Impacts on people and the environment: short-term impacts and long-term impacts

Earthquake and Volcanic Hazards – Impacts on People and the Environment

Learning Objective

Understand, analyse and evaluate the short‑term and long‑term impacts of earthquakes and volcanic eruptions on societies and natural systems, and assess how these hazards are identified, predicted and managed in line with the Cambridge AS & A‑Level Geography (9696) syllabus.

1. Hazardous Environments – Global Distribution & Tectonic Setting

1.1 Major Tectonic Plates (syllabus requirement)

• Pacific Plate – dominates the “Ring of Fire” (high seismicity & volcanism).
• Eurasian Plate – includes the Alpide belt (Mediterranean → Himalaya) and parts of the Atlantic margin.
• North American Plate – western margin (San Andreas) and Atlantic ridge.
• South American Plate – Andes subduction zone.
• African Plate – East African Rift and western Mediterranean.
• Indo‑Australian Plate – Sunda‑Andaman trench, New Zealand fault systems.
• Antarctic Plate – largely passive, but with some marginal volcanism.

1.2 Earthquakes

• Major seismic belts – Pacific “Ring of Fire”, Alpide belt, East African Rift.
• Plate‑boundary types
  • Transform – e.g., San Andreas (Pacific‑North American), North Anatolian (Eurasian‑Anatolian).
  • Convergent (subduction) – e.g., Japan (Pacific‑Eurasian), Chile (Nazca‑South American); produce the largest magnitudes.
  • Divergent – mid‑ocean ridges and continental rifts (e.g., East African Rift); generate shallow, moderate quakes.

1.3 Volcanic Eruptions

• Global volcanic arcs and belts – Pacific Ring of Fire, Mediterranean‑East African volcanic belt, Icelandic hotspot.
• Plate‑boundary contexts
  • Convergent (subduction) – explosive stratovolcanoes such as Mount Pinatubo (Philippines) and Vesuvius (Italy).
  • Divergent – fissure eruptions and basaltic shield volcanoes (Iceland, East African Rift).
  • Intraplate/hotspot – Hawaiian Islands, Yellowstone (North American Plate). Hotspot volcanism is a required syllabus topic.

2. Short‑Term Impacts

2.1 Earthquakes

• Primary effects
  • Ground shaking – collapse of buildings, bridges and lifelines.
  • Surface rupture – breaks pipelines, roads and railways.
  • Seismic‑induced tsunami – offshore thrust events generate destructive waves.
• Secondary effects
  • Liquefaction – loss of ground strength, causing sinking or tilting of structures.
  • Landslides & rockfalls on steep slopes.
  • Fire outbreaks from ruptured gas or electricity lines.
• Human consequences
  • Immediate fatalities and injuries.
  • Displacement – emergency shelters, temporary camps.
  • Disruption of water, electricity, health and communication services.

2.2 Volcanic Eruptions

• Explosive eruptions
  • Pyroclastic flows – high‑temperature, fast‑moving gas‑particle mixtures that incinerate everything in their path.
  • Ash fall – reduces visibility, contaminates water supplies, damages machinery and crops.
  • Volcanic bombs & lapilli – cause direct injuries and property damage.
• Effusive eruptions
  • Lava flows – destroy infrastructure, farmland and transport routes.
• Secondary hazards
  • Lahars – volcanic mudflows that travel kilometres downstream, burying settlements.
  • Volcanic gases (SO₂, CO₂, H₂S, HF) – respiratory problems, acid rain, fluorosis in livestock.
• Human consequences
  • Immediate fatalities and injuries.
  • Mass evacuations and temporary relocation.
  • Loss of agriculture, tourism and transport networks.

