Volcanic explosivity index (VEI)

Earthquake and Volcanic Hazards – Cambridge International AS & A Level Geography (9696)

1. Earthquake Hazards

1.1 Distribution and Tectonic Setting

• Plate‑boundary earthquakes – > 90 % occur along:
  • Convergent margins – subduction zones (e.g., Pacific “Ring of Fire”).
  • Divergent margins – mid‑ocean ridges and continental rifts.
  • Transform margins – strike‑slip faults such as the San Andreas.
• Intraplate earthquakes – less frequent but can be severe (e.g., New Madrid, USA; 1811‑12).

1.2 Magnitude and Intensity Scales

• Richter scale (M_L) – measures the maximum amplitude of seismic waves on a logarithmic scale; most useful for moderate‑size, local events.
• Moment magnitude scale (M_w) – based on seismic moment (fault‑area × average slip × rigidity); preferred for all sizes, especially large, worldwide earthquakes.
• Modified Mercalli Intensity (MMI) scale – qualitative description of shaking and damage (I – XII); used for hazard and vulnerability mapping.

1.3 Seismic Waves

• Body waves travel through the Earth’s interior:
  • P‑waves – primary, compressional, fastest.
  • S‑waves – secondary, shear, slower; cannot propagate through liquids.
• Surface waves travel along the Earth’s surface; cause the greatest damage (Rayleigh and Love waves).

1.4 Impacts of Earthquakes

• Ground shaking → building collapse, loss of life.
• Surface rupture → displacement of roads, pipelines, utilities.
• Secondary effects:
  • Liquefaction
  • Landslides
  • Tsunamis (if offshore)
  • Fires (from broken gas lines)

1.5 Vulnerability and Risk Mapping

• Vulnerability depends on building quality, population density, socioeconomic status and preparedness.
• Risk = Hazard × Exposure × Vulnerability – the three components are combined in GIS to produce layered risk maps for planning, insurance and emergency services.

1.6 Mitigation Strategies

• Hard‑engineering
  • Seismic‑resistant building codes (e.g., reinforced concrete frames, shear walls).
  • Base isolation and retrofitting of existing structures.
• Soft‑engineering
  • Public education and regular earthquake drills.
  • Early‑warning systems (e.g., Japan’s J‑Alert, US ShakeAlert).
  • Land‑use planning – avoiding construction on known fault traces.
• Emergency preparedness – evacuation routes, stockpiling of food/medicine, clear communication channels.

1.7 Case Study – Haiti Earthquake (2010, M_w = 7.0)

• Location: Shallow thrust fault within the Caribbean plate.
• Impacts: ≈ 230 000 deaths; > 300 000 buildings destroyed; collapse of health and water services.
• Key vulnerability factors: Poor construction standards, high population density, limited emergency response capacity.
• Mitigation lessons:
  • Enforce stricter building codes and retrofitting programmes.
  • Develop community‑based disaster risk reduction (DRR) schemes.
  • Strengthen international assistance frameworks for rapid response.

---

2. Volcanic Hazards

2.1 Distribution and Tectonic Setting

• Three principal volcanic belts:
  • Pacific Ring of Fire – subduction‑related stratovolcanoes (e.g., Mount Pinatubo, Japan).
  • Mid‑Ocean Ridges – basaltic shield volcanoes (e.g., Iceland, East African Rift).
  • Intraplate “Hot‑Spot” chains – e.g., Hawaiian Islands, Réunion.
• Convergent margins generate the most explosive eruptions; divergent and hotspot settings are typically effusive.

2.2 Types of Eruptions

Type	Typical Magma	Eruption Style	Principal Hazards
Effusive (Hawaiian)	Low‑viscosity basalt	Lava flows, low‑level ash	Flow damage, fire, minor ashfall
Strombolian	Basaltic‑andesitic	Regular mild blasts	Ballistic projectiles, localized ash
Vulcanian	Viscous andesite	Short, violent explosions	Ash columns, pyroclastic surges
Plinian	High‑viscosity rhyolite/dacite	High eruption column, sustained explosive activity	Pyroclastic flows, extensive ashfall, lahars, gas emissions

2.3 Volcanic Explosivity Index (VEI)

2.3.1 What the VEI Measures

• Logarithmic (base 10) scale that quantifies the **explosivity** of a volcanic eruption.
• Key parameters:
  • Erupted tephra volume (km³).
  • Eruption‑column height (km).
  • Qualitative description of the eruption’s intensity.
• Values range from 0 (non‑explosive) to 8 (mega‑colossal).

