Earthquake Mechanism and Stress

Earthquake Mechanism

• Earthquakes are ground shaking resulting from structures connected to the ground vibrating
• Ground motion, structural vibration, and building destruction are detailed processes in earthquakes (Celebi and Brown, 1994)

Stress and Forces in Earth's Crust

• Enormous lithosphere slabs move unevenly, causing earthquakes
• The Earth's brittle outer layer is under constant stress
• Stress is the force per unit area applied to a rock
• Four stress types act on materials:
  • Compression: Rocks are squeezed together; common at convergent plate boundaries
  • Tension/Extensional: Rocks are pulled apart; common at divergent plate boundaries
  • Shear: Forces are parallel but move in opposite directions; common at transform plate boundaries
  • Confining stress: Deeply buried rocks are pushed down by the weight of material above; cannot deform

Rock's Response to Stress

• Elastic Deformation: Rock returns to its original shape after stress is removed; reversible and not permanent
• Plastic Deformation/Ductile Deformation: Rock does not return to its original shape; irreversible and permanent
• Brittle Fracture: Rock breaks

Types of Folds

• Compressive stresses cause rocks to crumple into folds
• Rocks do not return to their original shape after deformation
• Monocline: Simple bend in rock layers; no longer horizontal
• Anticline: Upward-arched fold; oldest rocks at the center
• Syncline: Downward-bending fold; youngest rocks at the center

Types of Earthquakes

• Earthquakes vary in their behavior
• Earthquakes are sudden energy releases in the Earth's crust or upper mantle
• Seismicity is the earthquake frequency, type, and size experienced over time
• Moment magnitude is the most used scale for measuring earthquakes globally, primarily for those above magnitude 5

Seismic Waves

• Seismic waves are vibrations traveling through the Earth's interior or along its surface
• Body Waves: Travel through the Earth's interior
  • Primary (P) waves: Fastest; move through solids, liquids, or gases: Compressional
  • Secondary (S) waves: Slower; move only through solids: Transverse
• Surface Waves: Travel along the Earth's surface
  • Love waves: Transverse motion, horizontal; most destructive to structures
  • Rayleigh waves: Complex motion; up-and-down, and forward-backward; most felt during earthquakes

Tectonic Earthquakes

• Explained by elastic rebound theory
• Generated when strains in rock masses have accumulated and can't withstand the stresses
• Caused by the sudden fracturing of rock masses
• Example: San Andreas Fault (ruptured in 1906)

Motions of Plate Tectonics During Earthquakes

• Divergent Boundaries: Two plates move away from each other (e.g. Great Rift Valley in Africa, the Red Sea and the Gulf of Aden)
• Convergent Boundaries: Two plates move towards each other
  • Continental-Oceanic: Oceanic plates sink (subduction); create trenches and volcanoes
  • Oceanic-Oceanic: One oceanic plate sinks under another, forming volcanoes
  • Continental-Continental: Both plates are strong, causing collisions and mountain formation
• Transform Boundaries: Two plates slide past each other (e.g., San Andreas Fault)

Volcanic Earthquakes

• Related to volcanic activity
• Types:
  • Volcano-tectonic: Caused by stress changes in solid rock due to magma.
  • Long-period: Related to the unsteady transport of magma.

Explosion Earthquakes

• Caused by nuclear or chemical explosions.

Collapse Earthquakes

• Occur due to landslides or material collapsing below the Earth's surface.
• Can result from massive rock movement, steep slopes, heavy rainfall.

Earthquake Magnitude

• Measure of an earthquake's size.
• Proportional to the amount of released seismic energy
• Seismometer data is used to determine the earthquake characteristics (e.g., location, magnitude, time).
• Modern systems precisely amplify and record ground motion over 0.1 to 100 seconds.

Earthquake Intensity

• Measure of shaking and damage caused by an earthquake
• Varies by location
• Many intensity values are distributed around the epicenter

Earthquake Destruction

• Structural damage to buildings
• Injuries and casualties
• Costs
• Psychological effects
• Environmental impacts

Tsunamis

• Caused by earthquakes, landslides, or volcanic eruptions
• Powerful ocean waves generated
• Types: Local, Regional, Distant/Tele

Liquefaction

• Phenomenon where saturated soil temporarily loses strength during earthquakes
• Causes instability, damage to structures, and economic problems
• Happens when excess pore water pressure from cyclic loads weaken the soil.

Fires

• Secondary effect of seismic events; not directly from shaking itself
• Causes: ruptured gas lines, electrical faults, collapsed structures, ignition from friction
• Widespread and difficult to control

Floods

• Earthquakes can indirectly lead to floods through several mechanisms:
  • Dam failures
  • Landslides
  • Debris flows
  • Ground rupture effects on rivers