curve in the recorded trace of a standard torsion seismometer by a shock of magnitude 0. Gutenberg and Richter have drawn a showing the relation between A, and A and we extract below for convenience a small table showing this relation. The energy of the earthquake E being proportional to the square of the amplitude the relation between energy and magnitude is expressed by the relation log E-log E=2M where E. is the energy of an earthquake of magnitude 0. The following table gives the recorded maximum horizontal ground movements at a few observatories due to the Quetta earthquake and the corresponding calculated values of the magnitude. Taking 7.3 to be the magnitude of the earthquake, its energy E is given by log E=14:6+log E To determine E absolutely, we require to know E, the energy of a shock of magnitude 0. This was first estimated by Richter to be 106 ergs, but was later modified by Gutenberg and Richter to 107 to 108 ergs. Taking E, to be 107 ergs, E=4.0 x 1021 ergs. (3) We can also determine the approximate energy of the earthquake from the area over which the shock was felt. Assuming from his Pasadena experience that the lower limit of perceptibility of an earthquake corresponds to an acceleration of 250 milligals (0-25 cm/sec) or a recorded maximum amplitude of 5 mm. in the seismogram of a standard torsion seismometer, Richter gives the following table showing the relation between the radius of the felt area and the magnitude of the earthquake. According to West, the area over which the Quetta earthquake was felt was approximately 105,000 sq. miles and its mean radius is therefore 295 km. According to above table, the magnitude would only be 5.7 which is obviously too low. It is probable, as West has pointed out, that owing to the fact that the earthquake occurred at night when people were asleep it was not felt over as wide an area as it would have been during daytime. Remembering the fact that the energies of the most intense earthquakes such as the Assam earthquake of 1897 have been estimated to be above 1025 ergs, the present earthquake had less than 1 as much energy as the most intense shocks recorded in recent 2000 years. 1C. F. Richter, “ An Instrumental Earthquake Magnitude Scale,” Büll. Seism. Soc. Amer., Vol. 25, No. 1, p. 18, (1935). |