Earthquake Causes: Tectonic Stress and Seismic Energy Release

Earthquakes result from sudden release of accumulated elastic strain energy in Earth's crust, generating seismic waves.

Mechanism: Elastic Rebound Theory

1. Tectonic plates move (cm/year) but faults lock due to friction.
2. Strain accumulates in rocks on either side of the fault.
3. Stress exceeds rock strength → fault ruptures.
4. Rocks snap back (elastic rebound) → energy radiates as seismic waves.

Tectonic Settings

• Plate boundaries (90% of quakes):
  • Convergent (subduction): Megathrust quakes (largest, e.g., 2011 Tohoku, 2004 Sumatra).
  • Transform (strike-slip): Shallow, destructive (e.g., San Andreas, North Anatolian).
  • Divergent: Shallow, smaller (mid-ocean ridges, rift valleys).
• Intraplate: Within plates (e.g., New Madrid, 1811–12); ancient faults reactivated.

Key Concepts

• Focus (hypocenter): Point of rupture within Earth.
• Epicenter: Surface point above focus.
• Seismic waves: P (primary, compressional), S (secondary, shear), Surface (Love, Rayleigh — most damaging).
• Magnitude: Moment magnitude (Mw) — energy released (logarithmic).
• Intensity: Mercalli scale — shaking severity at a location.

Stress Accumulation

Interseismic strain builds over decades to centuries. GPS/InSAR measures crustal deformation to estimate seismic hazard. "Locked" faults are the greatest threat.

Earthquakes are inevitable on active faults; preparedness (building codes, early warning, drills) saves lives.