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Laboratory earthquakes: dynamic shear cracks in heterogeneous frictional interfaces

Final Report Summary - EARTHCRACKS (Laboratory earthquakes: dynamic shear cracks in heterogeneous frictional interfaces)

The general objective of the project was to expose a young physicist, Dr Julien Scheibert, to multidisciplinary research on earthquake dynamics. He came to perform original research and to receive training at the Norwegian Centre of excellence, 'Physics of Geological Processes' (PGP) at the University of Oslo. By working with geologists and physicists with a proven record of cross disciplinary research, Julien Scheibert was expected to broaden his competences and reach a deeper level of insight into fracture and friction in nature.

Dr Scheibert, in collaboration with numerous members of the host institution, led a combined experimental, theoretical and numerical approach to fracture and friction problems in both the physics and geology contexts:
- an original experimental setup was built in which a frictional elastomer-on-glass multicontact interface mimicking a seismic fault can be driven in highly controlled conditions from static contact to macroscopic sliding;
- a correlation software was implemented to extract, from time-lapsed images of the rough contact, the dynamics of the slip distribution along the interface;
- a theoretical analysis was developed in order to understand the role of the friction-induced contact asymmetry in the development of stick-slip motion, including precursors to sliding;
- a numerical approach was developed to understand the detailed effect of the boundary conditions on the stress build-up at the interface and the subsequent shear-crack propagation dynamics.

Put together, the results of all the above-mentioned actions allowed Dr Scheibert to develop the following comprehensive picture of the coupled physical processes underlying the transition from static to kinetic friction:
- Contrary to common belief, a frictional interface is highly non-homogeneous. One major outcome of the work was indeed to show that, on top of the initial heterogeneity due to geometry, the mere application of an increasing tangential force on the system will generically induce the simultaneous increase of a global leading / trailing edge asymmetry in the pressure distribution.
- In general, the stress heterogeneities at the interface due to the external loading conditions by far dominate the small scale disorder due i.e. to the topography irregularities of the contacting surfaces. Shear-crack nucleation occurs in the region where the shear stress reaches first the slipping threshold, which is proportional to the local pressure. This region is most often found at the trailing edge of the contact. Then the crack front propagates in such a way that it climbs up the threshold gradient until it eventually arrests.
- The description of the kinematics of the system, i.e. the states in which the system comes to rest, can be described essentially independently of the dynamics, i.e. the way in which fronts propagate.
- The shear-crack propagation velocities, on the contrary, are controlled by the kinematics, and depend crucially on the friction law.

The work carried out during this fellowship led to nine articles (two published in international journal, two in revision, two submitted and three currently in preparation), both in physics and geology journals, on fracture and friction topics. It also led to six talks in international conferences and various other oral communications.

The fellowship greatly increased the experimental and theoretical competence of Dr Scheibert and considerably extended his network. It contributed significantly to the fact that Dr Scheibert obtained, before the end of his fellowship, a permanent research position in CNRS, France.