Secondary or S-waves These are transverse waves and live like the oscillations in a piece of string vibrated sideways from one end. In this case, therefore the rocks vibrate perpendicular to the direction of wave propagation. These waves are also known as shear-waves as they are capable of changing the shape of the material without changing its volume.
As the liquid and gaseous substances do not exert resistance to the change in form, the shear waves are not propagated through these substances. However, these waves travel in solid media. These waves move less rapidly than the “P-waves”.
Besides, their velocity varies through the solid parts, proportional to the density of the materials. They are also having short wave-length and high-frequency. Since transverse vibrations cause a shaking of the earth’s surface, these are also known as shaking-waves.
Direction of Wave Propagation
Because the P and S-waves travel through the earth’s interior and spread outward from the focus in all directions they are known as Body-waves.
Surface or L-waves : They are also known as Rayleigh waves or Love waves or Long waves. These waves are generated by the energy brought to the surface by the P and S-waves. These are confined to the outer skin of the crust. Therefore, they are known as surface waves. These waves are transverse in nature and travel on the earth’s surface away from the epicentre like water waves spreading out from a stone thrown into pool.
These waves have much greater wave length in comparison to the P and S waves and accordingly known as Long or L-waves. They travel at comparatively low speeds. Their intensity decreases rapidly with depth.
They produce more ground movement and are responsible for most of the destructive force of the earthquake. Thus, these waves cause great damage to the lives and properties, during an earthquake.
In a particular earthquake-record the lines joining the places where the shock arrives at the same time are called homoseismals or Coseismals or Homoseists . In a similar way, a line joining all places experiencing the same earthquake intensity is known as an ‘Isoseismal-line or in other words, it is an isodiastrophic line of equal damage