The Origin of the attack is referencing where the is coming from, usually the model making the attack. The wording of the Trench is still in line with the old MK3 way of blasts, where the center of the blast was the origin, but that wording does not exist with blasts in MKIV.
That is not necessarily a correct assumption. The MK IV trend has been to remove the non-interactivity in model-to-model interactions. So, no more is it the case of “Oh, we both played Cygnar’s Storm theme. 60% of our attacks don’t affect one other” or “My 95% fire-damage Protectorate army is fighting Assault Kommandos. … Huh.”"
Terrain has always been hugely impactful on the game, and I suggest that allowing this type of non-interactivity is the whole point of having impactful terrain rules. Otherwise, the same non-interactivity argument could be applied to LOS-blocking terrain (“It’s not fair that my ranged models can’t draw LOS through this building”) and so forth.
So, in summary: having models that are just straight-up invincible while standing wide open in a field and getting shot in the face with exploding fiery mortars is bad. Having models that are carefully positioned in terrain be unaffected by certain types of damage, that’s fine; otherwise, why are you even playing with terrain?
This talks about origin, so we need to look at the Point of Origin rules (bolded bit for emphasis):
Let’s also look at the relevant bits of AOE attacks (bolded), because I think it’s pretty fair to call the AOE an “effect of the attack”:
I personally think the intent here is fairly clear and this is very easy to answer based on just the above items. I concede that it’s always possible for the rules to provide more details and spell everything out unambiguously, but I believe this is sufficient for our purposes.
So, here’s the crux of the question:
The only sensible answer to this is: the origin of the damage is the model directly hit by the AOE. Why? Because we have an example in the Measuring Range rules that immediately follow Point of Origin:
Interpreting the “origin of the damage” (i.e., Point of Origin) as anything but the directly hit target leads to nonsensical conclusions. For example, if the Point of Origin of damage is the attacker, why don’t the models nearest to the attacker suffer blast damage too? That’s obviously wrong.
So, we’ve established that the “origin of the damage” is the model directly hit by the blast. Knowing that, resolving the rest is trivial.
“Centered in the Crater” is pretty easy to determine. We know what “centered” means, because we use it with sprays all the time. “Centered in the Crater” must therefore mean “the center of the model’s base, the one from which the AOE damage is measured, must be in the crater.”
Is this absolutely, 100%, laboriously, unambiguously spelled out by the AOE rules or terrain rules? No. But we can use Point of Origin, Measuring Range, and common sense to draw parallels and figure out what it means.
(And, yes, it’s likely this could use updated MK IV wording. But we can figure out how it’s supposed to work until that happens.)