There are many scales divided for the measurement of earthquake intensity. Out of these Mercali-Richter scale is the most well known.
Mercali's Scale of Earthquake. It is qualitative and not a quantitative scale. Hence it is not considered to be accurate from the point of view of scientific measurement. This scale is dependent upon the experience gained by human sensory organs and the devastating effects of the earthquakes.
Richter Scale. Richter scale is made on the basis of the graph of earthquake measuring instruments. The intensity of the earthquake is measured by the energy, which is freed from rocks. It is expressed by numbers.
Earthquake represented by 7 causes great devastation. In the earthquake represented by 8, energy 200 thousand times that of number 5 and 3,5000 times that of number 6 is produced.
The isoseismal line is the line, which joins the points of an area having equal earthquake intensity. These lines are irregular.
Scientific Study of Earthquakes. Usually the earthquake originates at a point many km deep under the earth's surface. It is called Focus. Some scientists think that it is not a point but a limited area.
The point on the earth's surface vertically above the focus is called epicenter. The main area of earthquake effect surrounds this point.
Three types of waves are produced from the focus of an earthquake.
The first type of earthquake waves is called Primary waves. Areas of contraction and expansion are alternately produced in these waves. These waves are also called Longitudinal waves or P waves. These are really sound waves, which produce rumbling noise at the time of earthquake. The speed of these waves range from 5.4 to 13.8 km., per second.
Secondary Waves. These are the second type of waves. The vibration of these waves is at right angle to the direction of propagation of the waves. It is why these waves called transverse waves. They are known as S waves. The speed of these waves is 3.2 to 7.2 km., per second.
Tertiary Waves. These waves are called L-waves. Their speed is 4 to 4.3 km, per second. The speed of these waves is more stable than that of the other waves. These waves emit out of the epicenter and travel on the surface of the earth.
Seismograph. It is an instrument with which we measure the time taken by P,S and L waves in traveling distances. The route and the speed of traveling of these waves are different. Hence the time of the arrival of the three waves is measured at the station where seismograph is set up. The greater the distance of P arid S waves from the surface of the earth the greater is their speed but the traveling time is decreased. The difference between the time of arrival of P and S waves indicates the place of the origin of the earthquake. For example, if the difference in the time of arrival of P and S waves is 9 minutes and 10 seconds the epicenter should exist 7,724 km, away from that place.
The Depth of Focus. Seismologists believe that the 20% of the earthquakes are within a depth of 48 km. The earthquakes can be divided into three classes depending upon their depth.
(1) Shallow Focus Earthquakes. The foci of these earthquakes are within a depth of 70 km.
(2) Intermediate Focus earthquake. The foci of these earthquakes are between 70 and 300 km.
(3) Deep Focus earthquake. The depth of their foci is more than 300 km. The deepest focus found up to date is the one, which was 700 km deep.