Most seismicity on Earth comes from the classic movement of tectonic plates sliding and colliding above the planet’s mantle, but other, more localized forces can deform the crust as well. In a new study, Serpelloni et al. used GPS measurements from the Eastern Alps to monitor how rainfall and hydrological conditions contribute to ground deformation in this tectonically active region.
The Eastern Alps and the Dinarides feature areas characterized by classic karstic geography, in which water has gradually dissolved the limestone foundation that underlies the region’s mountains and valleys. As a result, the terrain is marked by abundant sinkholes, caves, and aquifers. In addition to providing a veritable playground for spelunkers, the researchers suspected that the karst, especially changes to aquifer water levels, might produce a measurable strain in the region.
Fortunately, the Eastern Alps are also rife with GPS stations that can precisely track ground displacement over time. Using data from these stations, the researchers applied a “blind source separation” algorithm, which allowed them to isolate ground displacement in the region that was caused by forces other than classic plate tectonics. The team eventually uncovered several different displacement signals across multiple spatial and temporal scales. In particular, they found a nonseasonal signal showing horizontal deformation that correlated to rainfall in the region on monthly timescales. They attribute this signal to pressure changes resulting from fluctuating water levels within the vertical fractures of different karst systems.
The researchers say that together, the results demonstrate that this technique can be used to measure millimeter-scale transient deformations caused by changes to groundwater flow and precipitation. In addition to giving more accurate measurements of tectonics in karst regions, the team also suggests that similar studies may be useful for measuring groundwater resources. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015252, 2018)
—David Shultz, Freelance writer