Zimanowski and co-workers pressed together two pieces of volcanic rock and then slid one across the other. Electrodes in each rock monitored the electromotive force that built up between. Although the sliding force was constant, the researchers found that the electrical signal peaked periodically, shortly before any movement was detected.

Based on this experiment, Zimanowski and colleagues developed a probe and used it to measure electrical signals at Mount Stromboli, an active volcano in Italy where landslides are frequent. The detector revealed a pattern in the signals similar to that in their experiment, and the team proposed a four-step process to explain it.

Initially, the researchers believe, strain builds up between the layers of rock. ‘Micro-cracks’ then form at the rock surfaces as they creep over each other, exposing fresh rock. This causes electrical charge to accumulate, which generates the ‘precursor signal’. Mechanical failure follows as the rocks slip past each other, and finally the interface relaxes and the electrical charge dissipates.

According to the German team, this technique would be suitable for monitoring unstable mountainsides, in particular because it gives advance warning of landslides and volcanoes. ‘The field probe is buried between 30 and 50 centimetres in the ground but its detection depth is presently one to two kilometres into the Earth’s crust’, Zimanowski told PhysicsWeb. Improvements to the probe could increase this to a depth of several kilometres, say the researchers, which could enable it to detect the precursor tremors of certain types of earthquake as well.