Earthquakes
Stored strain released along a fault.
An earthquake is the sudden release of energy when rock fails along a fault. Stress builds as plates move, rock deforms elastically, and when friction is overcome it snaps back — elastic rebound. The point of rupture is the focus; the point on the surface directly above is the epicentre.
Seismic waves
| Wave | Motion | Speed | Travels through |
|---|---|---|---|
| P (primary) | Compression, push-pull | Fastest, arrives first | Solid and liquid |
| S (secondary) | Shear, side to side | Slower | Solid only |
| Surface | Rolling and swaying | Slowest | The surface |
That S waves cannot cross liquid is how the liquid outer core was discovered: they leave a shadow zone on the far side of the planet.
Magnitude is not intensity
Magnitude measures energy released at the source — one number per earthquake, logarithmic, so magnitude 6 releases about 32 times the energy of magnitude 5. Intensity (Mercalli) measures shaking experienced at a place, so a single earthquake has many intensity values depending on distance, depth and ground type.
Deaths correlate with intensity and building stock, not magnitude. A shallow moderate quake under a city does more damage than a large deep one offshore.