During the first two years of the ERC project, we successfully installed both the biaxial apparatuses Crackdyn and Cyclequakes in the host laboratory Geoazur. We also set up complete data acquisition systems for both devices, including high-frequency acquisition for strain gauge measurements, accelerometers, and displacement sensors. Additionally, we synchronized all acquisition systems with the ultra-high-speed camera TMX6410. We developed an advanced optical system leveraging the photoelastic properties of PMMA, using a highly stable polarized light source. This setup, combined with the high-speed camera, enabled us to capture the rupture front during dynamic rupture propagation. The precise synchronization between the camera and high-frequency sensors allowed us to image and accurately quantify the breakdown work during instabilities, demonstrating that frictional ruptures observed in our experiments mirror those in natural earthquakes.
Coupled with the development of the experimental apparatuses, we also designed two distinct inversion procedures to reconstruct the evolution of fault slip during both the nucleation and propagation of seismic ruptures.
First, we developed a custom approach to compute the Green’s function of our experimental medium using finite element methods. This method allows us to model the strain response expected at strain gauge locations during biaxial experiments with the CrackDyn apparatus, as well as triaxial experiments on saw-cut granite samples. Building on this Green’s function solution, and in collaboration with Pierre Dublanchet, we created the KISLAB algorithm — an innovative tool enabling static and quasi-static inversion of fault slip during earthquake nucleation. KISLAB combines deterministic and Bayesian approaches to determine the optimal fault slip evolution and is now openly available on GitHub.
Secondly, we developed an inversion procedure to reconstruct both kinematic and quasi-static fault slip during frictional rupture propagation in Crackdyn experiments. This algorithm integrates two key forward modeling solutions: the first uses the widely adopted Okada solution for Green’s functions, commonly applied to natural earthquakes, while the second relies on numerical modeling via COMSOL, capturing the complete experimental setup, including local loading and sample holder effects. By leveraging both approaches, our algorithm reconstructs the slip front evolution during rupture propagation, aligning well with direct measurements from the high-speed camera and displacement sensors. In addition, this solution allowed us for the first time to compute accurate solution of the moment rate function during laboratory earthquakes, that could help us to explore the possible self-similarity of earthquakes in the laboratory.
These two complementary inversion methods establish an essential bridge between laboratory-controlled earthquake data analysis and inversions applied to natural earthquake datasets — a key milestone planned in WP2 of our research proposal.
