'seismic' originates from Greek, specifically the word 'seismos', where 'seismos' meant 'earthquake' or 'shake'. The adverb 'seismically' is formed from 'seismic' + the adverbial suffix '-ally'.

'seismos' entered scientific vocabulary via Late Greek/Latin (e.g. Neo-Latin 'seismus') and was adopted into English as 'seismic' in the 19th century; 'seismically' followed as the regular adverbial form of the adjective.

Initially it meant 'relating to earthquakes or earth-shaking'; over time it retained that technical meaning and additionally developed a figurative sense meaning 'causing or indicating a major change' (e.g. 'seismically important').