Simplifying to Capture the Essential: Towards Reliable Analog Experiments Mimicking Earthquakes

For more than half a century, analog experiments based on fracture or shear have tried to simplify the overwhelming complexity of earthquakes, aiming at capturing the essential ingredients of the dynamics. Two main complementary approaches are used: one that studies singular events of frictional failure between two blocks (often rocks), and another that focuses on reproducing the phenomenological relations describing seismicity: the Gutenberg-Richter law, the Omori law, the inter-event time distribution, etc.

For the first time, a shear experiment has been able to reproduce quantitatively and simultaneously the three main statistical laws mentioned earlier [1], opening a new pathway to the investigation of earthquake like dynamics at a laboratory scale.

The experimental system, developed at the Institut Lumière Matièreof the University of Lyon 1 in France, uses a monolayer of compressed and slowly sheared 3D-printed photoelastic disks to mimic the intermittent dynamics of a tectonic fault. A cylindrical geometry allows a continuous and limitless shear, provoking a significant number (more than a million in a 24 hours experiment) of "labquakes", consisting of acoustic emissions generated by frictional failures between grains. There is a striking quantitative agreement between the labquakesstatistics and the relations describing seismicity introduced earlier, which indicates that both systems are ruled by a similar physics. The main ingredient of the dynamics is an emerging and evolving heterogeneity of energy thresholds provided by the heterogeneous character of the force network within the granular system. The structure, dynamics and sizes of these heterogeneities in nature remain as open questions. However, the experimental results may serve as hints to eventually find them.

Potentially, this experimental system may be also capable of proposing answers related to the origin and robustness of these phenomenological relations. As it runs more than 10 000 times faster that a real fault, it may become a benchmark for finding and testing possible precursory behavior of catastrophic quakes.

[1] Lherminier et al, Continuously sheared granular matter reproduces in detail seismicity laws, Phys. Rev. Lett. 122, 218501 (2019).

PRL Editors' Suggestion, Editor's Choice in Science