What is seismology and seismic waves?

Seismology is the science which studies various aspects of seismic waves generated during the occurrence of earthquakes. Seismic waves are recorded with the help of an instrument, known as seismograph. It may be pointed out that seismology is the only source which provides us authenticated information about the composition of the earth’s interior. The place of the occurrence of an earthquake is called ‘focus’ and the place which experiences the seismic event first is called ‘epicenter’, which is located on the earth’s surface and is always perpendicular to the focus. On the other hand, the focus or the place of the origin of an earthquake is always inside the earth. The deepest focus has been measured at the depth of 700 km. from the earth’s surface. The different types of tremors and waves generated during the occurrence of an earthquake are called ‘seismic waves’, which are generally, divide in 3 broad categories e.g.                                                                                              1. Primary waves
                                             2. Secondary waves
                                             3. Surface waves etc.

Primary waves

Primary waves, also called as longitudinal or compressional waves or simply ‘P’ waves, analogous to sound waves wherein particles move both to and from the line of the propagation of the ray. P waves travel with fastest speed through solid materials. Though these also pass through liquid materials but their speed is slowed down.

Secondary waves

Secondary waves are also called as transverse or distortional or simply ‘S’ waves. These are analogous to water ripples or light waves, wherein the particles move at right angles to the rays. S waves cannot pass through liquid materials.

Surface waves

Surface waves are also called as long period waves or simply ‘L’ waves. These waves generally affect only the surface of the earth and die out at smaller depth. These waves cover longest distances of all the seismic waves. Though their speed is slower than P and S waves, but these are most violent and destructive.
          When an earthquake occurs, the seismic waves are recorded at the epicenter with the help of seismograph. In the beginning, a few small and weak swings are recorded. Such tremors are called ‘preliminary tremors’. After a brief interval the ‘second preliminary tremors’ of strong waves are recorded.