Here's my understanding of what happened:
Yhwach + Ichigo killed the Soul King that was keeping Soul Society and the other realms stable, and without his presence the
torrent of souls that pass through these realms will destroy them.
When the Soul King dies, the Soul Society instantly begins
crumbling and
shaking.
There is no release of Low 4-C levels of energy at the Soul King's corpse at the moment of his death and the Soul King's palace isn't visibly shaking or crumbling either. If the energy was radiating outwards, the point of the epicentre would have the biggest sign of damage / indication of an earthquake. There is a simple explanation for why there is no visible rumbling or crumbling at that point, and that's because the Soul King's body isn't releasing this energy which is causing the quaking.
As for the scenes in chapter 620, I believe that the Soul King's palace getting torn in two occured just as a result of
Yhwach ripping Mimihagi from the Soul King. The actual earthquakes did not start back up until
Yhwach actually absorbed Mimihagi (causing the quakes on the
next page).
So where is the epicenter if not in the Soul King's palace? I think that it would simply be the planets themselves; Earth, the Soul Society, Hueco Mundo, etc. which are each separately collapsing due to being unstable now by the removal of the Soul King.