1. Introduction
Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limits. If a strained rock breaks, it then snaps into a new position and, in the process of rebounding, generates vibrations called seismic waves.
Land waves produced by an earthquake have been reported with heights of more than 0.5 m and wave lengths of 8 m. The period between the passages of wave crests can be as much as 10 seconds. The vibrations can continue for as long as an hour before the wave dies out.
2. Focus and Epicenter
Earthquake waves are propagated in all directions from the center of origin or focus of a shock. The average depth of focus for normal earthquakes is about 30 km, though many have a deeper or shallow origin. Shallow-focus earthquakes occur from the surface to a depth of 70 km. They occur in all seismic belts and produce the largest percentage of earthquakes. Intermediate-focus earthquakes occur between 70 and 300 km. Both intermediate-focus and deep-focus earthquakes are limited in number and distribution. The center of origin of an earthquake is called focus and the point on the earth’s surface immediately above the focus is called epicenter.
3. Types of Seismic Waves
Three types of waves are generated by an earthquake shock:
3.1. Primary Waves (P-Waves)
These are a kind of longitudinal waves, identical in character to sound waves passing through liquid or gas. The particles involved in these waves move forward and backward in the direction of wave travel, causing relativity small displacements. These are the first waves to arrive at seismic station.
3.2. Secondary Waves (S-Waves)
These are a kind of transverse waves that are a little slower than the primary waves. The particles are displaces at right angles to the direction of wave propagation. These waves cause strong movements to be recorded on a seismograph.
3.3. Surface Waves (Rayleigh Waves)
The last waves to arrive are the surface waves and travel only in the outer layer of earth and are similar to waves in water. Particles involved in surface waves more in an orbit similar to that of particles in water waves i.e. in a circular path at the surface. The motion diminished with depth. Surface waves caused the most damage during an earthquake. Together with secondary effects from associated landslides, tsunamis, and fires, the results in approximately 10,000 lives and $100 million each year.
4. Elastic Rebound Theory
The origin of an earthquake can be illustrated by a simple experiment. Bend a stick until it snaps. Energy is stored in the elastic bending and is released if rupture occurs, causing the fractured end to vibrate and send out sound waves. Detailed studies of active faults show that this model, known as elastic-rebound theory applied to all major earthquakes. The elastic-rebound theory explains earthquakes as a result of either rupture or sudden movement along existing fractures.
5. Intensity
The intensity or destructive power of an earthquake is an evolution of the severity of ground motion at a given location. It is measured in relation to the effects of the earthquake on human life. Generally, destruction is described in terms of the damage caused to buildings, dams, bridges, and other structures.
6. Magnitude
The magnitude of an earthquake is the measure of the amount of energy released. It is much more precise measure than intensity. The total energy released by an earthquake can be calculated from the amplitude of the waves and the distance from the epicenter seismologists express magnitudes of earthquakes using the Richter scale, which arbitrarily assigns 0 to the lower limits of detection. Each step on the scale represents an increase in amplitude by a factor of 10. The vibrations of an earthquake with a magnitude of 2 are therefore 10 times greater than those of an earthquake with a magnitude of 1, and the vibrations of an earthquake with a magnitude of 8 are 1 million times greater in amplitude than those of earthquake with a magnitude of 2.
