Open: Occurs over a large geographic area. Usually these are natural events, including Tornadoes, Earthquakes, Ice-Storms, or Hurricanes.
Closed: Occurs within a small area. Often these are technological including building collapse, chemical exposures, or transportation accidents.
Finite: Has a defined start and end point. Usually after the defined time the chance of further victims is unlikely.
Ongoing: Often have a sudden start, but the forces causing the disaster are ongoing. This includes pandemics, famines, or sustained conflicts.