Earthquake measurement is not a simple problem. It is hampered by many things. Few people are capable of observing carefully what is happening around them when they are experiencing an earthquake. Moreover, equipment and structures often fail during severe earthquakes. Also, most quakes only last seconds or at most, minutes. Lastly, effects of the same event vary widely from location to location. Therefore it is difficult to coherently and accurately describe what happens during an earthquake based on eyewitness accounts. To measure earthquakes we use two different scales. One measures the energy of the waves, the other the resulting amount of damage. 

The Richter Scale

The first, the Richter scale, is an absolute scale that measures the amount of ground movement and the energy released by an earthquake, its magnitude. An earthquake of magnitude 1 releases approximately as much energy as that generated by the explosion of one pound of TNT. Each number on the Richter scale represents a ten-fold increase in ground motion and an associated energy release of approximately 30 times that of the previous number. This means that an earthquake of magnitude 7 will be nearly 1,000 times more powerful than a magnitude 5 event. Earthquakes of magnitude 6 or greater are considered severe, and represent potentially devastating events. Unfortunately, they are not all that rare.  Although the Richter scale gives a fairly accurate measurement of the amount of energy released, it does not give any information as to what happens to people and structures. For that, we use the Mercalli scale. 

The Modified Mercalli Scale

The Modified Mercalli Scale is a qualitative scale which describes the actual intensity or damage caused by an earthquake. On this scale, an earthquake of intensity I (we use Roman numerals for Mercalli intensities) can only be detected by instruments. An event of Intensity VI is felt by all, though damage to structures is slight; an intensity of XII corresponds to total destruction. At first glance it may appear that such a scale is not very scientific because it is descriptive and anecdotal in nature. However it has proved invaluable to urban planners who have come to understand that the hazards to human life and property are neither random or uniform, but follow definite patterns. 

In part, the intensity of an earthquake is a function of its magnitude and distance from the epicenter. Thanks to the use of the Mercalli scale, we have learned that there is a third factor which also affects the extent of damage. It has been observed that the geology of the area is just as critical, and that buildings erected on loose materials have a much greater tendency to fail than those built on solid rock. The Mercalli scale, therefore, has deep engineering, planning, and safety implications.