The area on the surface vertically over the focus is termed as epicentre, epifocus or epifocal area.
The vertical line passing through the centre and epicentre is seismic vertical. The area of disturbance is known as seismic area. Line joing the place, where the shock arrives at the same time is known as homoseismal or co-seismal and the line joining the places of equal intensity of the quake is known as isoseismal.
If the focus is a point, then the coseismals are circles, but generally, they are elliptical as the focus is commonly a line. The epicentre is determined from the study of distribution of co-seismals and iso-seismals.
The energy released by an earthquake propagates by waves. These are of three types. The primary, push or P-waves which are longitudinal and compressional and the secondary, shake shear or S- waves which are transverse and distortional in nature. The third waves are called free, long, L-waves, ground waves or Raleigh waves.
These waves are responsible for all the destruction and damage. The P and S-waves obey the laws of reflection and refraction. The P-waves are transmitted through solids and fluids, but the S-waves are obstructed by fluids. P-waves travel outward in straight lines in all directions from the focus. S-waves vibrate at right angles to the direction of propagation and are more destructive than the P-waves. After the main waves die down, after shocks are felt. The P- waves are the fastest and the L-waves are the slowest.
P-waves S-waves L-waves after shocks. Different types of earthquake waves