How Ants know about Earthquake?
No, they may not sense Earthquake by an organ but due to other reasons. This may be any other physical factors which can be measured.


  "...There is therefore
  little reason to believe that these ants react to earthquake
  precursors other, perhaps, than those that may affect colonies
  directly, by altering physical variables that can be directly measured
  by other means..." 


Fascinating though the behavior and physiology of ants may be, they cannot be employed as reliable predictors or even sensors of earthquakes.

Source: Shaken, not stirred: a serendipitous study of ants and earthquakes