Characteristics of hazards

A hazard is a process, condition or organism with the potential to harm people, property or the environment. A hazard event is one occurrence of that process at a particular time and place, and it becomes a disaster when its impacts seriously disrupt community functioning, with human, material, economic or environmental losses. The impacts may test or exceed local coping capacity, but that is not a required threshold in the definition. VCE Geography classifies hazards by cause.

Geological (geophysical) hazards originate in the Earth's crust and at its surface: earthquakes, volcanic eruptions, tsunamis, landslides, avalanches and coastal erosion. Hydro-meteorological hazards come from weather, climate and water: floods, droughts, tropical cyclones, storm surges, heatwaves and bushfires. Biological hazards involve living organisms: infectious diseases carried by water, air or animals, and invasions by introduced plants and animals such as blackberry or the cane toad.

Technological hazards are created or made worse by human systems: oil spills, chemical and radiation leaks, air pollution, flooding that follows land clearing, and the intensification of some weather extremes and rising seas attributed to human-induced climate change. The categories are not watertight. A flood may be hydro-meteorological in its trigger and technological in part of its cause, so always state the reasoning behind a classification.

Causes interconnect. A primary hazard often generates secondary hazards: an offshore earthquake can generate a tsunami if it displaces enough seawater, commonly through sudden vertical movement of the sea floor; earthquake shaking can also trigger landslides; a tropical cyclone can bring coastal storm surge and heavy rain that causes flooding, leaving standing water where mosquitoes can breed; drought can increase bushfire risk and leave dry, poorly vegetated soil vulnerable to erosion by strong winds.

When a question asks about interconnection between causes, trace the chain in order, name the type of each link and explain the physical or human mechanism that connects one link to the next.

The overview of hazards uses a shared vocabulary built from the key geographical concepts. Distribution is the arrangement of events across space and time: earthquakes cluster along plate boundaries, riverine floods occupy floodplains, and the burden of vector-borne disease is highest in tropical and subtropical regions, although each disease has its own distribution shaped by its vector, environment and social conditions. Location may be absolute (latitude and longitude, a grid reference) or relative (distance and direction from somewhere else).

Scale runs from local to regional, national and global, and the same hazard can be studied at more than one scale. Frequency is how often events of a given size recur, usually expressed as an average recurrence interval or an annual exceedance probability (AEP). Sequence is the order of stages, for example rainfall → saturated soil → runoff → rising river → peak → recession, or introduction → establishment → spread for an invasive species.

Magnitude is the size or energy of an event: moment magnitude for earthquakes, peak height in metres or discharge in m³/s for floods, wind-based category for tropical cyclones, or population density for a pest. Cyclone category does not measure rainfall or storm-surge severity. For a given location and hazard measure, exceedance of a larger threshold cannot be more frequent than exceedance of a smaller one; the actual frequency relationship must be estimated from appropriate data.

Hazards also do work within natural systems. Floods can deposit sediment and redistribute nutrients, replenish wetlands and groundwater, and support native fish breeding. Fire can stimulate regeneration in fire-adapted heath and eucalypt communities, but effects depend on the fire regime and species; fires too close together can prevent recovery. Volcanic ash can develop into productive soil, although weathering and nutrient availability affect fertility.

A hazard is identified by its potential to cause harm, including environmental damage; exposure and vulnerability influence the risk of actual losses.