PolystyQuakes : what can we learn from the use of polystyrene as analogue to earthquakes? From small to large scale and vice versa.

Laboratory investigations into the behavior of fault zones have been a significant focus in experimental rock mechanics over the past decades. Various approaches have been developed, ranging from analog models to testing natural samples in triaxial cells. The primary goal of the latter is to infer the physical mechanisms responsible for failure under realistic conditions encountered in natural settings, albeit on small sample sizes (e.g., centimeter scale). In contrast, analog modeling aims to replicate similar mechanical behavior by applying scaling laws to geometry and material properties. In the present study we aim at combining large scale experiments (meter scale) with small scale experiments (cm scale) in order to highlight the underlying physical mechanisms preceding the slip.

To address the spatial scale limitations of classical rock mechanics, we developed new experiments that bridge the gap between traditional rock mechanics and analog experiments. These experiments utilize the unique capabilities of the DIMITRI setup, a giant true-triaxial apparatus (1.5m × 1.5m × 1m). Due to the size of this experimental device, the maximum stress it can apply is limited to 2 MPa per principal stress. Consequently, we chose polystyrene as an analog for rocks. The low elastic properties of polystyrene slow down physical processes, enabling comprehensive observation of rupture phenomena, from initiation to failure arrest. Our objective is to investigate the interplay between different types of slip occurring along the interface.

In this study, we conducted stick-slip experiments on large-scale polystyrene blocks with a pre-cut surface area of 1.5 m². We applied shortening rates ranging from 1 to 10 mm/min. Our experiments successfully reproduced stick-slip behavior, allowing us to observe variations in frictional behavior along the interface and identify different types of slip, from slow slip to dynamic slip.

In parallel, we performed small scale experiments uniaxial stick slip experiments, under the same conditions than the previous, that we monitored with 2D X-ray radiography at high frequency (12Hz). Preliminary observations highlight density contrasts in the bulk material around the fault plane, offering insight into potential precursory signs of slip.

Therefore, this study including two scales of observation demonstrates the relevance of our material to study the physical mechanisms controlling various slip types occuring along the seismic cycle.