Main hazards: ground-shaking, liquefaction, landslides, tsunami, aftershocks

Earthquake & Volcanic Hazards (Cambridge AS/A Level Geography 9696)

This set of notes follows the Cambridge syllabus (9.1.1‑9.1.6). It covers the global‑to‑local distribution of earthquakes and volcanoes, the full range of earthquake‑related hazards, the links with volcanic activity, the factors that make societies vulnerable, and the management strategies used to reduce risk. Where appropriate, the notes highlight the scale of analysis, the seismic cycle, and the strengths and limitations of each mitigation measure – all required for AO2 and AO3.

1. Plate‑tectonic setting & scale of hazards

• Plate boundaries: 95 % of earthquakes and most volcanoes occur along convergent, transform or divergent margins.
  • Convergent (subduction) zones – Pacific “Ring of Fire”, Andes, Himalaya.
  • Transform faults – San Andreas (North America), Alpine‑Himalayan system.
  • Divergent (mid‑ocean ridge) zones – Mid‑Atlantic Ridge (mostly offshore, low‑magnitude quakes).
• Scale of analysis: hazards are examined at three scales:
  • Global – distribution of seismic belts & volcanic arcs.
  • Regional – seismic cycles, fault geometry, tectonic setting of a country or basin.
  • Local – site effects (soil type, topography), building stock, land‑use.
• Why it matters: The plate‑boundary context explains where hazards are most likely, guides hazard‑mapping, and informs land‑use planning at each scale.

2. Earthquake magnitude and intensity scales

• Moment magnitude (M_w) – preferred for all modern work.
  Equation: M_w = (2/3) log₁₀(M₀) – 6.07 (M₀ in N·m).
• Surface‑wave magnitude (M_s) – useful for shallow, crustal events; based on 20 s Rayleigh waves.
• Body‑wave magnitude (m_b) – employed for deep or teleseismic events; uses high‑frequency P‑waves.
• Intensity (Modified Mercalli, MMI) – describes observed effects at a location (I = imperceptible to XII = total destruction). Essential for AO2 questions on damage patterns.

3. Main earthquake‑related hazards

3.1 Ground‑shaking

• Cause: Sudden release of elastic strain energy along a fault generates body (P, S) and surface (Rayleigh, Love) waves.
• Quantitative description:
  • Magnitude (M_w, M_s, m_b) – energy released.
  • Peak Ground Acceleration (PGA) – a key parameter on seismic hazard maps.
  • Intensity (MMI) – observed damage.
• Typical impacts: Structural collapse, bridge failure, utility disruption, injuries and loss of life.
• Vulnerability factors: Poorly built unreinforced masonry, high population density, weak building‑code enforcement, low‑income governance.
• Mitigation (pros / cons):
  • Seismic‑resistant design (ductile frames, shear walls) – pros: reduces collapse; cons: higher construction cost.
  • Base isolation – pros: limits floor acceleration; cons: expensive, requires specialised expertise.
  • Retrofitting of existing buildings – pros: extends life of stock; cons: disruption during works, variable effectiveness.
  • Earthquake early‑warning (EEW) – pros: seconds to protect elevators, trains; cons: limited warning time for near‑field events.
• Example: The 2011 M_w 9.0 Tōhoku earthquake produced extreme ground‑shaking that damaged even modern Japanese structures, highlighting the need for extreme‑event design.

3.2 Liquefaction

• Cause: Saturated, loose sediments lose shear strength during strong shaking and behave like a fluid.
• Typical settings: River deltas (Ganges‑Brahmaputra), reclaimed coastal land (Tokyo Bay), alluvial valleys.
• Physical effects:
  • Loss of bearing capacity → settlement or tilting of foundations.
  • Sand boils (ejecta) and lateral spreading.
  • Disruption of buried services (water, gas, telecoms).
• Vulnerability: Low‑rise residential blocks on soft ground; critical infrastructure on reclaimed land.
• Mitigation (pros / cons):
  • Soil densification (vibro‑compaction, dynamic compaction) – pros: permanent improvement; cons: costly, disruptive.
  • Deep pile foundations – pros: bypasses weak layers; cons: expensive, may not be feasible for all structures.
  • Drainage to lower water table – pros: relatively cheap; cons: requires ongoing maintenance.
  • Ground‑improvement grouting – pros: targeted; cons: technically demanding.
• Case study: The 2015 M_w 7.8 Nepal earthquake caused extensive liquefaction in the Kathmandu valley’s alluvial deposits, damaging many low‑rise houses.

