Natural Hazard Categories and Classification

A natural hazard is a natural process or event, such as an earthquake, storm or flood, that has the potential to harm people, property or the environment, while a disaster occurs when a hazard strikes vulnerable people and causes serious damage that exceeds their ability to cope. In brief: hazards are commonly grouped as geophysical (earthquakes, volcanoes, tsunamis), meteorological (tropical cyclones, tornadoes), hydrological (floods, landslides), climatological (droughts, wildfires), biological (epidemics) and extraterrestrial (asteroid impacts). They are described by magnitude, frequency, duration, speed of onset, area affected and predictability.

Classifying hazards allows scientists and planners to compare them and choose suitable responses. This guide explains the difference between hazards and disasters, the main categories and characteristics, the scales used to measure hazards, compares the categories in a table, works through return-period and scale calculations, outlines the history of hazard research and corrects common mistakes.

What is the difference between a hazard and a disaster?

A hazard is a potential threat. An earthquake in an uninhabited desert is a natural event but not a disaster, because nobody is harmed. A disaster results from the combination of a hazard, the exposure of people and assets to it and their vulnerability. For a disaster database such as EM-DAT, an event is recorded when at least one of these is true: 10 or more people are killed, 100 or more are affected, a state of emergency is declared or an appeal for international assistance is made. Many “natural disasters” therefore have human causes: where people build, how they build and how well they are prepared.

What are the main categories of hazard?

• Geophysical: originate in the solid Earth, such as earthquakes, volcanic eruptions, tsunamis and dry mass movements (rockfalls, landslides on dry slopes).
• Meteorological: short-lived weather events, such as tropical cyclones (hurricanes and typhoons), tornadoes, storms, extreme temperatures and fog.
• Hydrological: related to water, such as river and coastal floods, flash floods and wet mass movements (mudflows, debris flows).
• Climatological: longer-term climate variability, such as droughts, heat waves, cold waves and wildfires.
• Biological: exposure to living organisms, such as epidemics and pandemics, plant and animal diseases and insect plagues.
• Extraterrestrial: from space, such as asteroid impacts and severe space weather (solar storms).

Another distinction is between tectonic hazards (from plate movements), atmospheric hazards, hydrological and slope hazards, and compound or cascading events, in which one hazard triggers another, as when the Tōhoku earthquake of 2011 caused a tsunami that flooded the Fukushima nuclear plant.

What characteristics describe a hazard?

• Magnitude: the size or strength of the event, such as the energy of an earthquake or the wind speed of a cyclone.
• Frequency and return period: how often events of a given size occur. A “100-year flood” has a 1% chance in any year. Small events are common, and large events are rare, so frequency and magnitude are inversely related.
• Duration: from seconds for an earthquake to months or years for a drought.
• Speed of onset: sudden (earthquake, flash flood) or slow (drought, sea-level rise). Slow-onset hazards allow more time to prepare.
• Areal extent and spatial concentration: a tornado affects a narrow path, a drought a whole region.
• Predictability: hurricanes can be tracked for days, while earthquakes cannot be predicted.

What scales measure hazards?

Different hazards use different scales. Earthquakes are measured by the moment magnitude scale (the modern replacement of the Richter scale, which is logarithmic) for energy released, and by the Mercalli scale for observed intensity (I to XII). Volcanic eruptions are rated on the Volcanic Explosivity Index (0 to 8). Tropical cyclones use the Saffir-Simpson scale (categories 1 to 5) based on sustained wind speed. Tornadoes use the Enhanced Fujita (EF) scale from EF0 to EF5, based on damage. Floods are described by water level or discharge and return period, and droughts by indices such as the Palmer Drought Severity Index.

Hazard categories compared

Worked example: return periods and scales

1. Return period and probability: an event with a return period of T years has an annual probability of 1 ÷ T. For a 50-year flood, it is 1 ÷ 50 = 0.02, or 2%.
2. Probability over a period: the chance that a 50-year flood occurs at least once in 25 years is 1 − (1 − 0.02)²⁵ = 1 − 0.98²⁵ = 1 − 0.603 = about 40%. For a 100-year flood over 30 years, it is 1 − 0.99³⁰ = 1 − 0.740 = 26%.
3. Magnitude and energy: an earthquake of magnitude 7 releases about 31.6 times the energy of magnitude 6, and 31.6² = 1,000 times the energy of magnitude 5.
4. Saffir-Simpson category: the sustained wind speeds are about 119–153 km/h (category 1), 154–177 (2), 178–208 (3), 209–251 (4) and 252 or more (5). A storm with sustained winds of 195 km/h is category 3, a “major hurricane.”
5. Frequency table: a region has recorded 40 events of magnitude 5 or greater, 4 of magnitude 6 or greater and 1 of magnitude 7 or greater in 100 years. The return period of M ≥ 7 is 100 ÷ 1 = 100 years, M ≥ 6 is 100 ÷ 4 = 25 years and M ≥ 5 is 100 ÷ 40 = 2.5 years, which shows that larger events are less frequent.
6. Disaster threshold: an event kills 8 people, affects 450 and has a state of emergency declared. It is recorded as a disaster under the EM-DAT criteria because more than 100 are affected and a state of emergency was declared.

How has hazard research developed?

Early thinking attributed disasters to divine will. The 1755 Lisbon earthquake and tsunami led thinkers to look for natural explanations, and the first scientific seismology developed in the 19th century. In the 20th century, the American geographer Gilbert White argued, after studying floods, that disasters result from human choices as well as natural events, and he promoted the idea of adjusting to hazards by planning and not just building defenses. The United Nations declared the 1990s the International Decade for Natural Disaster Reduction. In 2005, the Hyogo Framework, and in 2015, the Sendai Framework for Disaster Risk Reduction, put the focus on understanding risk and building resilience. Since the 1970s, the recorded number of weather-related disasters has risen, partly because of better reporting, more exposure and climate change.

Which mistakes are common?

Mistake 1: Using “hazard” and “disaster” as synonyms

A hazard is a potential threat, and a disaster is the harm that results when it meets vulnerable people.

Mistake 2: Thinking that a 100-year flood happens once a century

It means a 1% chance in any year. Two such floods can occur in consecutive years.

Mistake 3: Calling every hazard “natural”

Natural processes cause hazards, but human settlement, land use and climate change change the risk and the outcome.

Mistake 4: Comparing scales directly

Magnitude, intensity and category scales measure different things and are not interchangeable.