The energy of earthquakes

Summary Calculation of the energy released during earthquakes, including all great shocks from 1904 to 1954, indicates that the average annual release of energy in earthquakes is roughly 10 25 ergs. Since this is only about 0–1 per cent of the energy produced by disintegration of radioactive matter in the earth, processes maintained by the generation of heat could furnish the earthquake energy. In each of the three major depth ranges, (a) shallow shocks, depth h ≤ 60 km., (b) intermediate shocks, 60 < h ≤300 km., (c) deep shocks, h > 300 km., the frequency of earthquakes increases about exponentially with decreasing earthquake magnitude m down to at least m = 2, and in each the average energy release between 1904 and 1913 was greater than that in later decades. The largest energy calculated for a single shock during the 51 years, about 2 X 10 25 ergs, was found for two shallow shocks. With increasing focal depth h the maximum energy of a single shock decreases to about 6 X 10 23 ergs at h = 650 ± km. and to about 4 X 10 22 at 720 ± km. No deeper earthquakes are known. The rapid decrease in energy release near 700 km. could be caused by flow processes if the apparent coefficient of viscosity decreases to the order of 10 20 poises at a depth of about 700 km.