"seminar on" or "seminar in"?

‘Seminar on X’ and ‘seminar in X’ are both acceptable; which one will sound more natural depends on what X is. ‘Seminar on X’ can be regarded as a shorter version of ‘seminar on the topic of X’, while ‘seminar in X’ can be regarded as a shorter version of ‘seminar in the discipline of X’. Whether ‘on’ or ‘in’ will be more apt thus depends on whether one would be more inclined to say that X is a topic or that it is an academic discipline.

I would say that it depends on the material being reviewed.

If it's an informative seminar discussing elementary concepts of topography, it would seem to be a seminar on topography, like a college grad job fair.

If it's a seminar in topography, I'd assume it'd be an in-depth discussion that requires some background knowledge of the material, like a topoligists convention or mixer.