From the Latin for "shake", it's a synonym for earthquake
tremor synonyms, tremor pronunciation, tremor translation, English 
... [C14: from Latin: a shaking, from tremere to tremble, quake] .... often mistaken 
for Parkinsonism but is not life-threatening and can usually be kept under control 
... Verb, 1. tremor - shake with seismic vibrations; "The earth was quaking".