Seismic S-waves. Cutaway illustration of a section through the Earth, showing seismic waves propagating around the globe. Seismic waves (P-waves and S-waves) are waves of energy that travel through the Earth's layers as a result of an earthquake, explosion or volcano. Secondary waves (S-waves) are shear transverse waves which displace the ground perpendicular to the direction of propagation. S-waves are slower than P-waves (primary or pressure waves). They cannot pass through the Earth's liquid outer core, and are not detected more than approximately 103 degrees (approximately 11,570 kilometres) from the epicentre of an earthquake, creating an S-wave "shadow zone".