DEFIDEF

Earth rocks and man-made materials always contain defects, commonly in the form of small fractures of various sizes, often hardly visible to human eyes. Where the fracturing is dense, it modifies the mechanical properties of the rocks or materials, and these modifications can have critical consequences: weakening and even disruption of the materials (building walls, container walls, etc.), leakage of fluids they might host (water, oil, gas, radioactive elements, etc.), amplification of the waves they might be crossed with during an earthquake, etc. While research in material sciences has advanced our knowledge of the mechanical properties of man-made materials, little is known on the mechanical properties of natural fractured Earth rocks. The DEFIDEF project tackles this issue, focussing on the case of intensely fractured rocks around tectonic faults. The latter are those producing earthquakes, and therefore, quantifying the physical properties of the rocks that host them is of utmost importance to anticipate their rupture behaviour during an earthquake.
The objective of DEFIDEF is thus to derive a mathematical model for fractured rock volumes around faults. This remains a challenge, on the one hand because the natural fracture networks are dense and complex, and on the other hand because fracturing occurs over a wide range of spatial scales. To derive the elastic properties of the fractured rock masses, we modelled their overall mechanical behaviour by a homogenized macroscopic equivalent medium (i.e., a small volume in which homogeneous mechanical properties can be assumed). The modelling is calibrated with field observations and measurements (from the Faults_R_Gems^Pilot project). To deal with the variable scale challenge, we currently use an empirical statistical scaling model based on a double power law describing local fault density and length. Although a simplification of natural systems, this empirical model allows the description of complex fracture systems with a single set of statistical data. We are now in the process of making numerical tests to estimate the efficiency of the new method. The fracture and fault measurements that will be soon provided through the Faults_R_Gems^Pilot project will help us improve and validate the method.