Global hazards

Risk is the probability (likelihood) that a hazard will cause harm — loss of life, injury or damage — to a population.

It reflects both the chance of the hazard occurring and the exposure and vulnerability of the people and property that could be affected, and is often expressed as Risk = (Hazard × Vulnerability) / Capacity to cope.

Vulnerability is the extent to which a population is susceptible to the damaging effects of a hazard — it is raised by poverty, poor-quality housing, high population density and lack of warning. Higher vulnerability increases disaster risk.

Capacity to cope is a community's ability to withstand, respond to and recover from a hazard using its own resources — money, emergency services, planning, education and technology. Higher capacity to cope reduces disaster risk.

In the risk equation, Risk = (Hazard × Vulnerability) / Capacity to cope, so risk rises with vulnerability (in the numerator) and falls as capacity to cope (in the denominator) increases.

Geophysical hazards are driven by processes inside the Earth (tectonic/plate activity). They include earthquakes, volcanic eruptions and tsunami, which occur mainly at or near plate margins (for example earthquakes along the San Andreas Fault in California).

Hydro-meteorological hazards are driven by atmospheric and hydrological (climatic) processes. They include tropical cyclones, floods and drought, and their distribution is controlled by climate and weather systems rather than tectonics (for example typhoons striking the Philippines).

The key distinction is the cause: geophysical hazards come from plate tectonics/the Earth's interior, whereas hydro-meteorological hazards come from the atmosphere, weather and water. Some hazards (e.g. landslides) can be triggered by either.

The resource shows a clear inverse relationship between magnitude and frequency. Small-to-moderate earthquakes (magnitude 4.0–4.9) are extremely common — about 13 000 per year, whereas major earthquakes (magnitude 7.0–7.9) are rare — only about 15 per year. As magnitude rises, frequency falls steeply.

This can be explained using the logarithmic nature of the moment magnitude scale. Each whole step up the scale represents a ten-fold increase in the amplitude of ground shaking and roughly a 30-fold increase in the energy released. Moving from magnitude 4 to magnitude 7 is therefore an increase of about $10^3 = 1000$ times in amplitude and tens of thousands of times in energy.

Because each larger earthquake requires the build-up and sudden release of vastly more strain energy along a fault, such events happen far less often — the Earth simply cannot generate huge quakes as frequently as small ones. This is why the resource shows thousands of small tremors but only a handful of major earthquakes each year.

For hazard management this matters: the rare, high-magnitude events cause most of the damage and deaths, so places on active margins (e.g. California) must prepare for events that occur only occasionally but are devastating.

A mature tropical cyclone has a distinctive structure organised around a centre of very low pressure.

On landfall the cyclone causes damage in several ways:

• Strong winds (often exceeding 118 km/h, category 1+ on the Saffir–Simpson scale) destroy buildings, uproot trees and cut power and communications.
• Storm surge — the low pressure and onshore winds raise sea level by several metres, flooding low-lying coasts; this is usually the biggest killer (e.g. Typhoon Haiyan's surge in the Philippines, 2013).
• Torrential rainfall causes river flooding and landslides, especially on steep, saturated slopes.

The cyclone weakens once over land because it is cut off from its warm-ocean energy source, but by then it may already have caused a major disaster, particularly where the population is vulnerable and low-lying.

The type of plate margin strongly controls which geophysical hazards occur and how powerful they are, because each margin involves different plate movements and stresses.

Destructive (convergent) margins produce the most powerful and varied hazards. Where an oceanic plate is subducted beneath another plate, friction locks the plates so huge stress builds and is released as large, sometimes deep-focus earthquakes. Subduction also melts crust to form magma, giving explosive, high-VEI volcanoes (e.g. Mount Pinatubo, Philippines). If the seabed is displaced, a tsunami can be generated. These margins therefore produce earthquakes, volcanoes and tsunami, often of high magnitude.

Collision margins, where two continental plates meet and neither subducts, produce powerful earthquakes as crust buckles (e.g. the Himalayas), but little or no volcanic activity because there is no subduction to generate magma.

Constructive (divergent) margins, where plates move apart and magma rises, produce frequent but generally gentle (effusive), low-VEI volcanic eruptions and shallow, low-magnitude earthquakes — the plates are moving apart rather than locking, so less stress accumulates (e.g. Iceland on the Mid-Atlantic Ridge).

