networks, scientists typically rely on boreholes and seismic studies. For instance, the latter can detect fault motion that can change the flow of fluids and therefore groundwater geochemistry. Faults, however, are features that can be meters to kilometers long, whereas individual fractures can be far more diminutive, making them tougher to find. Instead, fractures are inferred—where there is a fault, there are likely fractures.
But detecting fracturing in bedrock below the surface sans local fault movement isn’t easy. “Most fracture processes are aseismic, and that just means they’re too quiet for most sensors to detect,” Gilbert says.
Monitoring watersheds, sourcing thorium
Gilbert’s team analyzed data from Colorado’s East River and Coal Creek, where a multi-year research project funded by the Department of Energy focuses on studying watersheds in the Upper Colorado River Basin. These watersheds reside in late Cretaceous sedimentary rock that has been altered by more recent igneous activity. Because of historic mining activity, the Coal Creek area exhibits enhanced concentrations of metals in its waters. Both East River and Coal Creek are carefully monitored—sometimes as often as daily, with other locations throughout the watersheds sampled less frequently. Numerous elements display seasonal trends. Thorium, however, does not.
Between 2016 and 2018, both the East River and Coal Creek showed 22 sudden thorium concentration increases above background levels, followed by a decay on the order of days to at most several weeks. The background thorium concentrations are very low, so these spikes really stood out, Gilbert says. Some thorium spikes occurred simultaneously in neighboring watersheds that are unconnected at the surface, hydrologically speaking. “That, to us, strongly suggested we have deep fractures that are connecting these two watersheds underground,” he says.
Nevertheless, the team considered several hypotheses for the source of the spikes. For instance, they demonstrated that thorium-bearing precipitation could not be responsible because the spike should be seen everywhere that rain fell. They also eliminated seasonal mineral dust storms as a possibility because such storms didn’t correlate with the spikes.
The team then considered whether the bedrock could be the source of the thorium. The East River watershed includes the Mancos Shale, which contains thorium. The team immersed powdered shale in simulated river water. Sure enough, they saw thorium release. “So, we concluded that the thorium excursions might most likely be caused by geomechanical processes that abruptly exposed shale to groundwater, processes like fracturing or fracture motion,” Gilbert says.
Fractures caused by distant faults
Though the team’s experiments established that fractures in the bedrock would result in water with high levels of thorium, the cause of the fractures remained mysterious. Falling trees can fracture rocks when wind speeds increase, but whipping winds did not correlate with thorium excursions. If the cyclical, centimeter-scale seasonal variations in elevation observed by GPS monitoring throughout the Rockies were responsible, elevation changes should occur in concert with thorium spikes. However, elevation values showed no correlation with thorium concentrations in East River.
The team then turned to seismic data. Four seismic stations—part of the Intermountain West network—were active between 2016 and 2018. The data from these four stations was downloaded from the SAGE Facility operated by EarthScope.
The team found that most thorium excursions did not correlate with any seismic signatures within about 50 kilometers. Looking beyond local seismicity, however, they found a small statistical correlation between thorium and seismic waves from distant earthquakes. Though waves from earthquakes at great distances are imperceptible to people, seismic stations—and some rocks—can experience and record these motions. It is these waves that Gilbert suspects of triggering the fracturing of rocks that contained thorium.
“Subsurface rocks are under stress,” says Gilbert, “and small changes to that stress caused by seismic wave propagation can cause fracture creation or movement in some locations.”
However, “our P-values are not fantastic,” Gilbert cautions. In other words, in their effort to statistically assess whether a correlation exists between seismicity and the thorium spikes, the correlation exists, but is weak. Nevertheless, he says, “I am excited by the idea that trace element geochemistry could report on subsurface fracturing processes that are currently invisible and silent.”