How are Earthquakes measured?

The severity of an earthquake can be measured either by measuring its magnitude or intensity. Magnitude is a measure of the strength of an earthquake at its source. It is assessed on an 8-point scale called the Richter Scale. Devised in 1935, by an American geophysicist, Charles Richter, the scale assigns a number to an earthquake depending on its strength. The Richter magnitude is calculated by using information obtained from a seismograph — the instrument that records an earthquake’s ground motion.

Intensity is a measure of the local effect caused by an earthquake which varies according to distance from the source of the earthquake. The intensity is measured in the Mercalli scale, formulated originally by an Italian seismologist Giuseppe Mercalli. The scale was subsequently modified and is still used as the Modified Mercalli Intensity Scale (MM). It classifies earthquake into 12 categories ranging from 1 (not felt except by few) to 12 (total destruction).