Seismic Waves

An earthquake rupture sends waves through a layered planet. Each layer bends, reflects or blocks them.

Time 0:00 Speed 60×

Earthquake

Magnitude 7.0 Focal depth 25 km

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Seismometers

Stn	Dist	P	S

Earth's layers

What am I seeing?

P waves (compressional) are fastest and travel through solids and liquids.
S waves (shear) are slower and cannot cross the liquid outer core, leaving an S-wave shadow.
Waves bend as the rock changes. Entering the core bends P waves so sharply that a P-wave shadow zone appears roughly 100°–140° from the quake.
Surface waves are slower but carry most of the shaking. Deep quakes barely make them.
Labels: PcP reflects off the core, PKP passes through it, PP bounces off the surface, pP is the depth phase.

Ray-traced through a simplified PREM velocity model. Wavefronts, travel times and the shadow zones are computed, not drawn by hand. Crust thickness is exaggerated for visibility; wavelengths and surface motion are exaggerated.