Tectonic Processes and Hazards - EQ1: Why Are Some Locations More at Risk?

Introduction

• Aim: Understand why some locations are more at risk from tectonic hazards.
• Reminder: This is an overview, not a replacement for detailed lessons.
• Tectonics and Hazards is a small part of the AEV course (16 marks).
• Importance of having a broad understanding to apply case studies in exams.

Tectonic Hazards

• Definition: Threats to human life/infrastructure due to plate boundaries and movements.
• Distribution:
  • Most earthquakes occur on plate boundaries.
  • Powerful earthquakes at conservative boundaries.
  • Ocean Fracture Zone (OFZ) - earthquake activity along mid-ocean ridges (e.g., Mid-Atlantic Ridge).
  • Continental Fracture Zone (CFZ) - earthquake activity along mountain ranges.
  • Pacific Ring of Fire - active volcanic/earthquake area.
  • Intraplate earthquakes and volcanic hotspots (e.g., Hawaii, Iceland).

Plate Margins

• Constructive Margins:
  • Plates move apart; magma rises, forming volcanoes.
  • Oceanic-oceanic or continental-continental plates.
  • Eruptions are effusive, low gas content, high viscosity.
• Destructive Margins:
  • Plates move towards each other; forms earthquakes, volcanic eruptions.
  • Oceanic-continental, oceanic-oceanic, continental-continental interactions.
  • Subduction zones create the most dangerous hazards.
• Conservative Margins:
  • Plates slide past each other (e.g., San Andreas Fault).
  • Frequent, shallow earthquakes; no volcanic activity.

Earth's Structure

• Layers: Crust, lithosphere, asthenosphere, mantle, outer core, inner core.
• Mantle Convection: Drives plate movements; internal heat engine of Earth.

Key Theories and Discoveries

• Continental Drift: Alfred Wegener (1912) - Pangaea supercontinent.
• Convection Currents: Arthur Holmes (1930s) - driving force of plate movements.
• Seafloor Spreading: Harry Hess (1960s) - mid-ocean ridges.
• Paleomagnetism: Earth's magnetic field creates patterns on seafloor.
• Subduction, Gravitational Sliding, Slab Pull:
  • Subduction: Dense plates sink under lighter plates.
  • Gravitational Sliding: Constructive margins, rising magma creates slopes.
  • Slab Pull: Dense oceanic plate pulled into mantle.

Hazard Characteristics by Plate Margin

• Constructive Margin:
  • Shallow, low-magnitude earthquakes; small, low gas content volcanoes.
  • Example: Mid-Atlantic Ridge.
• Destructive Margin (Oceanic-Continental):
  • Large earthquakes, violent volcanic eruptions.
  • Example: Nazca and South American Plate Boundary.
• Destructive Margin (Oceanic-Oceanic):
  • Frequent earthquakes, volcanic islands (e.g., Hawaii).
• Collisional Margin (Continental-Continental):
  • Large fault lines, high magnitude earthquakes; no volcanic eruptions.
  • Example: The Andes.
• Conservative Margin:
  • High magnitude earthquakes, shallow focus, no volcanic activity.
  • Example: San Andreas Fault.

Earthquake Hazards

• Primary Waves (P-waves): Fastest, least destructive.
• Secondary Waves (S-waves): Cause ground shaking, more destructive.
• Love Waves (L-waves): Cause significant damage, travel horizontally.

Earthquake-Induced Hazards

• Crustal Fracturing: Primary hazard; fractures and cracks in Earth's surface.
• Liquefaction: Secondary hazard; waterlogged ground destabilized.
• Landslides: Secondary hazard; soil dislodged on slopes.

Volcanic Hazards

• Primary Hazards:
  • Pyroclastic Flows: Fast, hot, dense ash and gas clouds.
  • Ashfall: Damages vegetation, buildings, water sources.
  • Lava Flows: Destroys anything in path.
  • Gas Eruptions: Poisonous gases emitted.
• Secondary Hazards:
  • Lahars: Volcanic mudflows.
  • Jökulhlaups: Flooding from glacial meltwater.

Tsunamis

• Causes: Submarine earthquakes, volcanic eruptions, landslides.
• Characteristics:
  • Disruption of seabed rock causes waves.
  • Waves increase in height as they approach shore.
  • Speeds between 250 and 950 km/h.

Conclusion

• Overview of EQ1 - tectonic processes and hazards.
• Importance of understanding risk factors and hazard development.
• Next session will cover EQ2.