[Analysis based on the data Max took with sensors on our first journey to Craterrock]
From the amount of melt in the crater, it's not just kinetic. But from the deepness of the crater, there's a kinetic factor as well. And from the radiation, it seems like there's some sort of plasma reaction going on. So, something like a really large and fast-moving plasma explosive is the most accurate guess. There are lots of extra masking effects, though, making this a shaky analysis. When whatever the inital cause was went up, it took every nearby reactor with it (a fair number, apparently). Plus it seems likely there was a deep large power plant of some sort that also went, which in the end may have caused more damage than the initial blast.
It's definitely not any sort of beam weapon though, too widespread, and too bell-curvey.