Processus de rupture dynamique d'un grand séisme et loi de friction sur les failles

We study the physics of rupture propagation on a set of active faults using observations of the 28 June 1992 Landers earthquake in California. This very well recorded event provides a wealth of information about the details of rupture propagation on earthquake faults that modify in a fundamental way the simple naive models of earthquake rupture that have been so popular in Earth Sciences. Different methods to invert seismic and geodetic data for the details of the rupture process are discussed and put to work in order to make a model of the Landers earthquake that explains essentially all available data down to a wavelength of about 3 km. It emerges from our studies that earthquakes propagate very rapidly but in a very complex and tortuous way. The kinematic description of the rupture history is used to constrain the parameters of friction on the fault. The numerical simulation completely reproduces the source properties, including wave radiation. A consequence of friction weakening is the existence of a phase of initiation prior to rupture propagation. This phase is associated with specific length and time characteristics. We show that the apparent friction on the fault during large earthquakes is the result of complex interactions between the fault segments. The weakening rate is therefore a scale-dependant property depending on the geometrical properties of the fault system at different scales.