EARTHQUAKE MAGNITUDE, INTENSITY, ENERGY, POWER LAW RELATIONS AND SOURCE MECHANISM J.R. Kayal Geological Survey of India, 27, J.L. Nehru Road Road, Kolkata – 700 016 email :
[email protected] EARTHQUAKE MAGNITUDE Magnitude is one of the basic and important parameters of an earthquake. It defines the size of an earthquake. The beginners of seismology are, in general, confused about different scales of magnitude, and sometimes they mix-up earthquake intensity with its magnitude. Journalists often report the magnitude value of an earthquake as its intensity; this is wrong. There are now different magnitude scales to define the size of an earthquake. After Richter (1935), various magnitude scales are proposed; all these scales are discussed below. Richter Magnitude (or Local Magnitude) ML Richter (1935) defined the local magnitude ML of an earthquake observed at a station to be ML = log A - log Ao ( ∆) (1) where A is the maximum amplitude in millimetres recorded on the Wood- Anderson seismograph for an earthquake at epicentral distance of ∆ km, and Ao (∆ ) is the maximum amplitude at ∆ km for a standard earthquake. The local magnitude is thus a number characteristic of the earthquake, and independent of the location of the recording station. Three arbitrary choices are made in the above definition: (i) the use of standard Wood-Anderson seismograph, (ii) the use of common logarithms to the base 10, and (iii) selection of the standard earthquake whose amplitudes as a function of distance are represented by Ao (∆). The zero level of Ao (∆) can be fixed by choosing its value at a particular distance. Richter chose the zero level of Ao (∆) to be 1 µm (or 0.001 mm) at a distance of 100 km from the earthquake epicentre.