Since the spell is written as "target: one object", it doesn't actually say how its within-ten-feet blast works. Strictly as-written, if the entire space between you and the runes were full of stone and walls of force, you'd still take the damage. With no save if the letters were large enough that you could read them from where you were standing.
But they almost certainly mean it to be an explosion... But is it a burst or a spread?
