Credible reports of nuclear explosion of any kind (test / weapon / power plant fault / anti-hurricane / etc) count.

The US embassies in other countries and other technicalities are not considered to be "in USA" for purposes of this market. If there is an explosion near borders with significant fallout, this will be considered on case-by-case basis (and market resolution might be lower-bounded at certain PROB%, keeping trading open in case there are more things happening).