Centre for Research on Epidemiology of Disasters (CRED)’s EM-DAT is the basic database to do disaster research.We can refer how they count the disasters.
CRED Disaster definition:
１．10 or more people reported killed
２．100 people reported affected
３．Declaration of a state of emergency
４．Call for international assistance
More than one of those are confirmed, the CRED counts the event as a disaster