Not sure if this is exactly what you are looking for, but to determine the level of risk you multiply the chance of occurrence by the consequence of the risk. For example, when planning Scouting events, I use the following:

Chance of Occurrence:

• 0: Improbably (will not occur)
• 1: Remote (unlikely to occur)
• 2: Occasional (possible to occur)
• 3: Probable (might occur)
• 4: Frequent (very likely to occur)

Consequence:

• 0: No harm
• 1: Slight injury (delay in resuming activity)
• 2: Injury requiring first aid (out for day)
• 3: Serious Injury requiring EMS
• 4: Life-threatening Injury

Risk Level (Chance of Occurrence x Consequence):

• 0-2: Minimal
• 3-5: Low
• 6-8: Moderate
• 9-16: High

So to perform a risk assessment for an outing, I fill out a matrix listing the activities and their potential hazards and then assign the appropriate chance of occurrence, consequence and risk level as well as listing any actions we can take to minimize the risk. The higher the level of risk, the more you need to look at ways to prepare for and mitigate that risk. For example, while a wild animal attack may have a grave consequence (4) it has a low probability (1) which makes it a low risk. Also, in my case, if I have one or more activities with a moderate level of risk it indicates I need to do an additional layer of paperwork and parental notification. If I have one or more activities that have a high level of risk I need to re-evaluate.

That said, I've used similar approaches to work projects too, to evaluate the risks involved that could jeopardize delivery or cost and the basic principle is the same; multiply the chance of occurrence by the consequence.

If you are talking wider-scale disasters, I'd use the same approach. It's too easy to focus on the consequence alone, but taking the probability into account you get a better picture of what you should really be looking at.