Wizard spell: Explosive Runes

[Natsumehack]

The problem is once everyone knows about this spell's existence, it's trivially easy to avoid: just don't read any graffiti.

What if a wizard could magically enchant random things, so that when those items are examined, they trigger the explosive effect? Would be harder to avoid.

'This is a buttbot. Inscribed on it in arcane writing are the words 'Gotcha, sucker'.' BOOM

How powerful are these explosions in the first place?