How is a natural hazard different from a natural risk?

Risk is the combination of a hazard acting on a set of assets that are exposed to that hazard and have different levels of vulnerability to it.

Let’s unpack that.

• Assets can be physical things (people, buildings, forests, etc.) or social or economic constructs (public safety, the economy). It’s easier to think about physical assets to start with.

• Hazards are measurable dangerous phenomena and have associated probabilities. Examples are flooding depth, wind-speed, intensity of ground shaking (note that I did not say floods, storms, or earthquakes: these are natural phenomena, not hazards).

• Exposure is the number of assets that find themselves in the same place as a dangerous level of hazard. A large city has many more assets than a small village.

• Vulnerability tells us how much a given asset can be damaged if it is exposed to a given level of hazard. A well-built concrete building is less vulnerable than a shanty-town house to the same level of wind-speed.

It’s the combination of hazard, exposure, and vulnerability that constitute risk.