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
Ref.
http://emdat.be/explanatory-notes
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