Earthquake hazards and impacts

Earthquake Hazards and Impacts (Cambridge 9696)

1. Why Earthquakes Occur

Earthquakes are the sudden release of elastic‑strain energy that has accumulated in the Earth’s crust, usually when rocks slip along a fault.

• Tectonic plate boundaries – transform, convergent and divergent margins.
• Intraplate fault zones – stress builds up away from plate edges.
• Volcanic processes – magma intrusion and eruption generate seismic waves.

1.1 Plate‑boundary types and typical seismic / volcanic activity

2. Global Distribution of Earthquakes & Volcanoes

Seismicity and volcanism are concentrated along plate boundaries, producing the classic “Ring of Fire” around the Pacific Ocean where > 75 % of world‑wide earthquakes occur. The distribution can be summarised by boundary type:

• Convergent margins – high‑magnitude megathrust quakes (Chile, Japan) and volcanic arcs.
• Divergent margins – low‑magnitude swarms and basaltic eruptions (Iceland, East African Rift).
• Transform faults – shallow, strike‑slip events (California, Turkey).
• Intraplate settings – occasional large quakes away from margins (New Madrid, USA) and hot‑spot volcanoes (Hawaii).

3. Measuring Earthquakes

1. Magnitude – quantifies the energy released.

   Moment Magnitude Scale (M_w) is the most widely used:

   $$M_w = \frac{2}{3}\log_{10}M_0 - 6.07$$

   where M₀ is the seismic moment (N·m).

2. Intensity – describes the effects of shaking at a specific location.

   The Modified Mercalli Intensity (MMI) scale runs from I (not felt) to XII (total destruction).

4. Primary (Direct) Hazards

• Ground rupture along the fault trace.
• Collapse of poorly designed or unreinforced buildings.
• Loss of life, injuries and immediate displacement.
• Disruption of essential services (water, gas, electricity, communications).

5. Secondary (Indirect) Hazards

• Liquefaction – saturated, loose soils lose strength and behave like a fluid.
• Landslides and rockfalls on steep slopes.
• Tsunamis generated by under‑sea megathrust events.
• Fires caused by ruptured gas lines or electrical faults.
• Aftershocks that re‑damage weakened structures.

6. Vulnerability Factors

6.1 Physical vulnerability

• Soil type – loose, water‑logged sediments increase liquefaction risk.
• Topography – steep slopes favour landslides.
• Groundwater depth – high water tables exacerbate both liquefaction and slope failure.

6.2 Human vulnerability

• Building quality – unreinforced masonry, informal settlements.
• Population density – high‑density urban areas suffer greater casualties.
• Preparedness level – early‑warning systems, emergency services and public education.
• Economic capacity – resources for rapid response and reconstruction.

7. Impacts of Earthquakes

8. Case Studies

8.1 Haiti – 12 January 2010 (M_w = 7.0)

• Epicentre 13 km deep, near Port‑au‑Prince – shallow depth amplified shaking.
• Primary hazards: massive building collapse due to unreinforced masonry and lack of building codes.
• Secondary hazards: landslides in mountainous terrain; limited tsunami risk.
• Impacts: ≈230 000 deaths, 300 000 injured, 1.5 million displaced; economic loss ≈120 % of GDP.
• Vulnerability: high population density, poor construction, weak emergency response capacity.

8.2 Japan – 11 March 2011 (M_w = 9.1, Tōhoku)

• Megathrust event on the subduction zone off Honshu; depth ≈30 km.
• Primary hazards: intense shaking, widespread building damage despite strict codes.
• Secondary hazards: 40 m tsunami and Fukushima nuclear accident.
• Impacts: 15 800 deaths, >2 million households affected; long‑term economic loss ≈US$235 bn.
• Vulnerability: coastal low‑lying communities; high preparedness and robust building standards limited casualties compared with the magnitude.

8.3 Chile – 27 February 2010 (M_w = 8.8)

• Megathrust earthquake on the Nazca‑South American subduction zone; depth ≈35 km.
• Primary hazards: severe ground shaking and extensive building collapse in Concepción.
• Secondary hazards: landslides, liquefaction in the coastal plain, and a locally generated tsunami (≈2 m).
• Impacts: 525 deaths, ≈800 000 people displaced, economic loss ≈US$30 bn (≈2 % of GDP).
• Vulnerability: concentration of population in the Central Valley, variable building quality, but strong national building code and effective emergency response reduced mortality.

9. Management of Earthquake Risk (AO3)

Risk‑reduction strategies can be grouped into hard engineering (physical works) and soft engineering (behavioural or organisational measures), as required by the syllabus.

9.1 Hard engineering measures

• Land‑use planning – avoid construction on active faults, liquefaction‑prone sediments and steep slopes.
• Seismic‑resistant building codes – ductile detailing, base isolation, adequate reinforcement; mandatory retro‑fitting of existing structures where feasible.

9.2 Soft engineering measures

• Early‑warning systems – detect P‑waves and give seconds‑long alerts for automatic shutdowns (trains, gas lines) and for people to “Drop, Cover, Hold”.
• Public education & drills – school programmes, community “earthquake‑ready” campaigns, regular practice of protective actions.
• Post‑disaster recovery planning – rapid damage assessment, provision of temporary shelters, “build back better” reconstruction to increase future resilience.

9.3 Evaluation of Effectiveness (AO3)

• Land‑use planning is highly effective where enforcement is strong (e.g., Japan’s zoning around the Nankai Trough). In many developing nations weak governance permits illegal building on high‑risk sites, limiting impact.
• Building codes dramatically reduce structural failure when properly designed, inspected and retrofitted. However, compliance costs can be prohibitive for low‑income households, creating a dual system of safe and unsafe structures.
• Early‑warning systems provide only seconds of notice, enough for automated shutdowns and for people to take cover, but they cannot prevent damage to poorly built buildings.
• Public education improves individual safety – most people survive if they “Drop, Cover, Hold” – yet behavioural change is uneven; repeated drills are needed to maintain knowledge.
• Recovery planning that incorporates “build back better” can increase long‑term resilience, but it requires substantial funding and political will. In Haiti, the lack of coordinated reconstruction prolonged vulnerability, whereas Chile’s post‑2010 rebuilding programme improved seismic standards.

10. Summary (AO1‑AO3)

Earthquakes result from the release of tectonic stress at plate boundaries and within plates. Their global distribution mirrors the four boundary types, with the Pacific “Ring of Fire” being the most active. Magnitude (M_w) quantifies the energy released; intensity (MMI) records local effects. Primary hazards (ground shaking, building collapse) and secondary hazards (liquefaction, landslides, tsunamis, fires, aftershocks) interact with physical and human vulnerability to produce a range of impacts on people, economies, the environment and societies. The three case studies – Haiti 2010, Japan 2011 and Chile 2010 – illustrate how construction quality, preparedness, governance and socio‑economic capacity shape outcomes. Effective risk reduction combines hard engineering (land‑use planning, seismic‑resistant codes) with soft engineering (early warning, education, recovery planning). Their success depends on local context, enforcement and resources – a key point for the Cambridge A‑Level geography examination.