Modern geophysics highlights that the slip behaviour response of faults is variable in space and time and can result in slow or fast ruptures. However, the origin of this variation of the rupture velocity in nature as well as the physics behind it is still debated. Here, we first highlight how the different types of fault slip observed in nature appear to stem from the same physical mechanism. Second, we reproduce at the scale of the laboratory the complete spectrum of rupture velocities observed in nature. Our results show that the rupture velocity can range from a few millimetres to kilometres per second, depending on the available energy at the onset of slip, in agreement with theoretical predictions. This combined set of observations bring a new explanation of the dominance of slow rupture fronts in the shallow part of the crust or in areas suspected to present large fluid pressure.

Injection of fluids into underground formations reactivates preexisting faults and modifies the seismic hazard, as demonstrated by the 2011 Mw 5.7 and the 2016 Mw 5.8 earthquakes in Oklahoma. Currently, the effect of injection remains poorly understood. We model the seismicity triggered by a fluid flowing inside a Dietrich‐Ruina heterogeneous 2‐D fault, which can generate irregular sequences of events with magnitudes obeying Gutenberg Richter distribution. We consider a punctual injection scenario where injection pressure increases at a constant rate until a maximum pressure is reached and kept constant. We show that such a fluid injection leads to a sharp increase in the seismicity rate, which correlates with the time series of the pore pressure rate, for a wide range of injection pressure. Increasing the final pressure leads to an increase in the amplitude and the duration of the seismicity rate perturbation but also to a decrease in the frequency of large‐magnitude events. The maximum seismicity rate during the sequence also increases with the injection pressure rate, as long as a pressure‐rate threshold is not exceeded. Beyond it, the effect of increasing the injection rate is to make large‐magnitude earthquakes more frequent. While the total number of induced earthquakes is essentially controlled by the maximum pressure, the total seismic moment liberated increases with both the maximum pressure and the pressure rate. The comparison of our model to Dietrich's (1994, https://doi.org/10.1029/93JB02581) model shows the important trade‐off existing between seismicity rate perturbations and magnitude content variations of fluid induced seismicity.