Detailed Study Notes on Tectonic Processes and Hazards

The Global Distribution and Nature of Hazards

• Hazard Definition: A potential threat to human life and property.

• Classification of Natural Hazards:

  • Hydro-meteorological: Hazards caused by climatic processes.
  • Geophysical: Hazards caused by land processes.

• Geophysical Hazard Distribution: These primarily occur near plate boundaries where plates move at varying speeds and directions, leading to collisions, earthquakes, and volcanic activity.

• Intra-plate Earthquakes: Earthquakes that occur near the middle of tectonic plates. While the causes are not fully understood, it is assumed they are caused by pre-existing weaknesses in the plates that become reactivated, creating seismic waves. An example is solid crust weakening over time and cracking under pressure.

• Volcanic Hotspots: Areas located in the center of plates, such as the Ring of Fire. First theorized by Tuzo Wilson in $1963$, these are localized areas of the lithosphere (crust and upper mantle) with unusually high temperatures caused by the upwelling of hot molten material from the core. At hotspots like Hawaii, magma rises as a plume of hot rock.

• Boundary Intensity: The most powerful earthquakes typically occur at convergent or conservative boundaries.

• Global Activity Belts:

  • OFZ (Oceanic Fracture Zone): A belt of activity through the oceans, along mid-ocean ridges through Africa, the Red Sea, and the Dead Sea.
  • CFZ (Continental Fracture Zone): A belt of activity following mountain ranges from Spain through the Alps to the Middle East and the Himalayas.

Tectonic Trends and Impact Reporting Challenges

• Trends since $1960$:

  • The total number of recorded hazards has increased.
  • The number of fatalities has decreased overall, despite spikes during mega-disasters.
  • The total number of people affected is increasing due to global population growth.
  • Economic costs have increased significantly. This is due to higher development levels (infrastructure in developed countries is more expensive to repair) and an increasing number of insurance policies.

• Difficulties in Reporting Disaster Impacts:

  • Direct vs. Indirect Deaths: It is hard to distinguish between those killed immediately and those who die later from diseases or other delayed impacts.
  • Location Accessibility: Data collection is difficult in isolated rural areas and extremely high-density urban areas.
  • Methodological Inconsistency: Different organizations use different data collection methods, leading to varying quotes for injuries and deaths.
  • Government Bias: Data may be manipulated for political reasons. For example, during the $2004$ Indian Ocean tsunami, the Burmese government claimed $0$ deaths to suggest their protection and aid efforts were successful.

Internal Structure of the Earth

• Crust (Lithosphere): The thinnest, least dense, and lightest uppermost layer. Oceanic crust is approximately $7\,km$ thick, while continental crust can reach up to $70\,km$ thick.

• Mantle (Asthenosphere): Located between $700\,km$ and $2890\,km$ below the crust. It is composed of silicate rocks rich in iron and magnesium. It is semi-molten, and a temperature gradient generates convection currents that move the lithospheric plates.

• Outer Core: Semi-molten rocks containing alloys of iron and nickel. It is located at a depth of $2890\,km$ to $5150\,km$.

• Inner Core: Solid due to extreme pressure, located over $5150\,km$ below the crust. It has a similar composition to the outer core.

• Internal Heat Sources:

  • Primordial Heat: Leftover heat from the Earth's formation.
  • Radiogenic Heat: Heat produced by radioactive decay.

Plate Tectonic Theory and Boundary Interactions

• Crustal Types:

  • Oceanic: High density, mainly basalt, thin, and newly created.
  • Continental: Low density, mainly granite, thick, and old.

• Destructive Plate Boundaries:

  • Continental and Oceanic: The denser oceanic plate subducts under the continental plate, creating a deep ocean trench. The crust melts in the asthenosphere, and the resulting pressurized magma forces through weak spots to create explosive composite volcanoes. Fold mountains form as sediment is pushed upward.
  • Oceanic and Oceanic: The heavier plate subducts, forming an ocean trench and underwater volcanoes. The cooling lava creates island arcs and fold mountains.
  • Continental and Continental: Because neither plate is dense enough to subduct fully, pressure causes the crust to pile up, forming fold mountains. Ancient oceanic crust may be subducted slightly.

• Constructive Plate Boundaries:

  • Oceanic and Oceanic: Plates separate, and magma rises to fill the gap, forming new land through seafloor spreading. This creates less explosive underwater volcanoes.
  • Continental to Continental: Land is forced apart, creating a rift valley. Magma rises to form volcanoes. Lifted rock areas are called horsts, and the valley is a graben.

