When did the meaning of epicenter (from Greek for 'upon the center') expand to just mean the center of something important or substantial?
Solution 1:
The OED’s earliest citation for this sense of the word is 1908:
Yesterday, as it were, the epicentre of the world's sea-power lay on the waters of north-western Europe.
— Japan Weekly Mail
This is very early, considering the OED’s earliest citation for the word overall is 1880. (And its synonym epicentrum, is listed as dating back to only 1874.)