What’s the difference between a new earthquake and an aftershock?
6.6 sounds like an earthquake to me.
I am no expert but from what I understand the initial quake is of a higher magnitude that the ones that follows. In short, think of the crust in the area “ringing” like ripples in a pond, the waves of energy flow away and then back causing further action.
The earths crust is fractured into pieces and where they beet they grind together, sometimes one goes under or over another, in other cases they grind against each other. You cannot make that much mass move without some residual oscillation as it settles.