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Understanding tectonic hazards requires moving beyond a purely physical analysis of earthquakes and volcanic eruptions to consider why some communities are more vulnerable than others. This lesson introduces hazard profiles, the Pressure and Release (PAR) model, and a detailed examination of the factors that determine vulnerability. This content is central to Edexcel A-Level Geography Enquiry Question 2 (EQ2): Why do some tectonic hazards develop into disasters?
Not all tectonic hazards become disasters. A hazard is a natural process with the potential to cause harm. A disaster occurs when a hazard overwhelms the capacity of a community to cope. The United Nations defines a disaster as:
"A serious disruption of the functioning of a community or a society at any scale due to hazardous events interacting with conditions of exposure, vulnerability and capacity, leading to human, material, economic and/or environmental losses and impacts." — UNDRR (2017)
The disaster risk equation formalises this:
Risk = (Hazard × Vulnerability) / Capacity
This equation demonstrates that disaster is not an inevitable consequence of a physical hazard — it results from the interaction between the hazard event and the social, economic and political context in which it occurs.
| Term | Definition | Example |
|---|---|---|
| Hazard | The physical event itself | An Mw 7.0 earthquake |
| Exposure | People and assets in the hazard zone | 3 million people living near the fault |
| Vulnerability | Susceptibility to harm | Poorly constructed buildings, poverty, lack of warning systems |
| Capacity | Ability to prepare, respond and recover | Building codes, emergency services, insurance, education |
| Risk | Probability of loss | Higher where vulnerability exceeds capacity |
A hazard profile is a framework for comparing different types of tectonic hazard across several key characteristics. This allows geographers to assess the relative risk posed by different hazards and to compare events at different locations.
| Dimension | Description | Earthquake Example | Volcanic Eruption Example |
|---|---|---|---|
| Magnitude | Physical size/energy of the event | Mw 9.1 (Sumatra 2004) | VEI 6 (Pinatubo 1991) |
| Speed of onset | How quickly the hazard develops | Seconds (no warning) | Days to weeks (often precursors) |
| Duration | How long the event lasts | Seconds to minutes (shaking) | Days to months (eruption) |
| Areal extent | Area affected by the hazard | 10s to 1,000s km² | Variable: local (lava) to global (ash/climate) |
| Predictability | How accurately onset can be forecast | Very low (no reliable short-term prediction) | Moderate (monitoring can detect precursors) |
| Frequency (recurrence) | How often events of this magnitude occur | Variable (seismic gaps provide some indication) | Variable (eruption history guides expectations) |
graph LR
A["Hazard Profile"] --> B["Magnitude"]
A --> C["Speed of Onset"]
A --> D["Duration"]
A --> E["Areal Extent"]
A --> F["Predictability"]
A --> G["Frequency"]
| Dimension | Earthquake | Volcanic Eruption |
|---|---|---|
| Magnitude | Can be extremely high (Mw 9+) | Variable (VEI 0–8) |
| Speed of onset | Virtually instantaneous (no warning) | Usually hours to weeks of precursory activity |
| Duration | Seconds to minutes | Hours to years |
| Areal extent | Moderate (shaking); large (tsunami) | Highly variable: pyroclastic flows (local), ash (regional), climate (global) |
| Predictability | Very low | Moderate — seismic, gas and deformation monitoring |
| Frequency | High (thousands of significant events per year globally) | Lower (50–70 eruptions per year globally) |
Exam Tip: Hazard profiles help explain why earthquakes are generally more deadly than volcanic eruptions. Earthquakes have virtually zero warning time and cannot be predicted reliably, whereas volcanic eruptions usually provide days or weeks of precursory signs, allowing evacuation. However, the unpredictability of earthquakes does not fully explain disaster outcomes — vulnerability and capacity matter enormously, as the PAR model demonstrates.
The Pressure and Release (PAR) model, developed by Wisner, Blaikie, Cannon and Davis (1994, revised 2004) in their influential book At Risk, provides a framework for understanding why disasters occur. The model argues that disasters happen when vulnerability and hazard intersect — and that vulnerability is the product of social, economic and political processes, not just physical exposure.
The PAR model identifies three progressively specific levels of vulnerability that "squeeze" a community toward disaster:
1. Root Causes — the deep-seated, structural factors in society that create vulnerability:
2. Dynamic Pressures — the processes that channel root causes into specific forms of vulnerability:
3. Unsafe Conditions — the specific circumstances that directly expose people to harm:
| PAR Level | Haiti (2010 earthquake, Mw 7.0) | Japan (2011 earthquake, Mw 9.1) |
|---|---|---|
| Root causes | Colonial legacy, political instability, extreme poverty, US-backed dictatorships, structural debt | Stable democracy, high GDP, strong institutions |
| Dynamic pressures | Rapid urbanisation (Port-au-Prince grew from 250,000 to 3 million in 50 years), deforestation (2% forest cover), no building codes enforced, brain drain of educated professionals | Advanced technology, continuous research investment, strong building codes since 1950, public education |
| Unsafe conditions | Dense informal settlements on unstable slopes, unreinforced masonry, no seismic building design, minimal emergency services | Seismically engineered buildings, tsunami walls, early warning systems, regular evacuation drills |
| Outcome | ~100,000–316,000 deaths; 1.5 million homeless | ~18,500 deaths (mostly tsunami); rapid recovery of infrastructure |
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