Foundation, Concrete and Earthquake Engineering

What is Seismic Body Waves

Whenever the earth is suddenly struck or disturbed  due to earthquake vibration are produced. These vibrations are setup or start from a limited area and are propagated outward in all directions. Thus an earthquake may be defined as the passage of these vibrations in the earth.

Body waves consist of two waves. These are

a) Primary waves ( P-waves)
b) Secondary Waves (S-waves)

a) Primary waves

1) Nature : These are longitudinal or compressional in nature. Therefore it is known as longitudinal waves or compressional waves.

2) Direction of Particle Vibration : The rock particles vibrates in the direction of propagation of the waves, with a push and pull effect.
Primary waves ( P-waves)

3) Speed
: It is the fastest waves and therefore first to be recorded at the recording station. It travels with about the same speed as sound through same rock.

4) Example : In granites, P -waves have speed of about 4.8 Km/Sec.
5) Penetration Capacity : These waves are capable of passing through solids as well as liquides.

b) Secondary Waves :

1) Nature: These are transverse or distortional in nature. Therefore it is known as transverse waves, shear waves or shake waves.
Secondary Waves (S-waves)
2) Direction of Particle Vibration: The rock particles vibrate at right angles to the direction of propagation like light waves.

3) Speed: These travel slower than the P-waves and are second to be recorded.

4) Example: In granites, S-waves have speed of about 3 km/sec.

5) Penetration capacity: These can pass through solids but it is in capable of passing through liquids.

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