Various facts appear to suggest that one continuous field of mechanical stress developed in the earth's crust has a certain upper limit for its voluminal extent. The ultimate mechanical stress energy that can be stored up in this whole volume until a break-down takes place in it may be identified with the energy of the largest possible earthquake. The energy deduced on this hypothesis agrees well with those of the actual largest earthquakes. The area A in which aftershocks occur in association with a major earthquake has been found by UTSU and SEKI regularly to increase with the magnitude M of that main shock. This relation, when combined with the magnitude-energy relation due to GUTENBERG and RICHTER, yields a formula
E=6×10²×A^1·5.
The numerical values of the coefficient and of the exponent of A in this formula can be well explained by the hypothesis stated above regarding the spatial distribution of the stress energy within the earth's crust.