How do seismic shadow zones reveal Earth's interior?

An earthquake sends seismic waves through Earth. Their arrival times and the locations where particular wave families are missing reveal properties of regions that cannot be directly sampled. P waves are compressional disturbances: material is alternately compressed and expanded along the direction of propagation. S waves are shear disturbances, with motion transverse to propagation. Both can travel through solid rock, but a liquid does not support the shear rigidity needed for an ordinary S wave.

Wave paths also depend on changes in propagation speed. At boundaries and through regions with changing properties, seismic rays can refract, or change direction. The liquid outer core both changes P-wave speeds and prevents a direct S wave from continuing across it as an S wave. The solid inner core is a separate central region.

These effects produce different direct-wave shadow patterns. In a simplified radial Earth, strong refraction leaves an approximate 104°–140° range without direct P arrivals. P energy can pass through the outer core and reappear at more distant stations. Direct S waves do not cross that liquid region, producing a wider far-side direct-S shadow. The illustrated P station at 120° lies within the direct-P shadow.

The inference rests on the combined global pattern, not one silent instrument. Real records contain reflections, diffraction, and converted phases in which a wave changes type. An S-related signal can involve P propagation through the outer core, so a direct-S shadow does not mean that every kind of seismic energy is absent. The diagram isolates the direct families rather than calculating full seismograms.