How Earthquakes Happen: Elastic Rebound, Seismic Waves & Resonance

Locked faults and elastic rebound

A fault is a fracture in the crust where two sides have moved relative to each other, and most earthquakes occur along the boundaries of Earth's roughly 15 tectonic plates, which drift at 1–15 cm per year — about the speed your fingernails grow. Between earthquakes the two sides of a fault are locked by friction while tectonic forces keep pushing, bending the rock elastically like a spring. Harry Fielding Reid's elastic rebound theory, developed after the 1906 San Francisco earthquake, describes what happens next: strain accumulates for years or centuries until it exceeds the fault's frictional strength, the fault ruptures at 2–4 km/s, and the rocks snap back to their undeformed shape, releasing the stored energy as seismic waves and heat before the fault re-locks and the cycle begins again.

P-waves, S-waves and early warning

P-waves (primary, compressional) are the fastest, travelling 5–8 km/s through the crust and arriving first. S-waves (secondary, shear) follow at 3–5 km/s, cannot pass through liquids, and cause more damage due to their larger ground motion. Slowest of all are surface waves — Love waves (horizontal shearing) and Rayleigh waves (a rolling motion), which often produce the strongest shaking felt at the surface. Because P-waves outrun the destructive S- and surface waves, the time gap grows with distance — about 12 seconds at 100 km — which is exactly what Japan's earthquake early-warning system exploits, detecting the P-wave and sending an alert seconds before the damaging shaking arrives.

Magnitude, and why it isn't the whole story

Each whole-number increase in moment magnitude represents roughly a 32× increase in energy released — an M8 releases about 1,000× more energy than an M6, and M9 versus M5 is roughly a millionfold difference. But intensity (the Modified Mercalli scale) — how much shaking people actually feel — depends on distance, soil type and building construction, not magnitude alone. That's why the physics of resonance matters more to survival than the number on the news.

Resonance: why some buildings collapse and others don't

A shaken building behaves as a damped oscillator: ẍ + 2ζω₀ẋ + ω₀²x = −ẍ(ground). Its natural frequency follows the rule-of-thumb f₀ ≈ 1/(0.1·N) for an N-storey frame, so a 2-storey block rings at about 5 Hz while a 15-storey tower rings at about 0.67 Hz. When the shaking frequency approaches f₀, sway is amplified by up to 1/(2ζ) — roughly 10× for masonry and 25× for a lightly damped steel frame — and it is that resonant amplification, not the raw magnitude, that breaks buildings. The 1985 Mexico City earthquake is the textbook example: it collapsed 8–18 storey buildings while shorter and taller ones nearby survived, because soft lake-bed soil amplified seismic waves at exactly the resonant frequency of those mid-rise structures.

Frequently asked questions

Try it live

Everything above runs in your browser — open Earthquake, build a structure, pick a material, add floors, then sweep the shaking frequency toward the building's natural frequency and watch the resonance climb. "Random quake" draws an event from a realistic Gutenberg–Richter catalogue. Nothing is installed, nothing is uploaded.