  Seismic faults release the stress accumulated during tectonic movement
through rapid ruptures or slow slip events. The slow slip events play a crucial
role in the seismic cycle as they impact the occurrence of earthquakes.
However, the mechanisms by which a slow-slip region affects the dynamics of
frictionally locked regions remain elusive. Here, building on model laboratory
experiments, we establish that a slow-slip region acts as a nucleation center
for seismic rupture, thereby enhancing earthquakes' frequency. We emulate
slow-slip regions by introducing a granular material patch along part of a
laboratory fault. By measuring the response of the fault to shear, we show that
the role of the heterogeneity is to serve as a seed crack for rapid ruptures,
reducing fault shear resistance. Additionally, by varying the external normal
load, we show that the slow-slip region extends beyond the heterogeneity,
demonstrating that fault composition is not the only requirement for slow-slip,
but that load also plays a role. Our findings demonstrate that fracture
concepts single out the very origin of earthquake nucleation and slip dynamics
in seismic faults. The interplay between slowly-slipping and locked regions
that we identify provides a promising avenue to monitor fault propagation and
mitigate seismic hazards.
