Social　media　records　the　pulse　of　social　discourse　and　drives　human　behaviors　in　temporal　and　spatial　dimensions,　as　well　as　the　structural　characteristics.　These　online　contexts　give　us　an　opportunity　to　understand　social　perceptions　of　people　in　the　context　of　certain　events,　and　can　help　us　improve　disaster　relief.　Taking　Twitter　as　data　source,　this　paper　quantitatively　measures　exogenous　and　endogenous　social　influences　on　collective　behaviors　in　different　events　based　on　standard　fluctuation　scaling　method.　Different　from　existing　studies　utilizing　manual　keywords　to　denote　events,　we　apply　a　clustering-based　event　analysis　to　identify　the　core　event　and　its　related　episodes　in　a　hashtag　network.　The　statistical　results　show　that　exogenous　factors　drive　the　amount　of　information　about　an　event　and　the　endogenous　factors　play　a　major　role　in　the　propagation　of　hashtags.　©　Springer-Verlag　2013.