3.3 Landslides & rock falls

• Cause: Seismic shaking reduces shear strength of slope material, overcoming resisting forces.
• Key controlling factors: slope angle & height, rock type & degree of weathering, water content, pre‑existing weaknesses (faults, previous slides).
• Impacts: Road/rail blockage, damming of rivers (temporary lakes), loss of life, long‑term alteration of drainage.
• Vulnerability: Communities on steep hillsides, agricultural terraces, transport corridors.
• Mitigation (pros / cons):
  • Slope grading & benching – pros: reduces angle of repose; cons: earthworks can be expensive.
  • Retaining walls & rock bolts – pros: provides immediate stability; cons: requires regular inspection.
  • Drainage control (horizontal drains, surface runoff channels) – pros: lowers pore pressure; cons: may clog over time.
  • Re‑vegetation – pros: low cost, long‑term; cons: slow to become effective.
  • Early‑warning monitoring (inclination sensors, remote sensing) – pros: allows rapid evacuation; cons: technology‑dependent.
• Example: The 1999 M_w 7.6 İzmit earthquake triggered > 2 000 landslides in north‑western Turkey, closing major highways for weeks.

3.4 Tsunami

• Generation mechanisms (syllabus requirement):
  • Uplift or subsidence of the seafloor during a thrust‑fault earthquake (most common).
  • Submarine landslides – e.g., 1998 Lituya Bay megatsunami.
  • Volcanic flank collapse – e.g., 1883 Krakatoa, 2018 Fogo Island.
  • Explosive volcanic eruptions displacing water (Krakatoa 1883).
• Wave physics: Vertical displacement creates a series of long‑wavelength waves travelling up to 800 km h⁻¹ in deep water. In deep water wave height < 1 m; as the wave shoals on continental shelves it shortens and can exceed 30 m.
• Typical impacts: Rapid coastal inundation, erosion, destruction of housing & infrastructure, salinisation of freshwater, massive loss of life.
• Vulnerability: Low‑lying coastal settlements, tourism resorts, ports, areas lacking evacuation routes.
• Mitigation (pros / cons):
  • International tsunami warning centres (e.g., Pacific Tsunami Warning System) – pros: provides minutes‑to‑hours warning; cons: depends on rapid data transmission.
  • Coastal setback zones & land‑use planning – pros: removes people from highest risk; cons: limits development space.
  • Sea‑walls – pros: protect specific assets; cons: high cost, can be overtopped.
  • Nature‑based buffers (mangroves, coral reefs) – pros: ecological co‑benefits; cons: effectiveness varies with wave height.
  • Public education, evacuation drills, vertical‑rise signage – pros: low cost, saves lives; cons: requires regular community engagement.
• Case study: The 2004 M_w 9.1 Sumatra‑Andaman earthquake generated a Pacific‑wide tsunami that killed > 230 000 people, illustrating the need for trans‑regional warning networks.

3.5 Aftershocks

• Definition: Smaller earthquakes occurring in the same fault zone after a main shock as stress is redistributed.
• Temporal pattern – Omori’s law: \(n(t)=\dfrac{k}{(c+t)^{p}}\) where \(n(t)\) = number of aftershocks per unit time at \(t\) days, \(k,c\) constants, \(p≈1\).
• Hazard significance: Can cause further collapse of weakened structures, impede rescue operations, and trigger secondary hazards (landslides, additional liquefaction).
• Vulnerability considerations: Buildings already damaged, temporary shelters, emergency responders.
• Mitigation (pros / cons):
  • Rapid post‑event structural assessments – pros: identifies unsafe buildings; cons: requires trained engineers.
  • Temporary shoring of critical facilities – pros: prevents collapse; cons: limited to short‑term.
  • Public advisories to avoid damaged zones – pros: simple; cons: compliance varies.
  • Continuous monitoring with dense seismograph networks – pros: provides up‑to‑date risk info; cons: expensive to maintain.
• Example: After the 2010 M_w 7.0 Haiti earthquake, aftershocks lasting several weeks hampered rescue work and caused additional casualties.