• Evidence for Plate Movement:

  • Seafloor Spreading: Theorized by Harry Hess in the $1940s$.
  • Paleomagnetism: Grains in cooling rock align with the Earth's magnetic poles. As poles switch periodically, symmetrical bands of alternating magnetic polarity are observed on either side of constructive boundaries.

• Force Mechanisms:

  • Ridge Push: Gravity pushes plates away from the higher elevation of mid-ocean ridges (also known as gravitational sliding).
  • Slab Pull: Theorized by Dan McKenzie; the weight of the subducting plate pulls the rest of the slab into the mantle. This is now considered the primary mechanism of plate movement, as convection currents are often seen as too weak.
  • Mantle Convection: Thermal energy from radioactive decay in the core creates rising and sinking magma currents.

• Conservative Plate Boundaries: Plates move parallel in different directions or at different speeds. No land is created or destroyed. High pressure builds until cracks or fault lines occur.

Mechanics of Earthquakes and Seismic Waves

• Process: Plates become stuck due to friction from surface irregularities. Convection currents continue to push against the stuck plates, building pressure until they eventually jolt forward, releasing energy as seismic (shock) waves.

• Key Terms:

  • Focus (Hypocentre): The underground point of origin.
  • Epicentre: The point on the surface directly above the focus.

• Seismic Wave Characteristics:

  • Primary (P) Waves: Compressional waves that travel through solids at $4-8\,km/s$. They vibrate in the direction of travel.
  • Secondary (S) Waves: Vibrate at right angles to travel. Only move through solid rock at $2.5-4\,km/hr$.
  • Love (L) Waves: Move near the ground surface with a rolling motion and vertical movement. Travel at $2-6\,km/hr$.
  • Rayleigh Waves: Compressional waves with vertical and horizontal displacement. Travel at $1-5\,km/hr$.

• Destructive Potential: Secondary and Love waves are most destructive because of their large amplitudes. Aftershocks are caused by different waves types arriving at different times due to their varying speeds.

Secondary Earthquake Hazards and Tsunami Dynamics

• Soil Liquefaction: Affects poorly compacted sand and silt. Water moisture separates from soil and rises, making the ground behave like a liquid, leading to subsidence and landslides.

• Landslides: Shaking weakens cliffs or hills. Loose rock and unconsolidated material collapse. Risk depends on topography, rainfall, and soil use.

• Tsunamis:

  • Generation: Oceanic crust jolts, displacing the water column upward. Gravity pulls the water back down, transferring energy into a fast-moving, low-amplitude wave.
  • Coastal Interaction: As the wave approaches the coast, friction with the seabed causes it to slow down and increase in height (amplitude). The resulting wall of water averages $10$ feet but can reach $100$ feet.
  • Vulnerability Factors: Subduction zones in the Ring of Fire (Asia and Oceania) are most at risk. Factors include population density, coastal defenses, wave amplitude, distance traveled, continental shelf gradient, and coastal shape (bays funnel waves).

Volcanic Hazards: Primary and Secondary Impacts

• Primary Hazards (Direct):

  • Lava Flows: Streams of surface lava. Hazard level depends on viscosity, which is determined by silicon dioxide content.
  • Pyroclastic Flows: Rapidly moving mixtures of hot rock, ash, and gases. They cause asphyxiation and move too fast to escape.
  • Tephra and Ash Falls: Rock and ash blasted into the air. Weight can collapse buildings.
  • Volcanic Gases: Potent gases like sulfur dioxide and carbon monoxide that travel long distances.

• Secondary Hazards (Result of Heat):

  • Lahars: Rapid rock, mud, and water flows caused by eruptions melting snow/ice or mixing with heavy rain.
  • Jokulhlaup: Sudden, dangerous floods caused by the melting of glaciers.
  • Acid Rain: Caused by the atmospheric release of sulfur dioxide.

Frameworks for Analyzing Tectonic Disasters

• Disaster Definition: A serious disruption of a community's functioning involving losses that exceed its ability to cope using its own resources.

• Risk Calculation: Risk is determined by the size/intensity of the hazard, the vulnerability of the population, and their capacity to cope.

• Degg’s Model: A disaster only occurs at the intersection of a hazardous geophysical event and a vulnerable population. If a population is not vulnerable, the event is not a disaster.

