Evolution of the Off-Fault Deformation during experimental strike-slip earthquakes

Large continental strike-slip earthquakes produce spectacular surface deformations. However, ground displacements are only partially measured in comparison with the amount of slip inferred at depth. Relatively few estimates of the proportion of surface deformation accumulated on faults and deformation distributed regionally around faults are available. However, new technological advances such as state-of-the-art space imaging techniques now greatly improve the quality of surface rupture measurements. Their application has revealed that a significant amount of deformation is accommodated as diffuse deformation in an area of several hundred meters around the fault. This distribution is suggested to depend on the fault complexity. It is therefore essential to understand this distribution and its relation with fault segmentation to study the impact of fault complexities in a seismic context, we recently developed an innovative experimental prototype using some granular materials in layers similar to the earth's crust. They consist of a basal layer of rubber powder that stores elastic energy provided by the displacement of a basal plate sliding parallel to a second, fixed plate. The second layer is made of raw, twice broken rice that brings the stick-slip behaviour required for locking the slip between ruptures, and a third layer of sand with the frictional behaviour of cold shallow sediments at the surface. The surface sand layer allows following the evolution of the fault surface trace from the R-shears stage to the anastomosed fault zone composed of a succession of segments separated by zones of complexities. Using image correlation, we analyse the surface displacements. Since the rice layer causes a stick-slip behaviour, the analysis of the surface displacement is done on several seismic cycles: if the surface displacement is lower than the displacement imposed by the motor, it is an inter-seismic period, if the surface displacement is faster than the displacement imposed by the motor then it is a seismic event.

Once this catalog of events is established, the analysis of the gradient of the displacement Ux parallel to the basal enables us to quantify the deformation: localized (On-Fault Deformation) or distributed (Off-Fault Deformation).

At the R-shear stage, we measure [50~80] % of Off-Fault Deformation. Once the strike-slip fault is formed, the percentage of OFD drops between 20 to 30 %. These results are comparable to measurements made by experiments devoid of a stick-slip behaviours (with only of sand). Moreover, if we compare these values to the proportions of OFD estimated for natural earthquakes, we find the same distribution.

These experiments show that the more mature the fault, the more it will rupture seismically, in time as in space.