Why disaster occurrence and impact varies, using the risk equation, vulnerability, resilience, the Pressure and Release model, hazard profiles and the distinction between primary, secondary and tertiary impacts.

What this dot point is asking

Edexcel wants you to explain why disaster occurrence and impact vary, even between hazards of similar magnitude. You need the risk equation, the ideas of exposure, vulnerability and resilience, the Pressure and Release model, hazard profiles, the distinction between primary, secondary and tertiary impacts, and the indices used to measure hazards and development.

The risk equation, exposure, vulnerability and resilience

The equation is usually written as:

This makes three ideas explicit. Exposure is the number of people and the value of assets located where a hazard can strike; coastal megacities and dense informal settlements raise exposure. Vulnerability is their susceptibility to harm, shaped by economic factors (income, insurance), social factors (age, health, education, population density), political factors (governance, corruption, planning enforcement) and environmental factors (unstable slopes, reclaimed land). Resilience is the capacity to absorb a shock and recover quickly, through savings, services, social networks and effective institutions.

These factors explain why urbanisation, poor building quality and informal housing raise the death toll. In Haiti, rapid urban growth had filled Port-au-Prince with unregulated, unreinforced concrete buildings, so a moderate quake pancaked them; high vulnerability and low capacity to cope turned a $M_w\,7.0$ hazard into a mega-disaster.

The Pressure and Release model

The PAR model explains a disaster as the meeting point of two forces: the natural hazard on one side, and a progression of vulnerability on the other.

The model is powerful because it shifts attention from the trigger to the social conditions that decide who suffers. In Haiti the root cause was entrenched poverty and a weak state; the dynamic pressure was unplanned urban growth; the unsafe condition was the housing stock. Releasing the pressure, by improving governance, planning and construction, reduces disaster risk even if the hazard is unchanged.

Hazard profiles, impacts and measurement

A hazard profile lets you compare hazards across several characteristics rather than magnitude alone: magnitude (size), speed of onset (sudden earthquakes versus slower volcanic build-up), duration, areal extent, spatial predictability (margins are mappable) and frequency. Profiling explains why a slow-onset, predictable event may cause fewer deaths than a sudden one of equal magnitude.

Impacts are layered. Primary impacts are direct (collapsed buildings, deaths from shaking). Secondary impacts are knock-on effects (tsunamis, fires, liquefaction, disease). Tertiary impacts are long-term (economic decline, migration, lost development). Hazards are measured with the moment magnitude scale (MMS) for earthquakes, the Volcanic Explosivity Index (VEI) for eruptions and the Mercalli scale for felt intensity, while the Human Development Index (HDI) indexes the development that shapes vulnerability.

Examples in context

Example 1: Tohoku, Japan (2011). A $M_w\,9.0$ subduction earthquake off Honshu generated a tsunami over 10 m high. Primary shaking deaths were limited by aseismic building, but the secondary tsunami caused most of the roughly $18{,}000$ deaths and the Fukushima Daiichi meltdown, a tertiary impact lasting decades. As a high-income, high-resilience country, Japan's economic loss neared \235$ billion yet relative mortality was low, showing high capacity to cope.

Example 2: Sichuan, China (2008). An $M_w\,7.9$ quake on a thrust fault killed around $87{,}000$ people. Poorly constructed schools collapsed, raising child deaths and exposing weak enforcement of building codes in a rapidly developing region, a clear PAR dynamic pressure. The 2004 Indian Ocean tsunami, which killed around $230{,}000$ across the basin, shows the same lesson at ocean scale: no warning system meant exposure could not be reduced.

Try this

Q1. Explain why two earthquakes of similar magnitude can produce very different death tolls. [4 marks]

• Cue. Use the risk equation: vulnerability and capacity to cope, shaped by development, building quality and warning, vary between places.

Q2. Outline how a hazard profile helps compare different tectonic hazards. [4 marks]

• Cue. Name the characteristics (magnitude, speed of onset, duration, areal extent, spatial predictability, frequency) and link onset and predictability to whether exposure can be reduced.