Magnitude of an Earthquake

an arbitrary quantity that characterizes the total energy of the elastic oscillations generated by earthquakes or explosions; the magnitude is proportional to the logarithm of the energy of the oscillations and usually is determined by the maximum ratio of the amplitude to the period of the oscillations recorded by seismographs. The magnitude of an earthquake makes it possible to compare the energy of sources of oscillations. A unit increase in the magnitude of an earthquake corresponds to a 100-fold increase in the energy of the oscillations. The strongest known earthquakes have a magnitude no higher than 9 (corresponding to approximately 10¹⁹ joules or 10²⁶ ergs). The intensity of an earthquake in points is estimated from the vibration and destruction on the earth’s surface and depends on the depth of the seismic focus and the geological conditions of the epicentral region, in addition to the magnitude. With a shallow seismic focus, destruction at the epicenter may begin at a magnitude of about 5, while in an earthquake of magnitude 7 there is almost no destruction if the focus is hundreds of kilometers deep.