4. Seismic hazard mapping & risk assessment

• Seismic hazard maps: Show the probability of exceeding a given Peak Ground Acceleration (PGA) over a specified time‑frame (e.g., 10 % chance in 50 years). Produced by:
  1. Compiling historic seismicity and instrumental records.
  2. Applying attenuation relationships (ground‑motion prediction equations) that relate magnitude, distance, and site conditions.
  3. Incorporating geological data (fault slip rates, rupture lengths).
• Risk‑matrix / Disaster‑risk index: Combines
  • Hazard probability (from maps),
  • Exposure (population, assets), and
  • Vulnerability (social, economic, political).
  Results are plotted in a colour‑coded matrix (low‑medium‑high risk) or expressed as a numeric index (e.g., UN‑ISDR Disaster Risk Index). This tool satisfies AO3 requirements for evaluating mitigation strategies.
• Application: Planners use hazard maps to set building‑code PGA thresholds; risk matrices guide where to prioritise retrofitting or land‑use restrictions.

5. Vulnerability & exposure

6. Management of earthquake & volcanic hazards

6.1 Prediction & monitoring

• Seismograph networks (global – IRIS, regional – USGS, Japan Meteorological Agency) record magnitude, depth, and epicentre.
• Real‑time GPS & InSAR detect crustal deformation indicating stress accumulation.
• Volcanic‑tectonic monitoring: Swarms of small quakes, harmonic tremor, and changes in gas emissions (SO₂, CO₂) often precede eruptions; these data are integrated with seismic records to forecast volcanic earthquakes.
• Limitations: Exact timing, location and magnitude of a main shock cannot be predicted; forecasts remain probabilistic (AO2).

6.2 Hazard mapping & risk assessment (expanded)

• Seismic zoning maps (e.g., USGS “Peak Ground Acceleration” maps) dictate building‑code requirements.
• Landslide susceptibility maps combine slope, lithology, and rainfall data.
• Tsunami inundation maps display maximum run‑up heights for different scenarios.
• Risk assessments calculate expected economic loss (GDP impact, reconstruction cost) and social impact (displacement, mortality) using the risk‑matrix approach.

6.3 Early‑warning systems

• Earthquake early‑warning (EEW): Uses the first P‑waves to issue alerts seconds before damaging S‑waves arrive (Japan J‑Alert, Mexico SASMEX).
• Tsunami warning centres: Issue alerts based on seismic magnitude, depth, fault type, followed by sea‑level sensor confirmation.
• Public‑alert technologies: mobile SMS, sirens, radio, community‑based alarms, and vertical‑rise evacuation signs.

6.4 Land‑use planning & engineering controls (pros / cons)

• Restrict development in high‑hazard zones (coastal buffers, landslide‑prone slopes).
  • Pros: removes people from greatest risk.
  • Cons: limits land availability, may affect livelihoods.
• Enforce seismic‑resistant building codes.
  • Pros: reduces casualties and economic loss.
  • Cons: higher construction costs; enforcement varies.
• Engineered solutions:
  • Base isolation – pros: excellent performance; cons: expensive, specialist design.
  • Energy‑dissipating devices (dampers) – pros: retrofittable; cons: maintenance required.
  • Deep foundations (piles, caissons) – pros: bypasses weak soils; cons: costly for low‑rise housing.
• Nature‑based solutions:
  • Mangrove restoration – pros: coastal protection + biodiversity; cons: requires space, long‑term growth.
  • Re‑forestation of vulnerable slopes – pros: cheap, reduces landslide risk; cons: takes years to become effective.

6.5 Evaluation of mitigation measures (strengths & limitations)

• Cost‑benefit analysis: Compare implementation cost with avoided loss (e.g., Japan’s base‑isolated buildings reduce expected damage by ~70 %).
• Strengths: Proven reduction in casualties, longer building lifespan, secondary benefits (energy efficiency, ecosystem services).
• Limitations: Uncertainty in hazard forecasts, uneven implementation across regions, social acceptance, maintenance requirements, and the “risk compensation” effect (people behave more recklessly when they feel protected).
• Evaluation tools: Post‑event damage surveys, loss‑ratio modelling, and the risk‑matrix to assess whether a mitigation strategy moves a region from high to medium/low risk.

7. Linking volcanic activity to earthquake hazards

• Volcanic‑tectonic earthquakes: Magma movement fractures rock, producing swarms of small‑to‑moderate quakes that often precede eruptions (e.g., Mt. Etna 2021 swarm).
• Lateral blasts & pyroclastic flows: Can destabilise surrounding slopes, triggering landslides and debris avalanches (1991 Mt. Pinatubo).
• Volcanic‑induced tsunamis: Submarine eruptions, island‑flank collapses, or massive pyroclastic flows displace water volumes (1883 Krakatoa, 2018 Fogo Island).
• Integrated monitoring: Joint seismic‑volcanic networks (seismometers, broadband sensors, gas analysers) improve early warning for both eruption‑related and tectonic hazards.

8. Summary table of main hazards