It's extremely unlikely that the US is going to unleash a preemptive nuclear strike against NK and at the first sign of an invasion you can bet NK will fire all guns on Seoul.
Dave H. Kim, Seoul resident; I research Korean foreign and security policy.
Answered 9 Aug 2016
Due to its reclusiveness and isolation from most of the global community, gathering intelligence on North Korea and estimating North Korea’s military capabilities is very difficult. Having said that, numerous analysts and think tanks have estimated several thousand artillery pieces—includes 130 mm, 170 mm and 240 mm artillery with a range of 27-plus km or more—to be near the DMZ and within striking distance of metropolitan Seoul. Higher estimates go well above 10,000 pieces. When we include rocket launchers, surface-to-surface missiles, and short-range ballistic missiles, the numbers are much, much higher.
So what do these mean? Calculations have been done that suggested well over 10,000 artillery rounds and potentially 4,000 long-range artillery rounds could be fired at Seoul
per minute. Many thousands of massive rockets could also be fired by multiple rocket launchers (MRLs) before quickly retreating into underground hiding places. Even with lower estimates, when one includes North Korea’s Scud-B/Scud-C variants,
Nodong and
Musudan ballistic missiles and other missiles, Seoul is effective a city held hostage, and North Korea doesn’t need to worry about an invasion any time soon. It also means North Korea’s “legitimate reason” for pursuing nuclear weapons—the need to deter U.S. nuclear threat with its own nuclear deterrent—is simply a poor excuse and pretext to do as it wants.
A great place start learning more on the above is
The Changing Military Balance in the Koreas and NE Asia (2015) by Anthony Cordesman.