An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth’s lithosphere that creates seismic waves.
Intensity: The severity of earthquake shaking is assessed using a descriptive scale – the Modified Mercalli Intensity Scale.
Magnitude: Earthquake size is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake. When an earthquake occurs, its magnitude can be given a single numerical value on the Richter Magnitude Scale. However, the intensity is variable over the area affected by the earthquake, with high intensities near the epicenter and lower values further away. These are allocated a value depending on the effects of the shaking according to the Modified Mercalli Intensity Scale.
Earthquakes can be measured in two ways:
- Magnitude – the amount of energy released at the earthquake source.
- Intensity – how much the ground shakes at a specific location. Although several scales have been developed over the years, the two commonly used today are the Moment Magnitude Scale, which measures magnitude (M), or size, and the Modified Mercalli Scale, which measures intensity.
Moment Magnitude Scale
- Measurements on the Moment Magnitude Scale are determined using a complex mathematical formula to convert motion recorded with a seismometer into a number that represents the amount of energy released during an earthquake.
- The energy released for each whole number measurement is about 31 times greater than that released by the whole number before.
- The smallest earthquakes recorded today on the Moment Magnitude Scale have negative magnitudes (such as M -2.0) because the scale’s range is based on that of the Richter Scale, developed in the 1930s when monitoring types of equipment were less sensitive.
- Scientists are now able to detect earthquakes smaller in magnitude than the “0” used as the Richter Scale baseline.
Measurements of Intensity on the Modified Mercalli Intensity (MMI)
- The scale ranges from I to XII and is based solely on damage assessment and eyewitness accounts.
- Intensity measurements near the source of an earthquake are generally higher than those at a distance.
- Determining intensity can be difficult in sparsely populated areas with few buildings because the intensity is calculated largely based on the effects that tremors have on human-made structures.
Although an earthquake’s magnitude and intensity measurements are not precisely comparable, they can, in general, be correlated when intensity measurements nearest the epicenter are used in the comparison. The magnitude of earthquakes occurring before the introduction of the Richter Scale are estimated based on reported damage and intensity. Seismologists categorize modern earthquakes by their magnitude, not by their perceived intensity.
Demarcation of Seismic Zones
Earthquake-prone areas of the country have been identified on the basis of scientific inputs relating to seismicity, earthquakes that occurred in the past, and the tectonic setup of the region. Based on these inputs, the Bureau of Indian Standards [IS 1893 (Part has grouped the country into four seismic zones, viz. Zone 11, 111, IV, and V. Of these, zone V is seismically the most active region, while zone 11 is the least.