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Earth sciences

Earth sciences

Three earthquake laws are reproduced in the lab

09 Jun 2019
Earthquake simulator
Stressed out: the chain-like network of forces between discs in the Lyon experiment. (Courtesy: S Lherminier et al./Physical Review Letters)

A new lab-based experiment that accurately reproduces three universal laws that govern the dynamics of earthquakes has been created by Osvanny Ramos and colleagues at the University of Lyon in France.  The team monitored the dynamics of thousands of discs, trapped between two concentric cylinders. Their work could provide valuable insights to scientists attempting to explore the complex mathematical behaviours of seismic events.

Seismologists often study the properties of earthquakes by recreating them on small scales in the lab. So far, this has largely involved compressing rocks or collections of grains, until cracking or slipping occurs. To an extent, these approaches can mimic the fracturing and stick-slip behaviours typical of seismic events. However, such experiments have yet to recreate three statistical laws that are obeyed by all earthquakes observed on Earth.

The first is the Gutenberg-Richter law, which describes a logarithmic relationship between the magnitude of earthquakes, and the number of them that occur over a given time. The Omori law relates the number of foreshocks and aftershocks to the magnitude of an earthquake. The third law describes how the time gap between two earthquakes is strongly correlated to the magnitudes of the earthquakes.

Stressed discs

The latest experiment focussed on a single layer of 3500 small discs (about 7 mm diameter) that were trapped in the narrow space between two concentric, vertical cylinders. The top of the discs was then weighed down by a heavy ring, while at the bottom, a spinning plate completed one full rotation every 18 hours. As they were subjected to increasing shear stress, the optical properties of the discs change. This allowed the team to use an array of cameras to image the forces between discs. In addition, the energy and seismic waves propagating through the discs were tracked by torque sensors and acoustic detectors attached to the top ring.

The cameras revealed a continuously changing network of chain-like forces as the discs shifted relative to each other; continually redistributing stress throughout the system. Over 24::h, the sensors revealed that the torque of the overall system dropped suddenly around 2000 times, while acoustic waves were emitted almost two million times. These observations were consistent with the Gutenberg-Richter law, the Omori law, and the inter-event times as quantified by seismologists.

Having successfully recreated these three statistical laws in the lab, Ramos and colleagues now hope to further modify their setup to explore the dynamics underlying real earthquakes in more detail. By fine-tuning the pressure exerted by the heavy ring, as well as the rotation speed of the bottom plate, they could soon pin down the precise dynamics which govern seismic events. In the future, such insights could help seismologists to identify precursors to large-scale earthquakes from seismic data.

The experiment is described in Physical Review Letters.

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