Earthquake and volcanic hazards: distribution, processes, impacts, management

Hazardous Environments – Earthquake and Volcanic Hazards

Learning Objectives

• Describe the global distribution of earthquakes and volcanoes and explain how each relates to specific plate‑tectonic settings.
• Explain the physical processes that generate earthquakes and volcanic eruptions and state the main magnitude/eruption‑size scales used in the syllabus.
• Analyse primary and secondary impacts on people, infrastructure and the environment, providing quantitative examples and a brief social‑economic breakdown.
• Evaluate the effectiveness of risk‑reduction and management strategies, highlighting strengths, limitations and cost‑benefit considerations.
• Apply knowledge through detailed case studies of a major earthquake and a major volcanic eruption.

1. Earthquake Hazards

1.1 Global Distribution

1.2 Earthquake Magnitude & Intensity Scales

Note: The syllabus expects students to recognise that MMI (formerly the Mercalli scale) links observed damage to ground‑motion intensity, whereas magnitude scales quantify the energy released at the source.

\(M_w = \frac{2}{3}\log_{10}E - 6.07\) where \(E\) = seismic energy (J)

1.3 Physical Processes

1. Elastic‑Rebound Theory – Tectonic stress accumulates in rocks along a fault until the shear strength is exceeded; the fault then slips suddenly, releasing stored elastic strain energy as seismic waves.
2. Energy Release – The amount of energy released (E) is proportional to the seismic moment; this underpins the moment‑magnitude scale.
3. Seismic Waves
   • P‑waves – Primary, compressional, fastest; travel through solids, liquids and gases.
   • S‑waves – Secondary, shear, slower; cannot travel through fluids.
   • Surface waves (Love & Rayleigh) – Travel along the Earth’s surface; cause the greatest ground shaking and damage.
4. Fault Types & Typical Motions
   • Strike‑slip (transform) – Horizontal displacement (e.g., San Andreas Fault).
   • Normal (divergent) – Vertical extension, crust thinning (e.g., East African Rift).
   • Reverse/Thrust (convergent) – Vertical compression, crust thickening (e.g., Himalaya).

1.4 Impacts – Primary, Secondary & Socio‑Economic Consequences

• Primary impacts
  • Ground shaking – building collapse, injuries, fatalities.
  • Surface rupture – displacement of roads, pipelines and utilities.
  • Liquefaction – loss of soil strength, tilting or sinking of structures.
• Secondary impacts
  • Triggered landslides (e.g., 2015 Gorkha, Nepal).
  • Tsunamis – 2011 Tōhoku generated a 38 m wave, >15 000 deaths.
  • Fires from ruptured gas lines, loss of lifelines (water, electricity).
• Socio‑economic consequences
  • Displacement of populations – temporary shelters, loss of housing.
  • Loss of livelihoods – agriculture, tourism and small‑business disruption.
  • Long‑term health impacts – injuries, mental health disorders, disease outbreaks in crowded shelters.
  • Economic cost – direct damage, reconstruction, loss of GDP.
• Quantitative examples
  • 2010 Haiti earthquake (M_w 7.0) – ≈230 000 deaths, US$ 8 bn economic loss.
  • 1995 Kobe, Japan (M_w 6.9) – 6 434 deaths, US$ 100 bn damage.

1.5 Human Vulnerability Factors

1.6 Management, Mitigation & Evaluation

1. Seismic Hazard Mapping
   • GIS‑based maps identify zones of high ground‑motion probability.
   • Strength: Guides land‑use planning and insurance pricing.
   • Limitation: Uncertainty in fault slip rates; costly to keep up‑to‑date.
2. Building Codes & Retrofitting
   • Base isolation, ductile detailing, design spectra based on expected PGA.
   • Strength: Japan’s post‑1995 code revision reduced fatalities by ~30 %.
   • Limitation: High construction costs; uneven enforcement in low‑income housing.
3. Early‑Warning Systems (EWS)
   • Detect P‑waves, issue alerts seconds‑to‑minutes before strong shaking.
   • Strength: Allows automatic shutdown of gas, rail, nuclear plants; “Drop‑Cover‑Hold” drills.
   • Limitation: Limited warning for near‑fault events; risk of public complacency.
4. Public Education & Preparedness
   • School drills, community workshops, media campaigns.
   • Strength: Improves behavioural response, reduces panic.
   • Limitation: Requires sustained funding and cultural adaptation.
5. Post‑Event Response & Recovery
   • Rapid damage assessment, emergency shelters, “Build Back Better” reconstruction guidelines.
   • Risk‑transfer mechanisms (insurance, government aid) mitigate long‑term economic impacts.

1.7 Case Study – 2011 Tōhoku Earthquake & Tsunami (Japan)

• Event details: M_w 9.1, offshore megathrust on the Pacific Plate subducting beneath the North American Plate.
• Primary impacts
  • Severe ground shaking (MMI IX‑X) on the east coast.
  • Coastal tsunami – 38 m maximum run‑up, inundated ≈2 500 km of shoreline.
  • Liquefaction in reclaimed ports, extensive infrastructure damage.
• Secondary impacts
  • Fukushima Daiichi nuclear accident – release of radionuclides, long‑term evacuation.
  • Displacement of >470 000 people; loss of agricultural land.
  • Economic loss ≈ US$ 235 bn (≈10 % of Japan’s GDP).
• Management evaluation
  • Advanced tsunami EWS gave up to 45 min warning – saved many lives.
  • Modern building codes performed well on the mainland; however, low‑rise coastal housing suffered higher damage.
  • Recovery highlighted the importance of integrated risk‑transfer (insurance, state aid) and the need for resilient energy infrastructure.

