Tectonics

Global Distribution of Hazards

• A hazard is a threat to human life and property.
• Natural hazards are either hydro-meteorological (climate-related) or geophysical (land-related).
• Geophysical hazards occur near plate boundaries due to plate movement, causing collisions, earthquakes, and volcanic activity.
• Earthquakes can also occur within plates (intra-plate) due to pre-existing weaknesses reactivating.
• Volcanic hotspots, like the Ring of Fire, are areas with unusually high temperatures due to magma upwelling from the Earth's core.
• The most powerful earthquakes often happen at convergent or transform boundaries, with significant activity along oceanic and continental fracture zones.

Tectonics Trends Since 1960

• Recorded hazards have increased.
• Fatalities have decreased, but there are spikes during major disasters.
• The number of people affected by tectonic hazards is increasing due to population growth.
• Economic costs associated with hazards have risen significantly due to increasing development and insurance.
• Reporting disaster impacts (e.g., fatalities) is challenging due to factors like: differences between direct and indirect deaths, difficulty accessing remote areas, different data collection methods, and potential government bias.

Tectonic Theory: Earth's Structure

• The Earth comprises four layers:
  • Crust (lithosphere): Thin, least dense, and lightest. Oceanic crust (7 km) is thinner than continental crust (up to 70 km).
  • Mantle (asthenosphere): Primarily silicate rocks, rich in iron and magnesium, semi-molten. Convection currents cause the mantle to circulate, potentially moving the lithosphere's plates. Depth: 700-2890 km.
  • Outer Core: Dense, semi-molten, containing iron and nickel alloys. Depth: 2890-5150 km.
  • Inner Core: Similar composition to outer core but solid due to immense pressure. Depth: > 5150km.
• The core's heat comes from primordial heat (formation) and radiogenic heat (radioactive decay).

Different Plate Boundaries

• At plate boundaries, plates move towards each other (destructive), away from each other (constructive), or parallel to each other (conservative).
• Different landforms and processes arise from these interactions.

Destructive Plate Boundaries

• Continental-Oceanic: Denser oceanic plate subducts, forming a deep ocean trench. Melted oceanic crust creates magma, causing volcanoes (composite) and fold mountains.
• Oceanic-Oceanic: Heavier plate subducts, forming a trench and island arcs (volcanoes).
• Continental-Continental: Both plates are less dense, causing immense pressure; ancient oceanic crust might subduct slightly, but no continental crust subducts. Fold mountains form from piling continental crust.

Constructive Plate Boundaries

• Oceanic-Oceanic: Magma rises, creating new land (e.g., seafloor spreading). Underwater volcanoes form.
• Continental-Continental: Land separates, forming rift valleys, potentially filled with water eventually separating continents. Horsts and grabens are formed.

Conservative Plate Boundaries

• Plates move parallel at varying speeds. No plates are destroyed, no new landforms are created. Pressure builds, resulting in fault lines on continental crust or water displacement on oceanic crust.

Plate Movements

• Oceanic crust (low-density, basalt, thin, recent) vs. Continental crust (high-density, granite, thick, ancient).
• Plate density determines subduction or uplift, affecting landscape and hazards.
• Mantle Convection: Radioactive decay heats the lower mantle, causing rising and sinking magma currents (convection currents).
• Slab Pull: Dense sinking oceanic crust pulls the rest of the plate.
• Understanding of plate movement is ongoing; slab pull is now believed to be a primary factor, not convection currents.

Earthquakes

• Plates are stuck due to friction, and pressure builds within the asthenosphere.
• Sudden release causes seismic waves (shock waves) originating at the focus (hypocenter) and propagating to the epicenter (surface above).

Seismic Waves:

• Primary (P-waves): Fastest, compressional, travels through solids.
• Secondary (S-waves): Slower, transverse, travels through solids only.
• Love waves: Surface waves, rolling motion, vertical movement, destructive.
• Rayleigh waves: Surface waves, vertical and horizontal displacement. Destructive.

Secondary Hazards of Earthquakes

• Soil liquefaction: Causes subsidence, landslides due to water separating from soil particles.
• Landslides: Shaking weakens slopes, causing debris flows.
• Tsunamis: Earthquakes displace water upwards causing waves that increase in height closer to shorelines.

Tsunamis

• Generated at convergent plate margins (often subduction zones).
• Impact depends on factors like population density, coastal defenses, event duration, wave amplitude, continental shelf gradient, and land shape.
• Warning and evacuation systems are vital.

Volcanoes

• Primary Hazards (fast onset): Lava flows (danger based on viscosity, dependent on SiO2 content), Pyroclastic flows (hot, dense, fast, asphyxiation hazard), Tephra and ash flows (building damage), volcanic gases (long-distance dispersal hazards, e.g., sulfur dioxide, carbon monoxide).
• Secondary Hazards (result of heat): Lahars (mudflows from melted snow/ice or heavy rainfall), Jokulhlaups (glacial outburst floods).
• Acid rain from released gases.

Classification and Theories of Tectonic Events

• Disaster: A serious disruption of community functioning, exceeding its coping abilities.
• Risk calculation: Consider a community's coping capacity, vulnerability, and the hazard's intensity.
• Degg's Model: A disaster occurs when a vulnerable population is exposed to a hazard.
• Different classifications: Volume of people affected, economic cost, comparison to previous events, or utilizing prediction models.
• Park Model: Graphical representation of human responses during a hazard using a recovery curve that displays time and quality of life impacted.
• PAR Model: Analyzes factors causing population vulnerability. Factors include: root causes (weak governance, reliance), dynamic pressures (lack of training), unsafe living conditions (infrastructure, location). Vulnerability is affected by physical, economic, social, and knowledge factors.

Tectonic Hazard Profiles

• Hazard profiles: Compare physical characteristics of hazards to help allocate resources.
• Characteristics include: frequency, magnitude, duration, speed of onset, fatalities, economic loss, spatial predictability.

Measuring Tectonic Events

• Volcanic Explosivity Index (VEI): Measures eruption explosiveness based on ejected material and duration (logarithmic scale).
• Modified Mercalli Intensity Scale (MMI): Measures earthquake destructiveness, based on human perception of shaking (subjective, I to XII scale).
• Moment Magnitude Scale: Measures earthquake energy release (absolute scale, 0–9).
• Richter Scale: Measures wave amplitude (absolute logarithmic scale).

Managing Tectonic Hazards

• Four approaches: Prevent, Prepare, Mitigate, Adapt
• Hazard Management Cycle: Preparedness, response, recovery, mitigation
• Monitoring and Prediction: Earthquake risk forecasting vs. specific prediction; volcanic prediction capabilities.
• Mitigation: Land-use zoning, building codes, protective defenses.
• Preparedness: Preparation plans, warning systems, supplies, education.
• Response: Rescue, evacuation, infrastructure restoration.
• Local and International Aid: Short-term and long-term assistance.
• Management Approaches (Modify Event, Modify Vulnerability, Modify Loss): Civil engineering modifications, monitoring systems, community preparedness, relocation, aid, insurance.

Development and Governance

• Development priorities often overshadow hazard mitigation in developing countries, making them more vulnerable.
• The Risk-Poverty Nexus: Poverty influences and results from hazards.
• Inequalities (asset, political, social, entitlement) can exacerbate vulnerability.

Tectonic Mega-Disasters

• Characteristics: large-scale impact, management challenges, international aid often required.
• Globalized businesses can be significantly disrupted.