10.1 Earthquake mechanisms and seismic waves

Earthquakes are Earth's way of releasing built-up stress. They occur when rocks break along faults, sending seismic waves rippling through the planet. Understanding how quakes form and spread is key to predicting and preparing for these powerful events.

Seismic waves come in different flavors, each behaving uniquely as they travel. Body waves zip through Earth's interior, while surface waves cause the most damage. The type of fault and its orientation shape how these waves move, influencing the shaking we feel on the surface.

Earthquake Generation and Seismic Wave Propagation

Elastic rebound theory for earthquakes

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• Elastic rebound theory developed by Harry Fielding Reid in 1910 explains energy storage and release in Earth's crust
• Stress accumulation occurs as tectonic plates apply force to rocks over time causing elastic deformation and storing potential energy
• Strain buildup happens when rocks reach their elastic limit and continued stress causes internal deformation
• Sudden rupture occurs when rocks break along a fault plane as stress exceeds rock strength releasing stored elastic energy
• Energy release generates seismic waves through rapid motion along the fault
• Return to equilibrium involves surrounding rocks snapping back to an undeformed state repeating the process in earthquake cycles

Types of seismic waves

• Body waves travel through Earth's interior
  • P-waves (Primary waves) move fastest longitudinally through solids liquids and gases at ~5-7 km/s in granite
  • S-waves (Secondary waves) travel second fastest transversely through only solids at ~3-4 km/s in granite
• Surface waves propagate along Earth's surface
  • Rayleigh waves move slowest in elliptical rolling motion causing most structural damage
  • Love waves travel faster than Rayleigh waves with horizontal shearing motion also damaging structures significantly

Fault types vs seismic waves

• Normal faults result from extensional forces with hanging wall moving down relative to footwall generating primarily vertical ground motion
• Reverse faults stem from compressional forces with hanging wall moving up relative to footwall producing strong vertical and horizontal ground motion
• Strike-slip faults arise from shearing forces with blocks moving horizontally past each other creating primarily horizontal ground motion
• Fault orientation affects seismic wave radiation patterns influencing ground shaking distribution
• Fault size and displacement correlate with earthquake magnitude producing stronger seismic waves
• Rupture propagation direction and speed influence wave characteristics causing directivity effects amplifying shaking in certain directions

Focus and epicenter of earthquakes

• Focus (hypocenter) marks the earthquake origin point within Earth where initial rock failure and energy release occur typically several kilometers below surface
• Epicenter represents the point on Earth's surface directly above the focus used as a reference for describing earthquake location
• Focal depth measures vertical distance between focus and epicenter classifying earthquakes as shallow intermediate or deep-focus
• Seismic waves radiate outward from focus in all directions with surface waves generated when body waves reach the surface
• Focal depth influences affected area with shallow earthquakes often causing more surface damage
• Understanding focus and epicenter helps identify active fault zones crucial for regional tectonics and seismic hazard assessment