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Figure 1: The three types of plate boundary. Arrows show the way each plate moves.
Constructive boundaries are also called divergent, destructive are convergent and conservative are transform. The blocks are not to scale.
A natural hazard is a naturally occurring physical event — such as an earthquake, volcanic eruption, tropical storm, flood, or wildfire — that poses a threat to human life, property, or livelihoods. The critical word is "threat": the event only becomes a hazard when people are in its path.
This distinction matters enormously. Consider two scenarios:
Lava flows destroy acres of land. Ash clouds rise kilometres into the atmosphere. Seismic activity shakes the seabed. But there are no people within 400 km. This is a natural event — not a natural hazard, because there are no humans at risk.
The same volcano, the same lava flows and ash. But now 3 million people live within the danger zone. Tens of thousands of people cannot be evacuated quickly because of narrow mountain roads. This is a natural hazard — and potentially a catastrophic one — because of the human vulnerability surrounding the physical event.
The key insight is that it is human society that turns a natural event into a natural hazard. An earthquake under the ocean with no nearby coastlines is not a hazard. The same earthquake triggering a tsunami that hits a densely populated coastline is one of the most deadly hazards on Earth.
When Does a Hazard Become a Disaster?
A natural disaster occurs when a natural hazard causes significant loss of life, destruction of property, or disruption to human society beyond the ability of the affected community to cope without external assistance.
The transition from hazard to disaster depends on three variables:
- Hazard magnitude — how powerful is the physical event? (earthquake magnitude, wind speed, flood depth)
- Vulnerability — how susceptible is the affected population? (poverty, building quality, location)
- Coping capacity — how able is the society to prepare, respond, and recover? (governance, wealth, emergency services, infrastructure)
A powerful hazard hitting a prepared, wealthy society may cause a manageable emergency. The same hazard hitting a poor, unprepared community becomes a catastrophe. Haiti and Christchurch prove this with devastating clarity.
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