A MODIFIED VERSION OF THE RATE- AND STATE- DEPENDENT FRICTION LAW DESCRIBING A FRACTURE VELOCITY DEPENDENCE OF FRICTION

Since earthquakes occur along pre-existing faults they have been commonly recognized as resulting from stick-slip frictional instabilities rather than fracture instabilities. Conversely, in this work we propose that the triggering factor of seismic release could be due to stick-slip fracture instabilities because we experimentally found that variations of growth velocity of a rupture have a strong control on friction. This statement is done on the basis of experimental results that were achieved by using an analogue model (Tape–Plate model), which is able to simulate different rupture typologies during the disjunction of an adhesive tape from a PVC plate at different, but constant imposed traction velocities. Under particular conditions of temperature and humidity, if the latter are less than 1.8mm/s a completely stable fracture propagation is observed; for values greater than that the rupture advancing obeys a stick-slip dynamic. The analytical description of the TP model evolution is given by a modified version of the rate- and state- dependent friction law. In other words, we affirm that, with little changes, this law can be also used as valid tools to investigate the fracturing processes. These statements imply that: a) the concepts of velocity weakening, steady state velocity weakening and velocity strengthening behaviours cannot be considered due only to sliding processes, but also to fracturing processes; b) we recognized the existence of characteristic rupture phenomena that could be considered as seismic rupture, or as triggering factor of seismic release.