2.3.2 Approximate Calculation

For a rapid estimate, use the erupted tephra volume \(V\) (in km³):

\[ \text{VEI} \approx \log_{10}(V) + 0.5 \]

Because the scale is logarithmic, a ten‑fold increase in volume raises the VEI by one unit.

2.3.3 VEI Scale

VEI	Tephra Volume (km³)	Column Height (km)	Typical Example
0	< 0.000001	< 1	Strombolian activity (e.g., Etna)
1	0.000001 – 0.001	1 – 5	Typical Hawaiian eruption (e.g., Kīlauea)
2	0.001 – 0.01	5 – 10	Minor Vulcanian eruption
3	0.01 – 0.1	10 – 15	Mount St Helens (1980 – early phase)
4	0.1 – 1	15 – 25	Mount Pinatubo (1991)
5	1 – 10	25 – 35	Mount Vesuvius (79 AD)
6	10 – 100	35 – 45	Mount Krakatoa (1883)
7	100 – 1 000	45 – 55	Mount Tambora (1815)
8	> 1 000	> 55	Super‑volcano eruptions (e.g., Toba ≈ 74 ka)

2.3.4 Sample Calculation

Assume a volcano erupts 5 km³ of tephra.

\[ \text{VEI} \approx \log_{10}(5) + 0.5 \approx 0.699 + 0.5 = 1.199 \approx 1 \]

Rounded to the nearest whole number, the eruption is classified as **VEI 1** (consistent with the volume range 0.000001 – 0.001 km³ for VEI 1). For larger volumes, the calculation yields higher VEI values as shown in the table.

2.4 Other Volcanic Hazards

• Ashfall – respiratory problems, aviation disruption, roof collapse, contamination of water supplies.
• Pyroclastic flows & surges – hot (up to 1000 °C), fast (30‑700 m s⁻¹) mixtures of gas and particles; cause immediate fatalities and destroy infrastructure.
• Lahars – volcanic mudflows triggered by rain, snowmelt or crater lake breach; travel long distances along river valleys.
• Volcanic gases – SO₂, CO₂, H₂S; can produce acid rain, climate cooling (sulphate aerosols) and asphyxiation.

2.5 Vulnerability and Risk Mapping for Volcanoes

• Hazard zones are delineated using:
  • Historical eruption footprints (flow, ash, lahar deposits).
  • Topography – slope, drainage networks, wind corridors.
  • Climatology – prevailing wind direction for ash dispersion.
• GIS layers combine these hazard zones with population, infrastructure and land‑use data to produce risk maps.
• Risk formula (identical to earthquakes): Risk = Hazard × Exposure × Vulnerability.

2.6 Mitigation and Management

• Hard‑engineering
  • Lahar diversion channels (e.g., Mount Pinatubo, Philippines).
  • Reinforced ash‑fall shelters and roof designs.
• Soft‑engineering
  • Vegetation planting on steep slopes to reduce landslide and lahar risk.
  • Public education on evacuation routes, respiratory protection and safe water use.
• Land‑use planning – exclusion zones around high‑risk vents; zoning restrictions for schools, hospitals and critical utilities.
• Early‑warning systems
  • Seismic monitoring of volcano‑tectonic earthquakes.
  • Gas emission sensors (SO₂, CO₂) and ground deformation (GPS, InSAR).
  • Satellite thermal imaging and plume tracking (e.g., Icelandic volcano monitoring network).

2.7 Case Study – Eyjafjallajökull (Iceland, 2010, VEI 4)

• Eruption type: Plinian with a 9 km high ash column.
• Hazards:
  • Fine ash plume disrupted European air traffic for > 6 weeks.
  • Local ashfall affected agriculture, water quality and human health.
• Risk mapping: Real‑time aviation hazard maps produced from satellite data and numerical wind‑field models.
• Mitigation actions:
  • Issuance of NOTAMs (Notice to Airmen) and establishment of ash‑clearance protocols.
  • Public health advisories on respiratory protection and safe water use.

2.8 Why the VEI Matters for Hazard Assessment

1. Risk mapping – Higher VEI values indicate larger tephra volumes and taller columns, expanding ashfall zones and increasing the probability of long‑range atmospheric impacts.
2. Emergency planning – Authorities use VEI to set evacuation radii, decide on air‑traffic restrictions, and allocate medical and logistical resources.
3. Long‑term impacts – Mega‑eruptions (VEI ≥ 7) can inject sulphur aerosols into the stratosphere, causing global cooling, crop failures and forced migration.

2.9 Limitations of the VEI

• Ignores eruption duration; a long‑lasting low‑intensity eruption may have a high total volume but a low VEI.
• Excludes gas emissions and lava‑flow volume, both of which can be major hazards.
• Estimates of historic tephra volume are often uncertain and rely on subjective interpretation of deposits.