Okay, I'm far from an expert, so my opinions should be taken with a grain of salt.

To my eyes, this is overpowered as a 1st level spell. Naturally, there are many circumstances where it is worthless, but no one is going to learn the spell unless they think they can use it. And here's what it can let you do:

1) Keep a bag of rocks with Explosive Runes cast on them. Throw them towards distant enemies, and detonate them the next turn. Since it only uses a 1st-level spell to detonate them, you've got a steady supply of 6d6 bombs to throw at people, with a few days' preparation. And if you're going TO, throw books on which you've cast Explosive Runes on every page. People already talk about that trick, but this makes it easier to pull off. Especially as an immediate action. I gather that this was the intent of the spell, but making it 1st level opens up greater abuse. (And on re-reading your appeal for comments, I realize that you were asking about that. I don't think you should be able affect multiple such traps with a level 1 spell.)

2) If you suspect a trap, you can cast the spell from maximum range and see what happens. If nothing happens, there's no trap; if it is a trap, you just disabled it from a safe range with a level 1 spell. A niche application, to be sure, but with a wand of Detonate Rune, you can safely check every book or inscription for traps before reading it. Perhaps there should be a caster level check to detonate someone else's rune this way?

That said, I don't know what spell level would balance this spell. And I think a lot of DMs would outright ban it, because of the TO potential.