Seismic waves
When a fault ruptures, energy spreads out as seismic waves. Seismometers record them in a fixed order, and the gaps between arrivals tell us where and how big the earthquake was.
P wave · ≈ 6 km/s · rock compresses along the direction of travel. Follow the coral particle.
P waves — first to arrive
Primary waves are compressional: rock is squeezed and stretched in the direction the wave travels, like a pulse along a spring. They are the fastest, around 6 km/s in the crust, and pass through solids, liquids and gas. You may feel them as a sharp jolt or hear a rumble.
S waves — the strong shaking
Secondary waves shear rock side to side, perpendicular to travel, at about 3.5 km/s. Liquids can’t be sheared, so S waves can’t cross Earth’s outer core — which is how we know it is liquid.
Surface waves — the long roll
Love and Rayleigh waves travel along the surface. They are the slowest but often the largest, and Rayleigh waves move the ground in a rolling ellipse like ocean swell. They cause much of the damage far from the epicentre.
Locating an earthquake
The delay between P and S arrivals grows by roughly 8 km for every second. Draw a circle of that distance around three or more stations and they intersect at the epicentre. Early-warning systems use the fast P wave to send alerts seconds before strong S-wave shaking arrives.