Measuring Earthquakes
Earthquake Intensity:
Earthquakes are measured. Before the advent of sensitive instruments earthquake were measured by the observed effects, e.g. movement of objects from their original position, degree of shaking felt in different areas, changes occurred in the landscape etc. A qualitative number was assigned to the earthquake, and it was called its intensity at the place of observation. Intensity varies between I and XII (or some times I and X). Areas of equal intensity were joined to draw contours, which were called isoseismals. Isoseismals indicated how the effects of earthquakes varied in different directions. Since, Intensity of an earthquake is assigned by its observed effects in the area of observation, intensity of an earthquake varies from place to place.
Seismograph
With advancing technology, instruments were invented to detect and measure very small movements of the ground. These instruments were called Seismometers. A continuous record of the ground motion as a function of time could be obtained at a location using the seismometer, a clock and the recording device. The combined equipment was called Seismograph and the record was called Seismogram.
Earthquake Magnitude:
Based on the measured amplitude of the earthquake ground motion from the seismogram a number was derived for to assign magnitude to the earthquake. Unlike intensity, which varies from place to place, the magnitude was a fixed number being a measure of the energy released at the earthquake source. Richter scale is conceptual scale on the basis of which a magnitude is assigned to an earthquake. The scale is so designed that a unit increase of magnitude represents nearly 30 times increase in energy released at the earthquake source. For any given magnitude M the energy E released in the earthquake is given as:
log10 E = 11.8 +1.5 M