• Classification Criteria:

  • Volume of People: The International Disaster Database defines a disaster as an event affecting $>100$ people or killing $>10$ people.
  • Economic Cost: Measured by jobs lost, repair costs, and productivity loss. The UN Sendai Framework aims to reduce these costs.
  • Statistical Averages: Hazards are compared to past events, though mega-disasters can skew averages. For example, if $100$ events in Iceland occur where $99$ kill $3$ people and $1$ kills $10,000$, the average ($103$ fatalities) does not accurately represent a typical event.

The Park Model and Hazard Profiles

• The Park Model: A graph of human response over time.

  • Stage 1 - Relief (Hours-Days): Immediate local response, medical aid, search and rescue, and global appeals.
  • Stage 2 - Rehabilitation (Days-Weeks): Restoring services, setting up temporary shelters/hospitals, distributing food/water, and coordinated foreign aid.
  • Stage 3 - Reconstruction (Weeks-Years): Rebuilding infrastructure, restoring the ecosystem, and implementing mitigation.
  • Interpretation: Steeper curves indicate faster deterioration and recovery. Deeper curves indicate lower quality of life and greater disaster scale.

• Pressure and Release (PAR) Model: Analyzes vulnerability factors:

  • Root Causes: Political and economic systems (e.g., weak governance, poverty).
  • Dynamic Pressures: Local factors (e.g., rapid urbanization, lack of training).
  • Unsafe Conditions: Physical factors (e.g., dangerous locations, lack of infrastructure like clean water and electricity).

• Vulnerability Types: Physical (living in hazard-prone areas), Economic (risk to assets/wealth), Social (inability to support the disadvantaged), Knowledge (lack of education/fatalist religious beliefs), and Environmental (high population density).

• Hazard Profiles: Compare physical characteristics like Frequency, Magnitude, Duration, Speed of Onset, Fatalities, Economic Loss, and Spatial Predictability to help allocate resources.

Quantitative Measurement of Tectonic Events

• Volcanic Explosivity Index (VEI): A logarithmic scale ($0-8$) measuring explosiveness based on the height of ejected material and duration.

• Modified Mercalli Scale: A subjective scale ($I$ to $XII$) measuring destruction based on human observation (e.g., furniture moving). It does not consider economic or environmental impacts.

• Moment Magnitude Scale: Measures energy release on a scale of $0-9$.

• Richter Scale: A logarithmic, absolute scale measuring the amplitude of seismic waves. It does not necessarily correlate with disaster severity (the highest readings aren't always the worst disasters).

Strategies for Hazard Management and Mitigation

• Hazard Management Cycle: Stages include Preparedness (education/training), Response (immediate evacuation/rescue), Recovery (long-term restoration), and Mitigation (barriers/warning systems).

• Modification Strategies:

  • Modify the Event: Micro-scale (strengthening individual buildings/private homes) or Macro-scale (tsunami walls, replanting mangroves, diverting lava, reinforcing roofs against ash).
  • Modify Vulnerability: High-tech monitoring (detecting tremors, tilt, and surface swelling), community preparedness/education, and adaptation (relocation).
  • Modify the Loss: Short-term aid (rescue/food), long-term aid (reconstruction), and insurance.

• Role of Stakeholders:

  • Communities: Vital for immediate recovery and clearing debris in remote areas.
  • NGOs & TNCs: Provide funds, coordinate rescue, and develop reconstruction plans. TNCs may support NGOs through corporate charity events.

Development, Governance, and the Risk Poverty Nexus

• Governance Factors: Meeting basic needs reduces secondary hazard risk (disease). Land-use planning prevents settlement in high-risk zones. Corruption increases vulnerability as funds are diverted from emergency services.

• Risk Poverty Nexus: Suggests poverty is both a cause and a consequence of hazards. It creates a positive feedback loop of economic loss.

• Inequalities Influencing Resilience:

  • Asset Inequality: Housing security and agricultural productivity.
  • Political Inequality: Power held by wealthy elites.
  • Social Status Inequality: Impacted by access to income and services.
  • Entitlement Inequality: Unequal access to public services, welfare, and the rule of law.

Tectonic Mega-Disasters and Globalized Impacts

• Characteristics: Large-scale, low-probability (rare) events affecting vast areas or populations, often requiring international aid.

• Global Supply Chain Disruption:

  • $2011$ Tohoku Earthquake & Tsunami: Created knock-on effects for TNCs like Toyota and BMW due to lost Japanese product sourcing, causing global economic uncertainty.
  • $2011$ Eyjafjallajökull Eruption: Ash clouds closed European airspace, halting trade. This led to Kenyan flowers wilting as they could not be transported to the EU market.