You can easily apply the tremor rule on each phase. I first thought this was mentioned like that in the rules.
But after rechecking it seems to only apply during combat phase.
Copy paste from the enhanced rulebook:
Tremors in the Warp
In the midst of battle a Wizard can easily become mentally
exhausted, and the insidious and treacherous powers of the chaos
empyrean are quick to capitalize on any weakness. Until play has
once again returned to exploration turns, for every attempted spell
after the initial three, the Wizard must make a Tremors in the Warp
test with progressively difficult odds of success. The depleted
Wizard must roll a D12 for each subsequent attempt after the third.
On the fourth attempt, he must roll a 4 or better to succeed or
something has gone terribly wrong. On the fifth attempt things go
awry on any roll less than 5; on the six attempt he fails on a roll of
less than 6, and so forth. When the Wizard fails this test he must
then roll on the table below to find out the results and may not cast
a spell this turn. The risk is always great but obviously there is a
point at which the Wizard will simply be unable to cast anymore
spells
Each spell does cost you quite some components..
So there's always the cost of healing..
Next campaign I might just apply the Tremor rule on each phase, both exploration and combat.