Foundation, Concrete and Earthquake Engineering

SEISMIC WAVES

Earthquake

A type of earth movement that locally gives rise to engineering problems is called the earthquake.Whenever the earth is suddenly struck or disturbed 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.

Earthquake waves

Two types of waves are produced during earthquake. These are

1. Body waves
2. Surface waves
1. Body waves

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.

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.

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.

2. Surface Waves

These waves travel along the earth's surface having similarity in behavior with sea waves.These waves also known as Long waves (L-waves).

Surface waves consist of two waves. These are

a) Love Waves

b) Rayleigh Waves

a) Love Waves

L
ove waves cause surface motions similar to that by S-waves, but with no vertical component. S-waves in associated with effects of Love waves cause maximum damage to structure by their racking motion on the surface in both vertical and horizontal directions.

b) Rayleigh Waves

R
ayleigh Wave makes a material particle oscillate in an elliptic path in the vertical plane (with horizontal motion along direction of energy transmission).

The speed of waves in decreasing sequence :

when P-waves and S-waves reach the earth's surface, most of their energy is reflected back. Some of the energy is returned back to the surface by reflections at different layers of soil and rock. Shaking is more severe (about twice as much) at the earth's surface than at substantial depths. This is often the basis for designing structures buried underground for similar levels of acceleration than those above the ground.