Seismic Waves & Subduction
When a fault slips, the energy spreads out as seismic waves. They're the actual shaking you feel, and the same waves that seismographs record to locate and size every quake on the tracker. There are three main kinds, and they arrive in a predictable order.
The three types of seismic wave
P-waves (primary)
The fastest waves, travelling at roughly 5–8 km/s through the crust. They are compressional — the rock squeezes and stretches in the same direction the wave moves, like a sound wave. P-waves arrive first and are usually felt as a sharp jolt or bang.
S-waves (secondary)
About 1.7 times slower than P-waves. They are shear waves — the ground moves side to side, perpendicular to the wave's travel. S-waves can't move through liquids, which is how scientists discovered the Earth has a liquid outer core. They carry more of the damaging shaking.
Surface waves
Slowest of all, but often the most destructive. They travel along the Earth's surface and roll or sway the ground (Love and Rayleigh waves), and because their energy is concentrated near the surface they cause much of the damage to buildings in a major quake.
How the wave order helps us — and warns us
Because P-waves outrun S-waves, the gap between them grows with distance. A single station can use that gap to estimate how far away the quake was, and several stations together pinpoint the epicentre. The same head start is what makes earthquake early-warning possible: systems like ShakeAlert detect the fast, gentler P-waves and can send an alert a few seconds to tens of seconds before the stronger S and surface waves arrive. Our tracker is not a warning system — it reports quakes after they happen — but it relies on exactly this wave physics behind the scenes.
Subduction: where the giants form
At a subduction zone, one tectonic plate is forced down beneath another into the mantle. The boundary between them — the megathrust — is a vast, gently sloping fault that can lock over hundreds of kilometres. When it finally ruptures, an enormous area slips at once, which is why subduction zones produce the planet's largest earthquakes: the 1960 Chile M9.5, 2004 Sumatra M9.1 and 2011 Japan M9.0 events were all megathrust ruptures.
Because the seafloor is suddenly thrust upward, subduction quakes are also the main source of tsunamis. That's why a large, shallow offshore quake on the tracker is flagged for tsunami potential — the displaced water, not the shaking, is often the deadlier hazard.
Depth and the slab
As the descending plate sinks, it keeps generating earthquakes hundreds of kilometres down — a sloping band of deep quakes called a Wadati–Benioff zone. That's why the tracker shows depth: a deep event often marks the subducting slab far below the surface, while a shallow one is closer to the boundary itself.
Filter the map to large quakes and watch them trace the world's subduction zones.
Open the Earthquake Tracker