I am prepared to be lambasted for this question.
When I saw the explosion, I thought the death toll would be greater than 42. I mean this explosion tore through several underground levels and caused damage at street level. Why weren't more people killed?
Admiral Nutjob announced the death toll at 42 before privately letting Kirk in on the target having been a secret Section 31 base below. I suspect 42 people died in and around the Kelvin Memorial Archive, not counting those beneath.