KenV makes some valid points.
The more you dig into the claim NK could level Seoul with artillery - the more nuanced the picture becomes.
According to some of the Stratfor reports, there's quite a high rate of misfires among their batteries.
Also, apparently they can't reach all of Seoul, just the Northern suburbs.
According to some of the reports I've read (which I'll try to find and post) it would be a pretty nasty barrage - but not the all out firestorm that the popular media makes out.
No doubt short range missiles would be used to fill the gaps that artillery couldn't hit - but one assumes they're a different engagement scenario.
Several of the US Senators who went to the recent classified briefing spoke of very thorough and considered planning for US/SK military options.
EDIT - Jeez that
MLRS is a nasty piece of kit! Love the nickname `The Grid Square removal system"!