Seismic vibrations travel as P-waves, S-waves, and surface waves (Rayleigh and Love surface waves). Structure determination is based on the differential transmission of seismic vibration through materials of different density and state (molten, solid).
P-waves, or pressure waves, or primary waves travel the fastest and propagate as alternating vertical compressions and contractions. P-waves, like sound waves, propagate through a liquid medium, so can travel through the Earth's molten core. S-waves, or shear waves, or secondary waves, are slower than P-waves and arrive later at a seismogram later than P-waves. Unlike P-waves, S-waves travel only through solid rock and cannot reach a seismograph on the opposite side of the planet from a seismic event (shadow), providing evidence for Earth's molten core. Surface waves result when P- and S-waves interact at the surface.
Rather than wait for natural seismic events, geologists employ controlled source seismology (small explosions) and receiver functions to map geological features such as rock types and discontinuities, geologic traps, faults, buried craters, salt domes. Receiver functions show seismic energy that has been converted to shear waves from direct P-waves at discontinuities within the earth.
NG interactive earthquakes :