Seismic occurrence (5)
I believe the answer is:
quake
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">seismic occurrence</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for quake that I've seen before include "Vibrate" , "Tremble (in one's boots?)" , "Tremble (with fear)" , "Earth tremor" , "Tremble, shake" .)