Conservative (transform) margins, where plates slide past each other, produce earthquakes but no volcanoes, because crust is neither created nor destroyed. Friction can still lock the fault and store large amounts of energy, so magnitudes can be high (e.g. the San Andreas Fault, California).

In conclusion, margin type determines the hazard 'menu': destructive margins are the most dangerous (earthquakes + explosive volcanoes + tsunami), collision and conservative margins produce mainly earthquakes, and constructive margins produce gentler activity. Magnitude tends to be greatest where friction locks plates and releases energy suddenly (destructive, collision and conservative margins), and lowest where plates move apart freely (constructive).

Whether a natural hazard becomes a major disaster is captured by the equation Risk = (Hazard × Vulnerability) / Capacity to cope. This immediately suggests that the physical magnitude of the hazard is only one of three controls, and that human factors — vulnerability and capacity to cope — are at least as important. This essay examines both sides before reaching a judgement.

The case that human factors dominate. Physical magnitude alone does not determine the death toll. The 2010 Haiti earthquake (Mw 7.0) killed an estimated 200 000–250 000 people because Haiti is a low-income country with widespread informal housing, weak building codes, poor emergency services and a fragile government — very high vulnerability and very low capacity to cope. By contrast, the 2010 Chile earthquake (Mw 8.8) — which released roughly 500 times more energy — killed only around 500 people, because Chile is wealthier, enforces earthquake-resistant building codes and has effective planning and warning. A far larger physical hazard produced a far smaller disaster, strongly supporting the view. The Philippines, a multiple-hazard hotspot, shows the same pattern: Typhoon Haiyan (2013) was so deadly partly because of dense coastal populations in flimsy housing with limited evacuation capacity, rather than wind speed alone.

The case that physical magnitude still matters. Human factors cannot explain everything. Some events are so large that even well-prepared, wealthy places suffer major disasters. The 2011 Tōhoku earthquake and tsunami in Japan (Mw 9.0) struck one of the best-prepared nations on Earth — with strict building codes, sea walls and warning systems — yet still killed around 18 000 people, because the tsunami's physical scale overwhelmed the defences. Volcanic magnitude matters too: a high-VEI eruption or a very large earthquake near a coast (generating a tsunami) creates hazards that no level of preparedness can fully neutralise. Rapid-onset, high-magnitude events also give little warning time, limiting how far capacity to cope can reduce losses.

The role of exposure and location. A further human/physical interaction is where people live. High-magnitude events in remote areas cause few deaths, whereas moderate events beneath megacities can be catastrophic — so population distribution and density (human factors) interact with magnitude to determine impact. California faces frequent earthquakes and wildfires of significant magnitude, but its wealth, strict codes and preparedness keep death tolls comparatively low, again showing capacity to cope moderating a real physical hazard.

Conclusion. On balance, human factors are usually the decisive control on the scale of a disaster: comparing Haiti (Mw 7.0, ~230 000 deaths) with Chile (Mw 8.8, ~500 deaths) shows that vulnerability and capacity to cope can matter far more than raw magnitude. However, this is not absolute — for the very largest, fastest events such as Tōhoku, physical magnitude can overwhelm even the best preparation. The most accurate view is therefore that human factors determine the scale of most disasters, but physical magnitude sets an upper limit that human factors cannot always overcome — exactly as the risk equation implies.

Some places are multiple-hazard hotspots — exposed to two or more hazards at once — so managing their disaster risk is especially challenging. Using the Philippines, this essay assesses how far management can reduce risk, structured around the equation Risk = (Hazard × Vulnerability) / Capacity to cope.

Why the Philippines is a hotspot. It lies on the Pacific Ring of Fire, where the Philippine and Eurasian plates converge, producing frequent earthquakes, active volcanoes (e.g. Pinatubo, Mayon) and tsunami. It also lies in the tropical western Pacific and is struck by about 20 typhoons a year (e.g. Typhoon Haiyan, 2013, which killed over 6 000 people), bringing flooding, storm surge and landslides. Its hazards therefore span both the geophysical and hydro-meteorological categories, and they can compound — typhoon rainfall triggering landslides or lahars on volcanic slopes.

