Tectonic Hazards

Plate tectonics fundamentals

The Earth's lithosphere is broken into large slabs called tectonic plates that float on the semi-molten asthenosphere. Heat from Earth's core drives convection currents in the mantle; where these rise, plates are pushed apart; where they sink, plates are pulled together. This movement — a few centimetres per year — causes earthquakes and volcanic eruptions at plate boundaries.

Types of plate boundary

Hot spots occur mid-plate over mantle plumes — e.g. Hawaii (shield volcanoes, gentle eruptions of basaltic lava).

Distribution of earthquakes and volcanoes

Both cluster along plate boundaries, visible on global hazard maps:

• The Pacific Ring of Fire (destructive boundaries around the Pacific) accounts for ~90% of the world's earthquakes and ~75% of active volcanoes.
• The Alpine–Himalayan Belt produces major collision-zone earthquakes (Nepal 2015, Turkey 2023).
• Mid-ocean ridges have frequent, low-magnitude earthquakes and effusive eruptions.

Measuring earthquakes

• Richter scale (logarithmic): each step = 10× ground motion amplitude. Magnitude 7+ is major; 8+ is great.
• Mercalli scale (ordinal I–XII): measures felt intensity at a given location — depends on distance from epicentre and local geology.
• Epicentre = point on Earth's surface above the focus (hypocentre); shallow focus earthquakes (<70 km) cause most damage.

Contrasting case studies: Nepal earthquake (2015) vs Japan earthquake/tsunami (2011)

Nepal earthquake, April 2015 — LIDC

• Magnitude 7.8; focus 15 km (shallow) on the Main Boundary Thrust, collision boundary between Indian and Eurasian plates.
• Primary impacts: ~8,900 deaths; 600,000 structures destroyed; Kathmandu Valley severely damaged; avalanche triggered on Everest.
• Secondary impacts: 2.8 million displaced; landslides blocked rivers; cholera threat; tourism collapsed.
• Factors increasing vulnerability: poor building quality (adobe/brick), steep terrain, limited emergency services, high poverty (HDI = 0.602), remote mountain villages inaccessible.
• Response: international aid (India, China, UK, USA); UN OCHA coordinated; slow to reach remote areas; rebuilding took years — only 50% of households rebuilt by 2020.

Japan (Tōhoku) earthquake + tsunami, March 2011 — HIC

• Magnitude 9.1; destructive boundary where Pacific Plate subducts under North American Plate; 30 km focus.
• Primary impacts: 15,900 deaths (mainly from tsunami, not ground shaking); coastal towns obliterated; Fukushima Daiichi nuclear plant meltdown.
• Secondary impacts: nuclear exclusion zone; $235 bn economic damage; supply-chain disruption worldwide.
• Factors reducing casualties from shaking: Japan's strict building codes (base isolation, reinforced concrete), early-warning system (80 s alert), earthquake drills.
• Response: Self-Defence Forces deployed 107,000 personnel within 3 days; $250 bn reconstruction package; temporary housing for 330,000 displaced; robust insurance system.

Key contrasts (HIC vs LIDC)

Management: prediction, preparation, protection

• Prediction: seismometers, GPS ground deformation monitoring, radon gas sensors. Earthquakes remain unpredictable in timing; volcanoes give more warning (increased seismicity, ground inflation, gas emissions).
• Preparation: building codes, evacuation drills, early-warning systems, land-use zoning away from fault lines.
• Protection: base isolation (Japan), cross-bracing, fire-resistant materials; tsunami sea walls (Japan built 14 m walls after 2011).

Edexcel B exam technique

Extended response (8 marks, L1–L3): structure as — introduce the pattern/process → apply to named case study A → apply to named case study B → evaluate/assess the contrast. Always use place-specific data (magnitudes, death tolls, GDP values).