The mechanical stresses induced in natural rock formations by the effects of the Sun and Moon have been measured in a new technique that exploits observations from a single seismic monitoring station. Devised by a pair of German researchers, the measurement technique has revealed the daily and annual stress cycles caused by solar heating and the tidal forces exerted by the Sun and the Moon, and the researchers believe that it could be used for applications ranging from the safeguarding of mines and construction sites to the monitoring of volcanoes and earthquakes.
The elastic properties of the minerals and rocks in the Earth’s crust are far from constant. When subjected to external forces, they undergo minute deformations that slightly alter their elastic moduli. In response to these strain variations, the velocities of seismic waves travelling through the crust – originating from sources including earthquakes, volcanoes, and ocean waves hitting the shoreline – are also subject to change.
In theory, researchers can explore the mechanical properties of complex subsurface rocks and minerals by monitoring changes in the velocities of seismic waves passing through them. So far, however, attempts to find the strain sensitivities of materials in the Earth’s crust have largely involved measuring these velocity variations within small samples of material in the lab, which can generate huge uncertainties when upscaling from small samples to geological formations.
To overcome this issue, Christoph Sens-Schönfelder at the GFZ German Research Centre for Geosciences, and Tom Eulenfeld at Friedrich Schiller University, analysed the ambient seismic vibrations observed over an 11-year period by a single seismometer: the Patache station in Chile’s Atacama Desert. By correlating repeated cycles of velocity variation in the waves, the duo identified several processes that cause geological elastic moduli to oscillate.
Regular oscillations revealed
Firstly, the researchers observed annual oscillations in seismic wave velocity, which they attribute to tidal forces induced in the crust as the Earth orbits the Sun. They also identified smaller, daily oscillations, caused by cycles of solar heating and night-time cooling of the Earth’s surface. In addition, the duo observed two separate oscillations of around 12 hours – a result of the lunar tidal influence, which varies due to the Moon’s slightly elliptical orbit.
Perhaps the most intriguing observations were small groups of velocity oscillations clustered around the specific frequencies that equate to the sums and differences of the lunar and solar signals. This has shown for the first time that the natural oscillation frequencies of completely different systems can interact with each other.
Sens-Schönfelder and Eulenfeld now hope to use this method to explore the mechanical properties of different rock and mineral types. In the future, they believe it could even be incorporated into planetary probes, enabling astronomers to explore the diverse geologies of the solar system.
Full details are reported in Physical Review Letters.
