The Seismic Cycle
Earthquakes are not random events but the culmination of a slow, relentless stress-accumulation process driven by plate tectonics. As tectonic plates converge, diverge, or slide past each other at rates of centimeters per year, shear stress builds on locked fault segments. When the accumulated stress exceeds the fault's frictional strength, sudden slip occurs — an earthquake — releasing the stored elastic energy as seismic waves and resetting the stress clock.
Coulomb Friction
The Coulomb failure criterion τ_f = μ × σ_n provides the simplest model for fault strength. Laboratory experiments by Byerlee (1978) showed that most rocks follow μ = 0.6–0.85 regardless of rock type — 'Byerlee's law'. However, real faults may be weaker due to elevated pore fluid pressure, clay minerals in fault gouge, or dynamic weakening during rupture. The effective normal stress σ_n = σ_total - P_pore means that pore pressure plays a critical role in fault mechanics.
Stress, Slip, and Magnitude
The stress drop during an earthquake — typically 1–10 MPa — is a small fraction of the absolute stress level. Fault slip D scales with the fault dimensions: larger faults produce more slip and higher magnitudes. Empirical scaling relations (Wells & Coppersmith, 1994) relate fault length, width, slip, and magnitude, providing the basis for seismic hazard assessment. A 50 km fault typically produces 1–2 m of slip in a Mw 7 earthquake.
Beyond Simple Models
Real fault behavior is far more complex than the simple elastic rebound model. Rate-and-state friction laws describe how fault friction evolves with slip velocity and contact time, explaining phenomena like earthquake nucleation, aftershock sequences, and aseismic creep. Some fault segments slip quietly (slow-slip events), while others remain locked for centuries before rupturing catastrophically. Understanding this spectrum of fault behavior is the central challenge of modern earthquake science.