Seismic activity occurs due to the motion of the Earth's continental crust over the underlying layers of magma. Earthquakes, the most devastating of natural disasters, are the result of intense seismic activity. The only way of preventing large-scale loss of life and property that occurs through earthquakes is creating advanced warning mechanisms or forecasting methods. For such a purpose and recording the levels of seismic activity, the Richter and Mercalli scales were created.
To accurately quantify the intensity of an earthquake, a proper scientific method was developed by scientists, devoted to earthquake study, who are called seismologists. There are a number of methods to measure the intensity. One could measure it according to the magnitude of actual seismic energy that is released or according to the intensity or effects of the earthquake on the surroundings. In fact, two separate types of measurement techniques exist, that depend on magnitude and intensity.
The magnitude-based systems, used to measure seismic activity all over the world are the body wave magnitude, Richter, moment magnitude, and surface wave magnitude scales. Those based on intensity are the European Macroseismic Scale (EMS), INQUA scale, Medvedev-Sponheuer-Karnik (MSK) scale, Shindo scale and the modified Mercalli scale. The magnitude and intensity scales used in United States of America are the Richter and the Mercalli scales, which are compared below.
The Mercalli scale in its original form predates Richter and has its origins in the Rossi-Forel scale used in the 19thcentury. It was modified by Giuseppe Mercalli, an Italian volcanologist, between 1884 and 1906. After a host of revisions and modifications from its original form, it was given its present form by Charles Richter, the guy who also created the Richter scale of earthquake magnitude measurement. After the modifications made by Richter, it came to be known as Modified Mercalli Intensity (MMI) scale. Inspired by the apparent magnitude scale used in astronomy to study the stars, Charles Richter, along with his associate Beno Gutenberg, created the Richter scale in 1935. It is the most widely used seismic activity measurement technique today.
The Richter and Mercalli scales are different in nature. The latter is largely subjective as it measures the intensity of earthquake, whereas the former is an objective and numerical measurement based on readings taken by seismometers. There is a Richter scale formula for calculating the magnitude of earthquake from the instrument readings. It is logarithmic in nature.
The Modified Mercalli Scale is divided into twelve levels with each ascending level describing a graver level of destruction. It begins with the 'Instrumental (Level I)', when tremors are felt only by the instruments and extends up to the highest intensity state (Level XII- called 'Cataclysmic'), characterized by total destruction.
The Richter scale range extends from zero to excess of 10 in numerical magnitude. Minor earthquakes fall in the range from 0-3.9, moderate ones from 5 to 5.9, strong earthquakes from 6 to 6.9, and most destructive ones occur in the range of 7 and above. A correlation between the magnitude and intensity of an earthquake can be made by correlating the Richter scale and Modified Mercalli scale measurements.