Laboratory studies of earthquakes easier with soft materials

Materials World magazine
13 Jan 2020

Studying earthquakes in a laboratory has become easier after researchers found soft materials that behave similarly under stress. Idha Valeur reports.

By using two types of soft materials and putting them under continual stress, researchers found that they behaved similarly to how the Earth’s crust is rebuilt amid earthquakes.

The researchers from the Indian Institute of Science (IISc), Raman Research Institute and ETH Zurich, used thin sheets of two soft materials.

‘Our study is on soft materials which have yield stress. The nematic gel made of 39 wt% surfactant Cetyltrimethylammonium Tosylate (CTAT) in water has optical anisotropy which was crucial to do polarised optical imaging along with the rheological measurements. The other system studied is made of 5 wt% Laponite clay in water which again has yield stress,’ IISc Honorary Professor at the Department of Physics, Ajay Sood, told Materials World.

The materials are sheared between two steel plates and when force was applied to the material, the restructuring of the inside of the material mirrored the seismograph data that earthquakes normally create.

This new approach differs from previous trials, as these have focused on more solid materials such as ceramics and rocks, but when force is applied to these materials they crack, deform and often split open before their changes can be studied. In this study, they used an optical microscope and camera to observe the changes over time at a close range in the soft materials.

‘Our study reports creep flow, i.e. we apply constant load below the yield stress and measure strain rate as a function of time. Normally, the expectation is that one will observe a low value of strain rate which will be constant with time. This is the case for very low applied stress – e.g. 0.01 Pa. Instead, we see huge bursts in shear rate. This has not been seen before,’ Sood explained, before adding, ‘The fluctuations and burst seen in our experiments are similar to ground acceleration measured in earthquakes. Hence our systems under shear are model systems to study earthquakes.’

Not only did the research show that the patterns in the material was similar to foreshocks and aftershocks, it also showed that the patterns they observed adhered to both the Gutenberg-Richter and the Omori law. The first indicates earthquakes’ strength and the second details the frequency reduction of aftershocks over time. With the parameter values defined by these laws calculated for the soft materials, the results revealed values close to ones reported for real earthquakes. The spike intervals were also a close match for ones observed in real-life earthquakes.

According to Sood, the team is now questioning whether machine learning and artificial intelligence methodology can be used to understand the data and whether they can have predictive power.