Geological control of the partitioning between seismic and aseismic sliding behaviours in active faults: Evidence from the Western Alps, France

Given that active faults can slide either continuously by aseismic creep or episodically during earthquakes, and that the same fault zone may evolve laterally from seismic to aseismic deformation, an important issue is to know whether seismic to aseismic transition can be geologically controlled. This article presents examples of contrasted mechanical behaviour along active faults that cross cut limestone and marl units within the sedimentary cover of the French Alps. By matching seismic events along strike-slip and normal faults with the nature and structure of the rocks, it is demonstrated that the partition between seismic and aseismic sliding at depth is geologically controlled: earthquakes nucleate in the strongest rocks, mainly limestones, whereas marls accommodate at least part of the tectonic loading by aseismic creep. By looking at exhumed rocks deformed in the same context it is possible to identify the mechanism of creep, which is shown to be pressure solution creep either as a permanent or post-seismic creep. As earthquakes slip are seen to propagate through the whole upper crust, creep processes do not necessarily prevent an earthquake rupture from propagating through creeping units. However, creep relaxes stress and consequently reduces the available elastic potential energy at the origin of earthquakes in such creeping zones. The key parameters of pressure solution creep laws are presented and discussed. Using these laws, it is possible to infer why marl may creep more easily than limestone or why highly fractured limestone may creep more easily than intact rock. This approach also identifies other rocks that could creep by pressure solution in subduction zones and indicates how creeping zones may act as barriers for earthquake rupture propagation. Finally, the criteria possibly revealing geological control of the transition between seismic and aseismic sliding at depth are discussed with respect to subduction zones.

► Partition between seismic and aseismic sliding is shown to be geologically controlled.
► Earthquakes nucleate in rocks less prone to creep, but may extend to creeping zones.
► Creep relaxes stress and consequently reduces the possible occurrence of earthquakes.
► Creep mechanism is pressure solution creep either as permanent or post-seismic creep.
► Criteria revealing geological control at depth are discussed in subduction zones.