Ways management can reduce the risk. Because the hazard term cannot be reduced — the plates and typhoons cannot be stopped — management works by lowering vulnerability and raising capacity to cope:

• Prediction and warning. The Philippines uses PAGASA for typhoon tracking and PHIVOLCS for volcano and earthquake monitoring; accurate typhoon forecasts and evacuation orders now save many lives, and Pinatubo's 1991 eruption was successfully predicted, allowing evacuation.
• Preparedness and education. Evacuation drills, hazard mapping, early-warning sirens and community education raise capacity to cope; storm shelters and improved evacuation reduced deaths in typhoons after Haiyan.
• Building design and land-use planning. Earthquake-resistant construction and restrictions on building in flood- or surge-prone coastal zones lower vulnerability.
• International aid and recovery help the country rebuild more resiliently ('building back better').

Limits to management. However, the ability to reduce risk is constrained:

• The Philippines is a middle-income country with large populations in informal housing and a heavy reliance on farming and fishing, so vulnerability remains high and defences are not universal.
• Multiple, compounding hazards are hard to plan for simultaneously — resources spent on typhoon defence may not protect against earthquakes.
• The sheer magnitude of some events (Haiyan's storm surge) can overwhelm even improved preparation, and rapid population growth and coastal urbanisation continually increase exposure.
• Climate change may be increasing the intensity of typhoons, raising the hazard term over time.

Conclusion. Management can significantly reduce the Philippines' disaster risk, but cannot eliminate it. Prediction, warning, preparedness and better building have clearly lowered death tolls — the successful evacuations before Pinatubo and improvements since Haiyan show this. Yet because the underlying hazards cannot be removed, vulnerability remains high in poorer communities, and hazards compound, a residual risk will always exist. The most accurate assessment is that management reduces the impact (deaths and disruption) far more effectively than it reduces the underlying hazard, so investment is best targeted at cutting vulnerability and raising capacity to cope, exactly the two terms in the risk equation that human action can change.

Geophysical hazards (earthquakes, volcanoes, tsunami) and hydro-meteorological hazards (tropical cyclones, floods, drought) differ fundamentally in how far they can be predicted, warned against and mitigated. Because the disaster risk equation, Risk = (Hazard × Vulnerability) / Capacity to cope, shows that the hazard itself cannot be stopped, 'management' means prediction, warning, preparedness and protection — and it is here that the two hazard types differ. This essay assesses the extent to which geophysical hazards are the harder to manage.

The case that geophysical hazards ARE harder to manage. The strongest argument concerns prediction and warning. Earthquakes cannot yet be reliably predicted — there is no way to say when a fault such as the San Andreas in California will rupture, so warning times are seconds at best. This makes evacuation before the event almost impossible, unlike storms. Tsunami, generated by sudden seabed displacement, can also strike nearby coasts within minutes. Geophysical events are therefore rapid-onset with little or no warning, which limits how far capacity to cope can reduce deaths.

The case that they are NOT always harder — hydro-meteorological hazards bring their own difficulties. However, some geophysical hazards can be managed well. Volcanoes are often predictable: monitoring of ground swelling, gas emissions and seismicity allowed the successful evacuation before Mount Pinatubo (Philippines, 1991), saving thousands of lives. Meanwhile hydro-meteorological hazards are not automatically 'easy'. Although tropical cyclones can be tracked for days (giving warning that geophysical hazards lack), their sheer scale can still overwhelm defences — Typhoon Haiyan (Philippines, 2013) was forecast, yet its storm surge still killed over 6 000 people because coastal populations were vulnerable and evacuation capacity limited. Drought is slow-onset and easy to detect but very hard to solve, because it stems from large-scale climate patterns such as ENSO and can last years.

The role of vulnerability and wealth. For both hazard types, management success depends on capacity to cope. California faces frequent, largely unpredictable earthquakes yet keeps death tolls low through strict building codes, land-use planning and preparedness — so a 'hard-to-predict' geophysical hazard is still well managed. In poorer parts of the Philippines, both typhoons and earthquakes cause high losses because vulnerability is high and resources limited. This suggests the decisive factor is wealth and preparedness, not simply the hazard category.

Conclusion. On balance, I partly agree: geophysical hazards — especially earthquakes and tsunami — are genuinely harder to manage because they cannot be predicted and are rapid-onset, so warning and evacuation are difficult (California's earthquake risk illustrates this). However, the statement is too sweeping. Volcanoes are often predictable (Pinatubo), while large hydro-meteorological events can overwhelm even forewarned communities (Haiyan) and drought is very hard to resolve. The most accurate view is that manageability depends less on whether a hazard is geophysical or hydro-meteorological and more on its speed of onset, predictability and, above all, the affected community's vulnerability and capacity to cope.