Measurement of Earthquakes

Measuring of an Earthquake:

Generally, earthquakes occur at depths of lesser than 100 kilometers under the ground. The earthquakes are frequently noted by the instruments like seismograph on daily basis. The location of the earthquakes are also determined by the seismographs. The point at which an earthquake is noted is known as the focus-point or hypocenter. Just like ripples in a pool of water, the vibrations spread out all around from the hypocenter. The location of the earth’s surface quite above the hypocenter is known as the epicenter. It is the epicenter, which normally bears the brunt of the destructive power of the earthquake vibrations. The epicenter is the place of maximum damage, being the nearest point to the hypocenter. The extent of the damage is directly dependent on the intensity of vibration or energy associated with the earthquake. Moreover, the destruction also depends on the density of the population near the epicenter. In the case of densely populated cities, an earthquake of high magnitude may be catastrophic, while if there is few people living around the epicenter, an earthquake will kill fewer people and bring lesser destruction. Another point must be noted that earthquakes do not directly kill people and animals, but the cause of deaths is our own constructed structures, which can not survive earthquakes. An earthquake is a natural phenomenon, which will happen at all costs. We can not make earthquakes not happen. We should construct buildings in a way that may resist earthquakes. We should follow the building codes in earthquake-prone zones.

Note: The nature of the soil is another factor, which determines the extent of the damage. If the soil is loose and damp, the damage is greater, but if it is hard and firm, it will bring lesser damage. Keeping in view the damage caused by earthquakes, an earthquake can be measured in two ways; in terms of its magnitude, and in terms of its intensity.

Measuring the magnitude of the earthquake (Richter Scale):

The energy of vibration determines the magnitude of an earthquake. If the energy released is higher, the earthquake will be of higher magnitude, while in the case of lower vibratory energy, the magnitude will be lower. The magnitude of an earthquake is measured by a seismograph on a scale known as the Richter Scale. The range of this scale starts from zero and the maximum magnitude on this scale can be recorded is 10. The energy of the vibrations increases by steps of about 30 on this scale. Each ascending scale on it will be multiplied by 30 in order to determine the amount of energy released. For example; vibrations of an earthquake of a magnitude 5, are 30 times more energetic than those of a quake measuring 4. Similarly, vibrations of an earthquake of magnitude 7 would be 30 times greater than the vibrations of an earthquake of magnitude 6. The following chart will clearly indicate the rate of change in energy at each ascending scale.

• 1 magnitude     =0.5 kg of TNT
• 2 magnitude     =0.5 ✕ 30            = 15kg of TNT
• 3 magnitude     =15 ✕ 30             = 450 kg of TNT
• 4 magnitude     =450 ✕ 30           = 13500 kg of TNT
• 5 magnitude     =13500 ✕ 30        = 40,5000 kg of TNT
• 6 magnitude     =40,5000 ✕ 30     = 12,150,000 kg of TNT
• 7 magnitude     =12,150,000 ✕ 30 = 364,500,000 kg of TNT
• (at each upper-scale the energy would be multiplied with 30)

Earthquakes measuring 9 or more on this scale are rare. Those measuring from 8 to 8.9 are quite devastating, while those between 7 and 7.9 are considered major. Even moderate (5 to 5.9) and strong (6.0 to 6.9) earthquakes are quite destructive in densely populated areas.

Measuring the intensity of earthquakes (Modified Mercalli scale):

The impact of an earthquake is known as its intensity. This scale measures the intensity of an earthquake over a range of I to XII. Earthquakes of intensity level I, termed instrumental, are merely recorded by instruments and are hardly felt by anyone. An earthquake of intensity level XII is termed catastrophic. The intensity of an earthquake varies from point to point. For example; the intensity of an earthquake of 8 magnitudes is higher at a point nearby the epicenter, than a point far from the epicenter. The closer we are to the epicenter, the higher the intensity will be.