The worst catastrophes are combinations of events, where a primary catastrophe causes secondary effects by triggering another ‘follow-on’ catastrophe. The escalation of consequences can be worse than if they had happened separately.
The potential for one class of threat to trigger or exacerbate the effects of another threat type is captured qualitatively in this matrix. This has been developed from historical examples of correlated catastrophe, or from consideration of plausible scenarios where one can lead to another.
The correlation categories are:
0. The two threat types are uncorrelated, and if they occurred coincidentally, their consequences would be broadly the same as if they occurred independently
1. No mechanism for this threat to directly cause an event of the second threat type, but the consequences of a coincidental second event shortly afterwards would be made significantly worse, for example because resources would be already committed and abilities to respond and contain would be weakened
2. There is some potential for an event to contribute to the causal mechanisms that would trigger the occurrence of an event of the second type
3. An event of this type potentially can directly trigger an event of the second type
4. An event of this type potentially can directly trigger another sub-category of threat within the same threat category