Waves Through the Earth
When rock suddenly fractures along a fault, the stored elastic energy radiates outward as seismic waves. These waves carry information about both the earthquake source and the material they pass through. Just as a doctor uses ultrasound to image the body's interior, seismologists use earthquake waves to map Earth's hidden structure — from the thin crust to the solid iron inner core 5,150 km below our feet.
P-Waves, S-Waves, and Surface Waves
Earthquakes generate three main wave types. P-waves (compressional) are fastest, arriving first — they squeeze and stretch rock in the travel direction. S-waves (shear) are slower, moving rock sideways. Both travel through Earth's deep interior. Surface waves (Love and Rayleigh waves) travel along the surface and cause the most destruction — rolling the ground like ocean waves. The critical discovery: S-waves cannot travel through liquids, which proved that Earth's outer core is molten.
The Simulation
This visualization shows a cross-section through Earth with its concentric layers: crust (thin outer shell), mantle (thick rocky layer), liquid outer core (orange), and solid inner core (yellow). The earthquake source appears as a red star at the specified depth. Watch wavefronts expand outward — P-waves (cyan) racing ahead of S-waves (red), with surface waves rolling along the top. A synthetic seismogram at the observer distance shows the characteristic wave arrivals.
Reading the Seismogram
The seismogram trace shows ground motion over time at your chosen distance. The P-wave arrives first as a sharp, small signal. The S-wave follows with larger amplitude. Surface waves arrive last with the longest period and highest amplitude — they cause the most building damage. The S-P time lag increases with distance: at 500 km it is about 62 seconds, at 5000 km about 620 seconds. Three stations' S-P times are enough to locate an earthquake's epicenter through triangulation — the same principle as GPS.