  Earthquakes vary in size over many orders of magnitude, yet the scaling of
the earthquake energy budget remains enigmatic. We propose that fundamentally
different "small-slip" and "large-slip" fracture processes govern earthquakes.
We combine seismological observations with a physics-based mechanical
earthquake model under flash-heating friction. We find that dynamic weakening
and restrengthening effects are non-negligible in the energy budget of small
earthquakes and establish a simple linear scaling relationship between
small-slip fracture energy and fault size. We use supercomputing to apply this
scaling and unveil volumetric "Mode-4" earthquake cascades involving $>700$
multi-scale fractures within a fault damage zone, capable of dynamically
triggering large earthquakes. Our findings provide an intuitive explanation of
seismicity across all scales with important implications for comprehending
earthquake nucleation and multi-fault rupture cascades.
