Events that are not actions have been previously used--at least by Fangzhen Lin [Lin98], Sheila McIlraith [McI00], and Javier Pinto.
Occurrence axioms are even more important in the treatment of concurrent events in situation calculus--to be the subject of another article.
This work benefited from discussions with Eyal Amir, Tom Costello, Ron Fadel, Hector Levesque, Vladimir Lifschitz, Fangzhen Lin, Sheila McIlraith, Leora Morgenstern, Aarati Parmar, Raymond Reiter, and Tran Son and the comments of three anonymous referees.
This research was partly supported by SRI Subcontract No. 34-000144 under SPAWAR Prime Contract No. N66001-00-C-8018.