Seismic occurrence (5)
I believe the answer is:
quake
'seismic occurrence' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for quake that I've seen before include "Vibrate" , "Tremble (in one's boots?)" , "Tremble (with fear)" , "Earth tremor" , "Tremble, shake" .)