3. Long‑Term Impacts

3.1 Earthquakes

3.2 Volcanic Eruptions

4. Risk Identification, Prediction & Early‑Warning

• Hazard‑mapping tools
  • Seismic hazard maps (peak ground acceleration, PGA).
  • Volcanic hazard maps – zones for lava, ash fall, lahars, pyroclastic density currents.
  • GIS layers showing fault lines, plate boundaries, population density and critical infrastructure.
  • Recurrence‑interval tables for major events (e.g., magnitude ≥ 7.0 every 50 years on the San Andreas).
• Prediction & monitoring
  • Earthquake: global and regional seismograph networks, GPS crustal deformation, foreshock‑mainshock‑aftershock patterns.
  • Volcano: seismicity, ground deformation (tiltmeters, InSAR), gas emissions (SO₂, CO₂), thermal imaging, satellite ash‑plume tracking.
• Early‑warning systems
  • ShakeAlert (US West Coast) – seconds to minutes of warning before strong shaking.
  • Tsunami warning centres – real‑time sea‑level gauges and DART buoys.
  • Volcanic alert levels (e.g., PHIVOLCS, USGS) communicated via radio, SMS, sirens and community networks.

5. Management & Mitigation Strategies

6. Detailed Case Studies (Evaluation of Management Success)

6.1 2010 Haiti Earthquake (Mw 7.0)

• Cause & setting – Strike‑slip fault within the Caribbean Plate; shallow focus (~13 km).
• Short‑term impacts – >230 000 deaths, massive building collapse, loss of hospitals and water supply.
• Prediction & warning – No reliable short‑term prediction; limited seismic monitoring.
• Management actions
  • International humanitarian response – field hospitals, UN‑led coordination.
  • Post‑event reconstruction plan (2009‑2015) emphasising “build back better”.
• Evaluation
  • Strengths: rapid mobilisation of aid, establishment of a central coordination mechanism (UNOCHA).
  • Weaknesses: weak pre‑existing building codes, poor enforcement, limited local capacity, prolonged delays in reconstruction, many displaced people remained in informal camps after five years.
  • Overall assessment: limited success in reducing long‑term vulnerability; highlighted the need for stronger seismic‑resistant design and community preparedness.

6.2 1991 Mount Pinatubo Eruption (Philippines)

• Cause & setting – Subduction of the Philippine Sea Plate beneath the Eurasian Plate; explosive stratovolcano.
• Short‑term impacts – 847 deaths (mainly from lahars), widespread ash fall, evacuation of ~200 000 people.
• Prediction & warning
  • Intensive monitoring (seismicity, gas emissions, ground deformation) detected unrest two months before eruption.
  • PHIVOLCS issued a four‑level alert system; evacuation orders were implemented.
• Management actions
  • Large‑scale pre‑emptive evacuation and relocation of villages.
  • Construction of lahar diversion dikes and early‑warning sirens.
  • Post‑eruption re‑forestation and livelihood programmes.
• Evaluation
  • Strengths: successful evacuation saved an estimated 70 % of the at‑risk population; effective coordination between USGS, PHIVOLCS and local authorities.
  • Weaknesses: some remote communities received delayed warnings; long‑term displacement created social stress.
  • Overall assessment: high success in reducing immediate loss of life; long‑term mitigation (diversion channels, land‑use changes) has reduced lahar risk in subsequent years.

7. Comparative Summary of Impacts & Management

8. Key Formula – Energy Release of an Earthquake

The relationship between moment magnitude (M) and energy released (E) in joules is approximated by:

$$E = 10^{1.5M + 4.8}$$

Thus, an increase of one magnitude unit corresponds to roughly 32 times more energy.

9. Suggested Diagrams

• World map showing major seismic belts, volcanic arcs, plate‑boundary symbols and hotspot locations.
• Cross‑section of a subduction zone illustrating megathrust earthquake generation and associated tsunami.
• Cross‑section of a stratovolcano displaying magma chamber, conduit, vent and typical hazards (lava flow, pyroclastic flow, ash plume, lahars).
• Flowchart of the earthquake early‑warning process (detection → alert → public response).
• Diagram of a lahar diversion system (check‑dam, channel, overflow protection).

10. Review Questions

1. Explain how liquefaction can intensify the short‑term impacts of an earthquake on urban infrastructure. Include at least two examples of secondary damage.
2. Discuss two ways in which volcanic ash can be both detrimental and beneficial to the environment and local economies.
3. Compare the long‑term socio‑economic and political impacts of a major earthquake with those of a major volcanic eruption.
4. Using the formula for earthquake energy release, calculate the energy difference between a magnitude 6.0 and a magnitude 7.0 event.
5. Evaluate the effectiveness of early‑warning systems for earthquakes and volcanoes, citing specific case studies from the notes.