2. Volcanic Hazards

2.1 Global Distribution

2.2 Volcanic Size & Eruption‑Intensity Scales

Note for AO2: Students should be able to interpret VEI values on eruption tables and relate effusion rates to lava‑flow length and hazard zones.

2.3 Physical Processes

1. Magma Generation
   • Decompression melting – occurs at divergent boundaries where mantle upwelling reduces pressure.
   • Flux melting – addition of water from a subducting slab lowers the mantle solidus, producing silica‑rich magmas.
   • Hot‑spot mantle plume – anomalously hot material rises from deep mantle, generating basaltic magma.
2. Magma Ascent & Volatile Exsolution – Buoyancy drives magma upward; decreasing pressure causes dissolved gases (H₂O, CO₂, SO₂) to exsolve, sharply increasing internal pressure.
3. Eruption Styles
   • Explosive – High silica, high viscosity, abundant gases; produces pyroclastic density currents (PDCs) and tall ash columns (Plinian eruptions).
   • Effusive – Low silica, low viscosity; fluid lava flows travel great distances (Hawaiian eruptions).
4. Volcanic Products
   • Lava flows (basaltic, andesitic, rhyolitic).
   • Pyroclastic density currents – hot, fast‑moving mixtures of gas and rock fragments.
   • Ash fall – fine particles that can travel thousands of kilometres.
   • Lahars – volcanic mudflows created when ash mixes with water.
   • Gases – SO₂, CO₂, H₂S; affect air quality, climate and human health.

2.4 Impacts – Primary, Secondary & Socio‑Economic Effects

• Primary impacts
  • Lava inundation – destruction of homes, roads, farmland.
  • Pyroclastic flows – temperatures > 700 °C, speeds up to 700 km h⁻¹; high mortality.
  • Ash fall – roof collapse, aviation hazards, disruption of electricity and water supplies.
• Secondary impacts
  • Lahars – can travel > 100 km, bury settlements (e.g., 1991 Pinatubo lahars in Pampanga).
  • Volcanic gases – respiratory problems, acid rain, long‑term soil acidification.
  • Atmospheric effects – SO₂ → sulfate aerosols → short‑term global cooling (Pinatubo lowered global temps by ~0.5 °C for 2 years).
• Socio‑economic consequences
  • Displacement of populations; loss of housing and livelihoods.
  • Tourism decline (e.g., Iceland’s 2010 ash cloud).
  • Economic losses from flight cancellations, agricultural damage and health costs.
• Quantitative examples
  • 1991 Mount Pinatubo (VEI 6) – ≈ 800 km³ tephra, > 5 million t of SO₂ released.
  • 2010 Eyjafjallajökull (Iceland) – ash cloud grounded > 100 000 flights, costing ≈ US$ 1.7 bn.

2.5 Human Vulnerability Factors

2.6 Management, Mitigation & Evaluation

1. Volcano Monitoring & Early‑Warning
   • Seismicity, ground deformation (GPS, InSAR), gas emissions, thermal imaging.
   • Strength: Allows authorities to raise alert levels and organise evacuations.
   • Limitation: False alarms can erode public trust; remote volcanoes may lack instrumentation.
2. Hazard Mapping & Land‑Use Planning
   • Define zones for lava flow, ash fall, pyroclastic density currents and lahars.
   • Strength: Informs zoning, restricts high‑risk development.
   • Limitation: Requires regular updates after each eruption; political pressure may override recommendations.
3. Aviation Alerts & Air‑Traffic Management
   • Volcanic Ash Advisory Centers (VAACs) issue flight‑level advisories.
   • Strength: Prevents engine damage and loss of aircraft.
   • Limitation: Economic impact of widespread cancellations; detection of fine ash remains challenging.
4. Public Education & Community Preparedness
   • Evacuation drills, distribution of hazard maps, school programmes.
   • Strength: Improves timely response and reduces panic.
   • Limitation: Requires ongoing funding and culturally appropriate messaging.
5. Post‑Eruption Recovery
   • Rapid damage assessment, provision of temporary housing, “Build Back Better” reconstruction.
   • Risk‑transfer mechanisms (insurance, disaster relief funds) to spread economic burden.

2.7 Case Study – 1991 Mount Pinatubo (Philippines)

• Event details: VEI 6, Plinian eruption following weeks of intense seismicity and dome growth.
• Primary impacts
  • Pyroclastic flows devastated the surrounding valleys.
  • Ash fall up to 30 cm thick, collapsing roofs and contaminating water supplies.
  • Lahars triggered by heavy monsoon rains, burying towns > 100 km downstream.
• Secondary impacts
  • Release of > 5 million t of SO₂ formed a global sulfate aerosol layer, cooling Earth’s surface by ~0.5 °C for two years.
  • Displacement of > 200 000 people; long‑term health issues from ash inhalation.
  • Economic loss estimated at US$ 800 million (infrastructure, agriculture, tourism).
• Management evaluation
  • Successful evacuation of > 200 000 residents after intensive monitoring and public education – no direct fatalities from the eruption.
  • Post‑event lahar mitigation (check‑dams, early‑warning sirens) reduced later casualties.
  • Lesson: Early warning combined with community participation can dramatically lower loss of life, even for high‑VEI eruptions.